LTC3868EUFD-1#TRPBF - ANALOG DEVICES

LTC3868-1

38681fd

TYPICAL APPLICATION

FEATURES

APPLICATIONS

DESCRIPTION

Low IQ, Dual

2-Phase Synchronous

Step-Down Controller

High Efﬁ ciency Dual 8.5V/3.3V Step-Down Converter

n Low Operating IQ: 170μA (One Channel On)

n Wide Output Voltage Range: 0.8V ≤ VOUT ≤ 14V

n Wide VIN Range: 4V to 24V

n RSENSE or DCR Current Sensing

n Out-of-Phase Controllers Reduce Required Input

Capacitance and Power Supply Induced Noise

n OPTI-LOOP

Compensation Minimizes COUT

n Phase-Lockable Frequency (75kHz to 850kHz)

n Programmable Fixed Frequency (50kHz to 900kHz)

n Selectable Continuous, Pulse-Skipping or

Burst Mode

Operation at Light Loads

n Very Low Dropout Operation: 99% Duty Cycle

n Adjustable Output Voltage Soft-Start

n Power Good Output Voltage Monitor

n Output Overvoltage Protection

n Output Latchoff Protection During Short Circuit

n Low Shutdown IQ: 8µA

n Internal LDO Powers Gate Drive from VIN or EXTVCC

n No Current Foldback During Start-Up

n Small 4mm × 5mm QFN and Narrow SSOP Packages

n Notebook and Palmtop Computers

n Portable Instruments

n Battery Operated Digital Devices

n Distributed DC Power Systems

Efﬁ ciency and Power Loss

vs Load Current

The LTC

3868-1 is a high performance dual step-down

switching regulator controller that drives all N-channel

synchronous power MOSFET stages. A constant frequency

current mode architecture allows a phase-lockable fre-

quency of up to 850kHz. Power loss and noise due to the

input capacitor ESR are minimized by operating the two

controller outputs out of phase.

The 170A no-load quiescent current extends operating

life in battery powered systems. OPTI-LOOP compensation

allows the transient response to be optimized over a wide

range of output capacitance and ESR values. The LTC3868-

1 features a precision 0.8V reference and a power good

output indicator. A wide 4V to 24V input supply range

encompasses a wide range of intermediate bus voltages

and battery chemistries.

Independent soft-start pins for each controller ramp the

output voltages during start-up. Current foldback limits

MOSFET heat dissipation during short-circuit conditions.

The output short-circuit latchoff feature further protects

the circuit in short-circuit conditions.

For a leadless 32-pin QFN package with the additional fea-

tures of adjustable current limit, clock out, phase modula-

tion and two PGOOD outputs, see the LTC3868 data sheet.

0.1µF

62.5k

3.3µH

680pF

150µF

4.7µF 22µF

50V

0.007Ω

20k 15k

VOUT1

3.3V

150µF

0.1µF

193k

7.2µH

680pF

0.01Ω

20k

15k

VOUT2

8.5V

3.5A

TG1 TG2

BOOST1 BOOST2

SW1 SW2

BG1 BG2

SGND

PGND

SENSE1+SENSE2+

SENSE1–SENSE2–

VFB1 VFB2

ITH1 ITH2

VIN INTVCC

SS1 SS2

VIN

9V TO 24V

38681 TA01

0.1µF 0.1µF

LTC3868-1

L, LT, LTC, LTM, Burst Mode, OPTI-LOOP, µModule, Linear Technology and the Linear logo

are registered trademarks and No RSENSE and UltraFast are trademarks of Linear Technology

Corporation. All other trademarks are the property of their respective owners. Protected by U.S.

Patents, including 5481178, 5705919, 5929620, 6100678, 6144194, 6177787, 6304066, 6580258.

OUTPUT CURRENT (A)

0.0001

EFFICIENCY (%)

POWER LOSS (mW)

0.001 0.01 0.1 1 10

38681 TA01b

100

1000

0.1

10000

EFFICIENCY

POWER LOSS

VIN = 12V

VOUT = 3.3V

FIGURE 12 CIRCUIT

LTC3868-1

38681fd

ABSOLUTE MAXIMUM RATINGS

(Note 1)

ORDER INFORMATION

LEAD FREE FINISH TAPE AND REEL PART MARKING* PACKAGE DESCRIPTION TEMPERATURE RANGE

LTC3868EUFD-1#PBF LTC3868EUFD-1#TRPBF 38681 28-Lead (4mm × 5mm) Plastic QFN –40°C to 85°C

LTC3868IUFD-1#PBF LTC3868IUFD-1#TRPBF 38681 28-Lead (4mm × 5mm) Plastic QFN –40°C to 85°C

LTC3868EGN-1#PBF LTC3868EGN-1#TRPBF LTC3868GN-1 28-Lead Plastic SSOP –40°C to 85°C

LTC3868IGN-1#PBF LTC3868IGN-1#TRPBF LTC3868GN-1 28-Lead Plastic SSOP –40°C to 85°C

Consult LTC Marketing for parts speciﬁ ed with wider operating temperature ranges. *The temperature grade is identiﬁ ed by a label on the shipping

container.Consult LTC Marketing for information on non-standard lead based ﬁ nish parts.

For more information on lead free part marking, go to: http://www.linear.com/leadfree/

For more information on tape and reel speciﬁ cations, go to: http://www.linear.com/tapeandreel/

Input Supply Voltage (VIN) ......................... –0.3V to 28V

Topside Driver Voltages

BOOST1, BOOST2 ................................. –0.3V to 34V

Switch Voltage (SW1, SW2) ........................ –5V to 28V

PLLIN/MODE,

(BOOST1-SW1), (BOOST2-SW2) ................ –0.3V to 6V

RUN1, RUN2 ............................................... –0.3V to 8V

Maximum Current Sourced into Pin from

Source >8V ......................................................100µA

SENSE1+, SENSE2+, SENSE1–

SENSE2– Voltages ...................................... –0.3V to 16V

9 10

TOP VIEW

SGND

UFD PACKAGE

28-LEAD (4mm s 5mm) PLASTIC QFN

11 12 13

28 27 26 25 24

SENSE1+

SENSE1–

FREQ

PLLIN/MODE

SGND

RUN1

RUN2

SENSE2–

BOOST1

BG1

VIN

PGND

EXTVCC

INTVCC

BG2

BOOST2

VFB1

ITH1

SS1

PGOOD1

TG1

SW1

SENSE2+

VFB2

ITH2

SS2

TG2

SW2

815

TJMAX = 125°C, θJA = 43°C/W

EXPOSED PAD (PIN 29) IS SGND, MUST BE SOLDERED TO PCB

TOP VIEW

GN PACKAGE

28-LEAD PLASTIC SSOP

ITH1

VFB1

SENSE1+

SENSE1–

FREQ

PLLIN/MODE

SGND

RUN1

RUN2

SENSE2–

SENSE2+

VFB2

ITH2

SS2

SS1

PGOOD1

TG1

SW1

BOOST1

BG1

VIN

PGND

EXTVCC

INTVCC

BG2

BOOST2

SW2

TG2

TJMAX = 125°C, θJA = 90°C/W

PIN CONFIGURATION

FREQ Voltages ..................................... –0.3V to INTVCC

EXTVCC ...................................................... –0.3V to 14V

ITH1, ITH2, VFB1, VFB2 Voltages ..................... –0.3V to 6V

PGOOD1 Voltage ......................................... –0.3V to 6V

SS1, SS2, INTVCC Voltages ......................... –0.3V to 6V

Operating Temperature Range (Note 2).... –40°C to 85°C

Junction Temperature (Note 3) ............................. 125°C

Storage Temperature Range ................... –65°C to 150°C

Lead Temperature (Soldering, 10 sec)

SSOP ................................................................ 300°C

LTC3868-1

38681fd

SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS

VIN Input Supply Operating Voltage Range 4 24 V

VFB1,2 Regulated Feedback Voltage (Note 4) ITH1,2 Voltage = 1.2V l0.788 0.8 0.812 V

IFB1,2 Feedback Current (Note 4) ±5 ±50 nA

VREFLNREG Reference Voltage Line Regulation (Note 4) VIN = 4.5V to 24V 0.002 0.02 %/V

VLOADREG Output Voltage Load Regulation (Note4)

Measured in Servo Loop,

∆ITH Voltage = 1.2V to 0.7V

l0.01 0.1 %

(Note4)

Measured in Servo Loop,

∆ITH Voltage = 1.2V to 2V

l–0.01 –0.1 %

gm1,2 Transconductance Ampliﬁ er gm(Note 4) ITH1,2 = 1.2V, Sink/Source = 5µA 2 mmho

IQInput DC Supply Current (Note 5)

Pulse-Skipping or Forced Continuous

Mode (One Channel On)

RUN1 = 5V and RUN2 = 0V, VFB1 = 0.83V (No Load) or

RUN1 = 0V and RUN2 = 5V, VFB2 = 0.83V (No Load)

1.3 mA

Pulse-Skipping or Forced Continuous

Mode (Both Channels On)

RUN1,2 = 5V, VFB1,2 = 0.83V (No Load) 2 mA

Sleep Mode (One Channel On) RUN1 = 5V and RUN2 = 0V, VFB1 = 0.83V (No Load) or

RUN1 = 0V and RUN2 = 5V, VFB2 = 0.83V (No Load)

170 250 µA

Sleep Mode (Both Channels On) RUN1,2 = 5V, VFB1,2 = 0.83V (No Load) 300 450 µA

Shutdown RUN1,2 = 0V 8 25 µA

UVLO Undervoltage Lockout INTVCC Ramping Up

INTVCC Ramping Down

l3.6

3.8

4.2

VOVL Feedback Overvoltage Protection Measured at VFB1,2, Relative to Regulated

VFB1,2

71013 %

ISENSE+SENSE+ Pin Current Each Channel ±1 µA

ISENSE–SENSE– Pin Current Each Channel

VOUT1,2 < INTVCC – 0.5V

VOUT1,2 > INTVCC + 0.5V 550

±1

950

µA

DFMAX Maximum Duty Factor In Dropout, FREQ = 0V 98 99.4 %

ISS1,2 Soft-Start Charge Current VSS1,2 = 0V 0.7 1 1.4 µA

VRUN1,2 On RUN Pin On Threshold VRUN1, VRUN2 Rising l1.21 1.26 1.31 V

VRUN1,2 Hyst RUN Pin Hysteresis 50 mV

VSS1,2 LA SS Pin Latchoff Arming Threshold VSS1, VSS2 Rising from 1V 1.9 2 2.1 V

VSS1,2 LT SS Pin Latchoff Threshold VSS1, VSS2 Rising from 2V 1.3 1.5 1.7 V

IDSC1,2 LT SS Discharge Current Short-Circuit Condition VFB1,2 = 0V

VSS1,2 = 5V

71013 µA

VSENSE(MAX) Maximum Current Sense Threshold VFB1,2 = 0.7V, VSENSE1–,2– = 3.3V 43 50 57 mV

Gate Driver

TG1,2 Pull-Up On-Resistance

Pull-Down On-Resistance

2.5

1.5



BG1,2 Pull-Up On-Resistance

Pull-Down On-Resistance

2.4

1.1



TG1,2 tr

TG1,2 tf

TG Transition Time:

Rise Time

Fall Time

(Note 6)

CLOAD = 3300pF

ELECTRICAL CHARACTERISTICS

The l denotes the speciﬁ cations which apply over the full operating

temperature range, otherwise speciﬁ cations are at TA = 25°C. VIN = 12V, VRUN1,2 = 5V, EXTVCC = 0V unless otherwise noted.

LTC3868-1

38681fd

ELECTRICAL CHARACTERISTICS

The l denotes the speciﬁ cations which apply over the full operating

temperature range, otherwise speciﬁ cations are at TA = 25°C. VIN = 12V, VRUN1,2 = 5V, EXTVCC = 0V unless otherwise noted.

SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS

BG1,2 tr

BG1,2 tf

BG Transition Time:

Rise Time

Fall Time

(Note 6)

CLOAD = 3300pF

TG/BG t1D Top Gate Off to Bottom Gate On Delay

Synchronous Switch-On Delay Time

CLOAD = 3300pF Each Driver 30 ns

BG/TG t1D Bottom Gate Off to Top Gate On Delay

Top Switch-On Delay Time

CLOAD = 3300pF Each Driver 30 ns

tON(MIN) Minimum On-Time (Note 7) 95 ns

INTVCC Linear Regulator

VINTVCCVIN Internal VCC Voltage 6V < VIN < 24V, VEXTVCC = 0V 4.85 5.1 5.35 V

VLDOVIN INTVCC Load Regulation ICC = 0mA to 50mA, VEXTVCC = 0V 0.7 1.1 %

VINTVCCEXT Internal VCC Voltage 6V < VEXTVCC < 13V 4.85 5.1 5.35 V

VLDOEXT INTVCC Load Regulation ICC = 0mA to 50mA, VEXTVCC = 8.5V 0.6 1.1 %

VEXTVCC EXTVCC Switchover Voltage EXTVCC Ramping Positive 4.5 4.7 4.9 V

VLDOHYS EXTVCC Hysteresis 250 mV

Oscillator and Phase-Locked Loop

f25k Programmable Frequency RFREQ = 25k, PLLIN/MODE = DC Voltage 105 kHz

f65k Programmable Frequency RFREQ = 65k, PLLIN/MODE = DC Voltage 375 440 505 kHz

f105k Programmable Frequency RFREQ = 105k, PLLIN/MODE = DC Voltage 835 kHz

fLOW Low Fixed Frequency VFREQ = 0V, PLLIN/MODE = DC Voltage 320 350 380 kHz

fHIGH High Fixed Frequency VFREQ = INTVCC, PLLIN/MODE = DC Voltage 485 535 585 kHz

fSYNC Synchronizable Frequency PLLIN/MODE = External Clock l75 850 kHz

PGOOD1 Output

VPGL PGOOD1 Voltage Low IPGOOD = 2mA 0.2 0.4 V

IPGOOD PGOOD1 Leakage Current VPGOOD = 5V ±1 µA

VPG PGOOD1 Trip Level VFB with Respect to Set Regulated Voltage

VFB Ramping Negative

Hysteresis

–13 –10

2.5

–7 %

VFB with Respect to Set Regulated Voltage

VFB Ramping Positive

Hysteresis

710

2.5

13 %

tPG Delay for Reporting a Fault (PGOOD Low) 25 µs

Note 1: Stresses beyond those listed under Absolute Maximum Ratings

may cause permanent damage to the device. Exposure to any Absolute

Maximum Ratings for extended periods may affect device reliability and

lifetime.

Note 2: The LTC3868E-1 is guaranteed to meet performance speciﬁ cations

from 0°C to 85°C. Speciﬁ cations over the –40°C to 85°C operating

temperature range are assured by design, characterization and correlation

with statistical process controls. The LTC3868I-1 is guaranteed over the

full –40°C to 85°C operating temperature range.

Note 3: TJ is calculated from the ambient temperature TA and power

dissipation PD according to the following formula:

J = TA + (PD • θJA)

where θJA = 43°C for the QFN package and θJA = 90°C for the SSOP

package.

Note 4: The LTC3868-1 is tested in a feedback loop that servos VITH1,2 to a

speciﬁ ed voltage and measures the resultant VFB1,2.

Note 5: Dynamic supply current is higher due to the gate charge being

delivered at the switching frequency. See Applications information.

Note 6: Rise and fall times are measured using 10% and 90% levels. Delay

times are measured using 50% levels.

Note 7: The minimum on-time condition is speciﬁ ed for an inductor

peak-to-peak ripple current ≥ 40% of IMAX (See Minimum On-Time

Considerations in the Applications Information section).

LTC3868-1

38681fd

TYPICAL PERFORMANCE CHARACTERISTICS

Efﬁ ciency and Power Loss

vs Output Current Efﬁ ciency vs Load Current

Load Step (Burst Mode Operation)

Load Step

(Forced Continuous Mode)

Load Step (Pulse-Skipping Mode) Inductor Current at Light Load Soft Start-Up

Efﬁ ciency vs Input Voltage

OUTPUT CURRENT (A)

0.0001

EFFICIENCY (%)

0.001 0.01 0.1 1 10

38681 G02

100

VIN = 5V

VIN = 12V

VOUT = 3.3V

FIGURE 12 CIRCUIT

INPUT VOLTAGE (V)

EFFICIENCY (%)

38681 G03

10 20

515 25 28

FIGURE 12 CIRCUIT

VOUT = 3.3V

IOUT = 4A

OUTPUT CURRENT (A)

0.0001

EFFICIENCY (%)

POWER LOSS (mW)

0.001 0.01 0.1 1 10

38681 G01

100

1000

0.1

10000

FIGURE 12 CIRCUIT

VIN = 12V

VOUT = 3.3V

Burst Mode

OPERATION

PULSE-

SKIPPING

FCM

VOUT

100mV/DIV

AC-

COUPLED

2A/DIV

20µs/DIV 38681 G04

VOUT = 3.3V

FIGURE 12 CIRCUIT

VOUT

100mV/DIV

AC-

COUPLED

2A/DIV

20µs/DIV 38681 G05

VOUT = 3.3V

FIGURE 12 CIRCUIT

VOUT

100mV/DIV

AC-

COUPLED

2A/DIV

20µs/DIV 38681 G06

VOUT = 3.3V

FIGURE 12 CIRCUIT

Burst Mode

OPERATION

2A/DIV

FORCED

CONTINUOUS

MODE

PULSE-

SKIPPING

MODE

2µs/DIV 38681 G07

VOUT = 3.3V

ILOAD = 200µA

FIGURE 12 CIRCUIT

VOUT2

2V/DIV

VOUT1

2V/DIV

20ms/DIV 38681 G08

FIGURE 12 CIRCUIT

LTC3868-1

38681fd

TYPICAL PERFORMANCE CHARACTERISTICS

Total Input Supply Current

vs Input Voltage

EXTVCC Switchover and INTVCC

Voltages vs Temperature INTVCC Line Regulation

Maximum Current Sense Voltage

vs ITH Voltage SENSE– Pins Input Bias Current

Maximum Current Sense

Threshold vs Duty Cycle

Foldback Current Limit Quiescent Current vs Temperature Shutdown Current vs Temperature

INPUT VOLTAGE (V)

SUPPLY CURRENT (µA)

350

38681 G10

200

100

10 15 25 28

400

300

250

150

300µA LOAD

FIGURE 12 CIRCUIT

VOUT = 3.3V

ONE CHANNEL ON

NO LOAD

TEMPERATURE (°C)

–45

EXTVCC AND INTVCC VOLTAGE (V)

5.4

38681 G11

4.8

4.4

–20 5 55

4.2

4.0

5.6

5.2

5.0

4.6

80 105 130

INTVCC

EXTVCC RISING

EXTVCC FALLING

INPUT VOLTAGE (V)

5.0

INTVCC VOLTAGE (V)

5.1

5.2

5101520

38681 G12

25 28

VSENSE COMMON MODE VOLTAGE (V)

–600

SENSE– CURRENT (µA)

–500

–550

–450

–400

–300

–350

–200

–250

510 15

38681 G14

–50

–100

–150

FEEDBACK VOLTAGE (V)

MAXIMUM CURRENT SENSE VOLTAGE (mV)

0.6

38681 G16

0.3

0.1 0.7

0.4

0.2 0.8

0.5 0.9

ITH PIN VOLTAGE

CURRENT SENSE THRESHOLD (mV)

0.6 1.0

38681 G13

0.2 0.4 0.8 1.2 1.4

–20

–40

PULSE-SKIPPING

FORCED CONTINUOUS

Burst Mode OPERATION

(FALLING)

Burst Mode OPERATION

(RISING)

5% DUTY CYCLE

DUTY CYCLE (%)

MAXIMUM CURRENT SENSE VOLTAGE (mV)

38681 G15

020 40 50 100

10 30 90

TEMPERATURE (°C)

–45

110

QUIESCENT CURRENT (µA)

120

190

200

240

220

–20 30 55 130

38681 G17

230

210

150

160

130

140

180

170

580 105

PLLIN/MODE = 0

VIN = 12V

VOUT = 3.3V

ONE CHANNEL ON

TEMPERATURE (°C)

–45

SHUTDOWN CURRENT (µA)

30 80

38681 G18

–20 5 55 105 130

LTC3868-1

38681fd

TYPICAL PERFORMANCE CHARACTERISTICS

Soft-Start Pull-Up Current

vs Temperature

Shutdown (RUN) Threshold

vs Temperature

Regulated Feedback Voltage

vs Temperature

SENSE– Pin Input Current

vs Temperature

Shutdown Current

vs Input Voltage

Oscillator Frequency

vs Temperature

Undervoltage Lockout Threshold

vs Temperature

TEMPERATURE (°C)

–45

SS PULL-UP CURRENT (µA)

1.15

38681 G19

1.00

0.90

–20 5 55

0.85

0.80

1.20

1.10

1.05

0.95

80 105 130

TEMPERATURE (°C)

–45

0.90

RUN PIN VOLTAGE (V)

0.95

1.05

1.10

1.15

1.40

1.25

555 80

38681 G20

1.00

1.30

1.35

1.20

–20 30 105 130

TEMPERATURE (°C)

–45

REGULATED FEEDBACK VOLTAGE (mV)

806

38681 G21

800

796

–20 5 55

794

792

808

804

802

798

80 105 130

TEMPERATURE (°C)

–45

–600

SENSE– CURRENT (µA)

–550

–200

–150

–50

–20 30 55 130

38681 G22

–100

–400

–350

–500

–450

–250

–300

580 105

VOUT = 3.3V

VOUT = 28V

INPUT VOLTAGE (V)

38681 G23

10 15 25 28

INPUT CURRENT (µA)

Oscillator Frequency

vs Input Voltage

INPUT VOLTAGE (V)

OSCILLATOR FREQUENCY (kHz)

352

354

356

38681 G28

350

348

10 15 25 28

346

344

TEMPERATURE (°C)

–45

3.4

INTVCC VOLTAGE (V)

3.5

3.7

3.8

3.9

4.4

4.1

555 80

38681 G25

3.6

4.2

4.3

4.0

–20 30 105 130

TEMPERATURE (°C)

–45

FREQUENCY (kHz)

700

38681 G24

400

200

–20 5 55

100

800

600

500

300

80 105 130

FREQ = INTVCC

FREQ = GND

LTC3868-1

38681fd

PIN FUNCTIONS

(QFN/SSOP)

SENSE1–, SENSE2– (Pin 2, Pin 8/Pin 4, Pin 10): The (–)

Input to the Differential Current Comparators. When greater

than INTVCC – 0.5V, the SENSE– pin supplies current to

the current comparator.

FREQ (Pin 3/Pin 5): The Frequency Control Pin for the

Internal VCO. Connecting this pin to GND forces the VCO

to a ﬁ xed low frequency of 350kHz. Connecting this pin

to INTVCC forces the VCO to a ﬁ xed high frequency of

535kHz. Other frequencies between 50kHz and 900kHz can

be programmed using a resistor between FREQ and GND.

An internal 20µA pull-up current develops the voltage to

be used by the VCO to control the frequency

PLLIN/MODE (Pin 4/Pin 6): External Synchronization

Input to Phase Detector and Forced Continuous Mode

Input. When an external clock is applied to this pin, the

phase-locked loop will force the rising TG1 signal to be

synchronized with the rising edge of the external clock.

When not synchronizing to an external clock, this input,

which acts on both controllers, determines how the

LTC3868-1 operates at light loads. Pulling this pin to

ground selects Burst Mode operation. An internal 100k

resistor to ground also invokes Burst Mode operation

when the pin is ﬂ oated. Tying this pin to INTVCC forces

continuous inductor current operation. Tying this pin to

a voltage greater than 1.2V and less than INTVCC – 1.3V

selects pulse-skipping operation.

SGND (Pin 5, Exposed Pad Pin 29/Pin 7): Small-signal

ground common to both controllers, must be routed

separately from high current grounds to the common (–)

terminals of the CIN capacitors. The exposed pad (QFN

only) must be soldered to the PCB for rated thermal

performance.

RUN1, RUN2 (Pin 6, Pin 7/Pin 8, Pin 9): Digital Run

Control Inputs for Each Controller. Forcing either of these

pins below 1.26V shuts down that controller. Forcing both

of these pins below 0.7V shuts down the entire LTC3868-1,

reducing quiescent current to approximately 8µA. Do not

ﬂ oat these pins.

INTVCC vs Load Current

Latchoff Thresholds

vs Temperature

TYPICAL PERFORMANCE CHARACTERISTICS

LOAD CURRENT (mA)

INTVCC VOLTAGE (V)

5.10

5.15

5.20

160

38681 G26

5.05

5.00

4.95 20 40 60 80 100 120 140 180 200

EXTVCC = 0V

VIN = 12V

EXTVCC = 8V

TEMPERATURE (°C)

–45

1.2

INTVCC VOLTAGE (V)

1.3

1.5

1.6

1.7

2.3

2.2

1.9

555 80

38681 G27

1.4

2.0

2.1

1.8

–20 30 105 130

ARMING THRESHOLD

LATCH-OFF THRESHOLD

LTC3868-1

38681fd

PIN FUNCTIONS

(QFN/SSOP)

INTVCC (Pin 17/Pin 19): Output of the Internal Linear Low

Dropout Regulator. The driver and control circuits are

powered from this voltage source. Must be decoupled to

power ground with a minimum of 4.7µF ceramic or other

low ESR capacitor. Do not use the INTVCC pin for any

other purpose.

EXTVCC (Pin 18/Pin 20): External Power Input to an

Internal LDO Connected to INTVCC. This LDO supplies

INTVCC power, bypassing the internal LDO powered from

VIN whenever EXTVCC is higher than 4.7V. See EXTVCC

Connection in the Applications Information section. Do

not exceed 14V on this pin.

PGND (Pin 19/Pin 21): Driver Power Ground. Connects to

the sources of bottom (synchronous) N-channel MOSFETs

and the (–) terminal(s) of CIN.

VIN (Pin 20/Pin 22): Main Supply Pin. A bypass capaci-

tor should be tied between this pin and the signal ground

pin.

BG1, BG2 (Pin 21, Pin 16/Pin 23, Pin 18): High Current

Gate Drives for Bottom (Synchronous) N-Channel

MOSFETs. Voltage swing at these pins is from ground

to INTVCC.

BOOST1, BOOST2 (Pin 22, Pin 15/Pin 24, Pin 17): Boot-

strapped Supplies to the Topside Floating Drivers. Capacitors

are connected between the BOOST and SW pins and

Schottky diodes are tied between the BOOST and INTVCC

pins. Voltage swing at the BOOST pins is from INTVCC to

(VIN + INTVCC).

SW1, SW2 (Pin 23, Pin 14/Pin 25, Pin 16): Switch Node

Connections to Inductors.

TG1, TG2 (Pin 24, Pin 13/Pin 26, Pin 15): High Current

Gate Drives for Top N-Channel MOSFETs. These are the

outputs of ﬂ oating drivers with a voltage swing equal to

INTVCC – 0.5V superimposed on the switch node voltage

SW.

PGOOD1 (Pin 25/Pin 27): Open-Drain Logic Output.

PGOOD1 is pulled to ground when the voltage on the VFB1

pin is not within ±10% of its set point.

SS1, SS2 (Pin 26, Pin 12/Pin 28, Pin 14): External Soft-

Start Input. The LTC3868-1 regulates the VFB1,2 voltage

to the smaller of 0.8V or the voltage on the SS1,2 pin. An

internal 1µA pull-up current source is connected to this

pin. A capacitor to ground at this pin sets the ramp time

to ﬁ nal regulated output voltage. This pin is also used as

the short-circuit latchoff timer.

ITH1, ITH2 (Pin 27, Pin 11/Pin 1, Pin 13): Error Ampliﬁ er

Outputs and Switching Regulator Compensation Points.

Each associated channel’s current comparator trip point

increases with this control voltage.

VFB1, VFB2 (Pin 28, Pin 10/Pin 2, Pin 12): Receives the

remotely sensed feedback voltage for each controller from

an external resistive divider across the output.

SENSE1+, SENSE2+ (Pin 1, Pin 9/Pin 3, Pin 11): The (+)

input to the differential current comparators are normally

connected to DCR sensing networks or current sensing

resistors. The ITH pin voltage and controlled offsets between

the SENSE– and SENSE+ pins in conjunction with RSENSE

set the current trip threshold.

LTC3868-1

38681fd

FUNCTIONAL DIAGRAM

TOP

BOOST

TG CB

CIN

PGND

BOT BG

INTVCC

VIN

OUT

38681 FD

RSENSE

DROP

OUT

DET BOT

TOP ON

SHDN

SLEEP

0.425V

ICMP

2(VFB)

0.45V

3mV

SLOPE COMP

DUPLICATE FOR SECOND

CONTROLLER CHANNEL

SENSE+

SENSE–

PGOOD1

VFB1

0.88V

0.72V

–

FREQ

–

SWITCH

LOGIC

VFB

CC2

0.80V

TRACK/SS

0.88V

0.5µA

RUN

ITH

–

CSS

1µA

SHDN

10µA

FOLDBACK

SHDN

RST

2(VFB)

SHORT CKT

LATCH-OFF

PLLIN/MODE

20µA

VCO

LDO

INTVCC

5.1V

SYNC

DET

100k

CLK2

CLK1

VIN

EXTVCC

LDO

PFD

4.7V

5.1V

–

SGND

11V

LTC3868-1

38681fd

OPERATION

(Refer to the Functional Diagram)

Main Control Loop

The LTC3868-1 uses a constant frequency, current mode

step-down architecture with the two controller channels

operating 180 degrees out of phase. During normal op-

eration, each external top MOSFET is turned on when the

clock for that channel sets the RS latch, and is turned off

when the main current comparator, ICMP, resets the RS

latch. The peak inductor current at which ICMP trips and

resets the latch is controlled by the voltage on the ITH pin,

which is the output of the error ampliﬁ er, EA. The error

ampliﬁ er compares the output voltage feedback signal at

the VFB pin (which is generated with an external resistor

divider connected across the output voltage, VOUT

, to

ground) to the internal 0.800V reference voltage. When the

load current increases, it causes a slight decrease in VFB

relative to the reference, which causes the EA to increase

the ITH voltage until the average inductor current matches

the new load current.

After the top MOSFET is turned off each cycle, the bottom

MOSFET is turned on until either the inductor current starts

to reverse, as indicated by the current comparator IR, or

the beginning of the next clock cycle.

INTVCC/EXTVCC Power

Power for the top and bottom MOSFET drivers and most

other internal circuitry is derived from the INTVCC pin. When

the EXTVCC pin is left open or tied to a voltage less than

4.7V, the VIN LDO (low dropout linear regulator) supplies

5.1V from VIN to INTVCC. If EXTVCC is taken above 4.7V,

the VIN LDO is turned off and an EXTVCC LDO is turned on.

Once enabled, the EXTVCC LDO supplies 5.1V from EXTVCC

to INTVCC. Using the EXTVCC pin allows the INTVCC power

to be derived from a high efﬁ ciency external source such

as one of the LTC3868-1 switching regulator outputs.

Each top MOSFET driver is biased from the ﬂ oating boot-

strap capacitor, CB, which normally recharges during each

cycle through an external diode when the top MOSFET

turns off. If the input voltage VIN decreases to a voltage

close to VOUT

, the loop may enter dropout and attempt

to turn on the top MOSFET continuously. The dropout

detector detects this and forces the top MOSFET off for

about one-twelfth of the clock period every tenth cycle to

allow CB to recharge.

Shutdown and Start-Up (RUN1, RUN2 and

SS1, SS2 Pins)

The two channels of the LTC3868-1 can be independently

shut down using the RUN1 and RUN2 pins. Pulling either of

these pins below 1.26V shuts down the main control loop

for that controller. Pulling both pins below 0.7V disables

both controllers and most internal circuits, including the

INTVCC LDOs. In this state, the LTC3868-1 draws only 8µA

of quiescent current.

The RUN pin may be externally pulled up or driven directly

by logic. When driving the RUN pin with a low impedance

source, do not exceed the absolute maximum rating of

8V. The RUN pin has an internal 11V voltage clamp that

allows the RUN pin to be connected through a resistor to a

higher voltage (for example, VIN), so long as the maximum

current into the RUN pin does not exceed 100µA.

The start-up of each controller’s output voltage VOUT is

controlled by the voltage on the SS pin for that channel.

When the voltage on the SS pin is less than the 0.8V

internal reference, the LTC3868-1 regulates the VFB volt-

age to the SS pin voltage instead of the 0.8V reference.

This allows the SS pin to be used to program a soft-start

by connecting an external capacitor from the SS pin to

SGND. An internal 1µA pull-up current charges this ca-

pacitor creating a voltage ramp on the SS pin. As the SS

voltage rises linearly from 0V to 0.8V (and beyond up to

the absolute maximum rating of 6V), the output voltage

VOUT rises smoothly from zero to its ﬁ nal value.

Short-Circuit Latchoff

After the controller has been started and been given ad-

equate time to ramp up the output voltage, the SS capaci-

tor is used in a short-circuit timeout circuit. Speciﬁ cally,

once the voltage on the SS pin rises above 2V (the arming

threshold), the short-circuit timeout circuit is enabled (see

Figure 1). If the output voltage falls below 70% of its nomi-

nal regulated voltage, the SS capacitor begins discharg-

ing with a net 9µA pulldown current on the assumption

that the output is in an overcurrent and/or short-circuit

condition. If the condition lasts long enough to allow the

SS pin voltage to fall below 1.5V (the latchoff threshold),

the controller will shut down (latch off) until the RUN pin

voltage or the VIN voltage is recycled.

LTC3868-1

38681fd

OPERATION

(Refer to the Functional Diagram)

The delay time from when a short-circuit occurs until the

controller latches off can be calculated using the follow-

ing equation

LATCH ~ CSS (VSS – 1.5V)/9µA

where VSS is the initial voltage (must be greater than 2V)

on the SS pin at the time the short-circuit occurs. Normally

the SS pin voltage will have been pulled up to the INTVCC

voltage (5.1V) by the internal 1µA pull-up current.

Note that the two controllers on the LTC3868-1 have sepa-

rate, independent short-circuit latchoff circuits. Latchoff

can be overridden/defeated by connecting a resistor 150k

or less from the SS pin to INTVCC. This resistor provides

enough pull-up current to overcome the 9µA pull-down

current present during a short-circuit. Note that this resis-

tor also shortens the soft-start period.

Foldback Current

On the other hand, when the output voltage falls to less

than 70% of its nominal level, foldback current limiting

is also activated, progressively lowering the peak current

limit in proportion to the severity of the overcurrent or

short-circuit condition. Even if a short-circuit is present

and the short-circuit latchoff is not yet enabled (when

SS voltage has not yet reached 2V), a safe, low output

current is provided due to internal current foldback and

actual power wasted is low due to the efﬁ cient nature of

the current mode switching regulator. Foldback current

limiting is disabled during the soft-start interval (as long

as the VFB voltage is keeping up with the SS voltage).

Light Load Current Operation (Burst Mode Operation,

Pulse-Skipping or Forced Continuous Mode)

(PLLIN/MODE Pin)

The LTC3868-1 can be enabled to enter high efﬁ ciency

Burst Mode operation, constant frequency pulse-skipping

mode, or forced continuous conduction mode at low load

currents. To select Burst Mode operation, tie the PLLIN/

MODE pin to ground. To select forced continuous opera-

tion, tie the PLLIN/MODE pin to INTVCC. To select pulse-

skipping mode, tie the PLLIN/MODE pin to a DC voltage

greater than 1.2V and less than INTVCC – 1.3V.

When a controller is enabled for Burst Mode operation,

the minimum peak current in the inductor is set to ap-

proximately 30% of the maximum sense voltage even

though the voltage on the ITH pin indicates a lower value.

If the average inductor current is higher than the load

current, the error ampliﬁ er, EA, will decrease the voltage

on the ITH pin. When the ITH voltage drops below 0.425V,

the internal sleep signal goes high (enabling sleep mode)

and both external MOSFETs are turned off.

In sleep mode, much of the internal circuitry is turned off,

reducing the quiescent current. If one channel is shut down

and the other channel is in sleep mode, the LTC3868-1

draws only 170µA of quiescent current. If both channels

are in sleep mode, the LTC3868-1 draws only 300µA of qui-

escent current. In sleep mode, the load current is supplied

by the output capacitor. As the output voltage decreases,

the EA’s output begins to rise. When the output voltage

drops enough, the ITH pin is reconnected to the output

of the EA, the sleep signal goes low, and the controller

resumes normal operation by turning on the top external

MOSFET on the next cycle of the internal oscillator.

When a controller is enabled for Burst Mode operation,

the inductor current is not allowed to reverse. The reverse

INTVCC

0.8V 1.5V

1µA

–9µA

SS VOLTAGE

LATCHOFF

COMMAND

SS PIN

CURRENT

OUTPUT

VOLTAGE

LATCHOFF

ENABLE

SOFT-START INTERVAL

ARMING tLATCH

38681 F01

1µA

Figure 1. Latchoff Timing Diagram

LTC3868-1

38681fd

OPERATION

(Refer to the Functional Diagram)

current comparator, IR, turns off the bottom external

MOSFET just before the inductor current reaches zero,

preventing it from reversing and going negative. Thus,

the controller is in discontinuous operation.

In forced continuous operation or when clocked by an

external clock source to use the phase-locked loop (see

Frequency Selection and Phase-Locked Loop section),

the inductor current is allowed to reverse at light loads

or under large transient conditions. The peak inductor

current is determined by the voltage on the ITH pin, just

as in normal operation. In this mode, the efﬁ ciency at light

loads is lower than in Burst Mode operation. However,

continuous operation has the advantages of lower output

voltage ripple and less interference to audio circuitry. In

forced continuous mode, the output ripple is independent

of load current.

When the PLLIN/MODE pin is connected for pulse-skipping

mode, the LTC3868-1 operates in PWM pulse-skipping

mode at light loads. In this mode, constant frequency

operation is maintained down to approximately 1% of

designed maximum output current. At very light loads, the

current comparator, ICMP, may remain tripped for several

cycles and force the external top MOSFET to stay off for

the same number of cycles (i.e., skipping pulses). The

inductor current is not allowed to reverse (discontinuous

operation). This mode, like forced continuous operation,

exhibits low output ripple as well as low audio noise and

reduced RF interference when compared to Burst Mode

operation. It provides higher light load efﬁ ciency than

forced continuous mode, but not nearly as high as Burst

Mode operation.

Frequency Selection and Phase-Locked Loop

(FREQ and PLLIN/MODE Pins)

The selection of switching frequency is a trade off between

efﬁ ciency and component size. Low frequency opera-

tion increases efﬁ ciency by reducing MOSFET switching

losses, but requires larger inductance and/or capacitance

to maintain low output ripple voltage.

The switching frequency of the LTC3868-1’s controllers

can be selected using the FREQ pin.

If the PLLIN/MODE pin is not being driven by an external

clock source, the FREQ pin can be tied to SGND, tied to

INTVCC or programmed through an external resistor. Tying

FREQ to SGND selects 350kHz while tying FREQ to INTVCC

selects 535kHz. Placing a resistor between FREQ and

SGND allows the frequency to be programmed between

50kHz and 900kHz.

A phase-locked loop (PLL) is available on the LTC3868-1

to synchronize the internal oscillator to an external clock

source that is connected to the PLLIN/MODE pin. The

phase detector adjusts the voltage (through an internal

lowpass ﬁ lter) of the VCO input to align the turn-on of

controller 1’s external top MOSFET to the rising edge of

the synchronizing signal. Thus, the turn-on of controller

2’s external top MOSFET is 180 degrees out of phase to

the rising edge of the external clock source.

The VCO input voltage is prebiased to the operating fre-

quency set by the FREQ pin before the external clock is

applied. If prebiased near the external clock frequency,

the PLL loop only needs to make slight changes to the

VCO input in order to synchronize the rising edge of the

external clock’s to the rising edge of TG1. The ability to

prebias the loop ﬁ lter allows the PLL to lock-in rapidly

without deviating far from the desired frequency.

The typical capture range of the phase-locked loop is from

approximately 55kHz to 1MHz, with a guarantee over all

manufacturing variations to be between 75kHz and 850kHz.

In other words, the LTC3868-1’s PLL is guaranteed to lock

to an external clock source whose frequency is between

75kHz and 850kHz.

The typical input clock thresholds on the PLLIN/MODE

pin are 1.6V (rising) and 1.1V (falling).

Output Overvoltage Protection

An overvoltage comparator guards against transient over-

shoots as well as other more serious conditions that may

overvoltage the output. When the VFB pin rises by more

than 10% above its regulation point of 0.800V, the top

MOSFET is turned off and the bottom MOSFET is turned

on until the overvoltage condition is cleared.

LTC3868-1

38681fd

OPERATION

(Refer to the Functional Diagram)

Figure 2. Input Waveforms Comparing Single-Phase (a) and 2-Phase (b) Operation for Dual Switching Regulators

Converting 12V to 5V and 3.3V at 3A Each. The Reduced Input Ripple with the 2-Phase Regulator Allows

Less Expensive Input Capacitors, Reduces Shielding Requirements for EMI and Improves Efﬁ ciency

Power Good (PGOOD) Pin

The PGOOD1 pin is connected to an open drain of an

internal N-channel MOSFET. The MOSFET turns on and

pulls the PGOOD1 pin low when the corresponding VFB1 pin

voltage is not within ±10% of the 0.8V reference voltage.

The PGOOD1 pin is also pulled low when the RUN1 pin

is low (shut down). When the VFB1 pin voltage is within

the ±10% requirement, the MOSFET is turned off and the

pin is allowed to be pulled up by an external resistor to a

source no greater than 6V.

Theory and Beneﬁ ts of 2-Phase Operation

Why the need for 2-phase operation? Up until the

2-phase family, constant frequency dual switching regula-

tors operated both channels in phase (i.e., single phase

operation). This means that both switches turned on at

the same time, causing current pulses of up to twice the

amplitude of those for one regulator to be drawn from the

input capacitor and battery. These large amplitude current

pulses increased the total RMS current ﬂ owing from the

input capacitor, requiring the use of more expensive input

capacitors and increasing both EMI and losses in the input

capacitor and battery.

With 2-phase operation, the two channels of the dual

switching regulator are operated 180 degrees out of phase.

This effectively interleaves the current pulses drawn by the

switches, greatly reducing the overlap time where they add

together. The result is a signiﬁ cant reduction in total RMS

input current, which in turn allows less expensive input

capacitors to be used, reduces shielding requirements for

EMI and improves real world operating efﬁ ciency.

Figure 2 compares the input waveforms for a representative

single phase dual switching regulator to the LTC3868-1

2-phase dual switching regulator. An actual measure-

ment of the RMS input current under these conditions

shows that 2-phase operation dropped the input current

from 2.53ARMS to 1.55ARMS. While this is an impressive

reduction in itself, remember that the power losses are

IIN(MEAS) = 2.53ARMS IIN(MEAS) = 1.55ARMS

38681 F01

5V SWITCH

20V/DIV

3.3V SWITCH

20V/DIV

INPUT CURRENT

5A/DIV

INPUT VOLTAGE

500mV/DIV

LTC3868-1

38681fd

Figure 3. RMS Input Current Comparison

INPUT VOLTAGE (V)

INPUT RMS CURRENT (A)

3.0

2.5

2.0

1.5

1.0

0.5

010 20 30 40

38681 F03

SINGLE PHASE

DUAL CONTROLLER

2-PHASE

DUAL CONTROLLER

VO1 = 5V/3A

VO2 = 3.3V/3A

proportional to IRMS2, meaning that the actual power wasted

is reduced by a factor of 2.66. The reduced input ripple

voltage also means less power is lost in the input power

path, which could include batteries, switches, trace/con-

nector resistances and protection circuitry. Improvements

in both conducted and radiated EMI also directly accrue as

a result of the reduced RMS input current and voltage.

Of course, the improvement afforded by 2-phase opera-

tion is a function of the dual switching regulator’s relative

duty cycles which, in turn, are dependent upon the input

OPERATION

(Refer to the Functional Diagram)

voltage VIN (Duty Cycle = VOUT/VIN). Figure 3 shows how

the RMS input current varies for single-phase and 2-phase

operation for 3.3V and 5V regulators over a wide input

voltage range.

It can readily be seen that the advantages of 2-phase op-

eration are not just limited to a narrow operating range,

for most applications is that 2-phase operation will reduce

the input capacitor requirement to that for just one channel

operating at maximum current and 50% duty cycle.

LTC3868-1

38681fd

APPLICATIONS INFORMATION

Figure 4. Sense Lines Placement with Inductor or Sense Resistor

(5a) Using a Resistor to Sense Current

(5b) Using the Inductor DCR to Sense Current

Figure 5. Current Sensing Methods

The Typical Application on the ﬁ rst page is a basic

LTC3868-1 application circuit. LTC3868-1 can be conﬁ gured

to use either DCR (inductor resistance) sensing or low

value resistor sensing. The choice between the two cur-

rent sensing schemes is largely a design trade off between

cost, power consumption and accuracy. DCR sensing is

becoming popular because it saves expensive current

sensing resistors and is more power efﬁ cient, especially

in high current applications. However, current sensing

resistors provide the most accurate current limits for the

controller. Other external component selection is driven

by the load requirement, and begins with the selection of

RSENSE (if RSENSE is used) and inductor value. Next, the

power MOSFETs and Schottky diodes are selected. Finally,

input and output capacitors are selected.

SENSE+ and SENSE– Pins

The SENSE+ and SENSE– pins are the inputs to the cur-

rent comparators. The common mode voltage range on

these pins is 0V to 16V (Absolute Maximum), enabling

the LTC3868-1 to regulate output voltages up to a nominal

14V (allowing margin for tolerances and transients).

The SENSE+ pin is high impedance over the full common

mode range, drawing at most ±1µA. This high impedance

allows the current comparators to be used in inductor

DCR sensing.

The impedance of the SENSE– pin changes depending on

the common mode voltage. When SENSE– is less than

INTVCC – 0.5V, a small current of less than 1µA ﬂ ows out

of the pin. When SENSE– is above INTVCC + 0.5V, a higher

current (~550µA) ﬂ ows into the pin. Between INTVCC – 0.5V

and INTVCC + 0.5V, the current transitions from the smaller

current to the higher current.

Filter components mutual to the sense lines should be

placed close to the LTC3868-1, and the sense lines should

run close together to a Kelvin connection underneath the

current sense element (shown in Figure 4). Sensing cur-

rent elsewhere can effectively add parasitic inductance

and capacitance to the current sense element, degrading

the information at the sense terminals and making the

programmed current limit unpredictable. If inductor DCR

sensing is used (Figure 5b), resistor R1 should be placed

close to the switching node, to prevent noise from coupling

into sensitive small-signal nodes.

COUT

TO SENSE FILTER,

NEXT TO THE CONTROLLER

INDUCTOR OR RSENSE

38681 F04

VIN VIN

INTVCC

BOOST

PLACE CAPACITOR NEAR

SENSE PINS

SENSE+

SENSE–

SGND

LTC3868-1

VOUT

38681 F05a

VIN VIN

INTVCC

BOOST

*PLACE C1 NEAR

SENSE PINS

INDUCTOR

DCRL

SENSE+

SENSE–

SGND

LTC3868-1

VOUT

38681 F05b

R2C1*

(R1||R2) • C1 = L

DCR RSENSE(EQ) = DCR R2

R1 + R2

LTC3868-1

38681fd

APPLICATIONS INFORMATION

Low Value Resistor Current Sensing

A typical sensing circuit using a discrete resistor is shown

in Figure 5a. RSENSE is chosen based on the required

output current.

The current comparator has a maximum threshold

VSENSE(MAX) of 50mV. The current comparator threshold

voltage sets the peak of the inductor current, yielding

a maximum average output current, IMAX, equal to the

peak value less half the peak-to-peak ripple current, ∆IL.

To calculate the sense resistor value, use the equation:

RSENSE =VSENSE(MAX)

IMAX +ΔIL

When using the controller in very low dropout conditions,

the maximum output current level will be reduced due to

the internal compensation required to meet stability cri-

terion for buck regulators operating at greater than 50%

duty factor. A curve is provided in the Typical Performance

Characteristics section to estimate this reduction in

peak output current depending upon the operating duty

factor.

Inductor DCR Sensing

For applications requiring the highest possible efﬁ ciency

at high load currents, the LTC3850 is capable of sensing

the voltage drop across the inductor DCR, as shown in

Figure 5b. The DCR of the inductor represents the small

amount of DC resistance of the copper wire, which can be

less than 1m for today’s low value, high current inductors.

In a high current application requiring such an inductor,

power loss through a sense resistor would cost several

points of efﬁ ciency compared to inductor DCR sensing.

If the external R1||R2 • C1 time constant is chosen to be

exactly equal to the L/DCR time constant, the voltage drop

across the external capacitor is equal to the drop across

the inductor DCR multiplied by R2/(R1 + R2). R2 scales the

voltage across the sense terminals for applications where

the DCR is greater than the target sense resistor value.

To properly dimension the external ﬁ lter components, the

DCR of the inductor must be known. It can be measured

using a good RLC meter, but the DCR tolerance is not

always the same and varies with temperature; consult the

manufacturers’ data sheets for detailed information.

Using the inductor ripple current value from the Inductor

Value Calculation section, the target sense resistor value

is:

RSENSE(EQUIV) =VSENSE(MAX)

IMAX +ΔIL

To ensure that the application will deliver full load current

over the full operating temperature range, choose the

minimum value for the maximum current sense threshold

voltage (VSENSE(MAX)) in the Electrical Characteristics

table.

Next, determine the DCR of the inductor. When provided,

use the manufacturer’s maximum value, usually given at

20°C. Increase this value to account for the temperature

coefﬁ cient of copper resistance, which is approximately

0.4%/°C. A conservative value for TL(MAX) is 100°C.

To scale the maximum inductor DCR to the desired sense

resistor (RD) value, use the divider ratio:

RD=RSENSE(EQUIV)

DCRMAX atT

L(MAX)

C1 is usually selected to be in the range of 0.1µF to 0.47µF.

This forces R1||R2 to around 2k, reducing error that might

have been caused by the SENSE+ pin’s ±1µA current.

The equivalent resistance R1||R2 is scaled to the room

temperature inductance and maximum DCR:

R1|| R2 =L

DCR at 20°C

()

•C1

The sense resistor values are:

R1=R1|| R2

; R2 =R1• RD

1–RD

LTC3868-1

38681fd

APPLICATIONS INFORMATION

The maximum power loss in R1 is related to duty cycle,

and will occur in continuous mode at the maximum input

voltage:

LOSS R1=V

IN(MAX) –V

OUT

()

•V

OUT

Ensure that R1 has a power rating higher than this value.

If high efﬁ ciency is necessary at light loads, consider this

power loss when deciding whether to use DCR sensing or

sense resistors. Light load power loss can be modestly

higher with a DCR network than with a sense resistor, due

to the extra switching losses incurred through R1. However,

DCR sensing eliminates a sense resistor, reduces conduc-

tion losses and provides higher efﬁ ciency at heavy loads.

Peak efﬁ ciency is about the same with either method.

Inductor Value Calculation

The operating frequency and inductor selection are inter-

related in that higher operating frequencies allow the use

of smaller inductor and capacitor values. So why would

anyone ever choose to operate at lower frequencies with

larger components? The answer is efﬁ ciency. A higher

frequency generally results in lower efﬁ ciency because

of MOSFET gate charge losses. In addition to this basic

trade-off, the effect of inductor value on ripple current and

low current operation must also be considered.

The inductor value has a direct effect on ripple current. The

inductor ripple current ∆IL decreases with higher induc-

tance or higher frequency and increases with higher VIN:

ΔIL=1

()

VOUT 1– VOUT

VIN

⎛

⎝

⎜⎞

⎠

⎟

Accepting larger values of ∆IL allows the use of low

inductances, but results in higher output voltage ripple

and greater core losses. A reasonable starting point for

setting ripple current is ∆IL = 0.3(IMAX). The maximum

∆IL occurs at the maximum input voltage.

The inductor value also has secondary effects. The tran-

sition to Burst Mode operation begins when the average

inductor current required results in a peak current below

30% of the current limit determined by RSENSE. Lower

inductor values (higher ∆IL) will cause this to occur at

lower load currents, which can cause a dip in efﬁ ciency in

the upper range of low current operation. In Burst Mode

operation, lower inductance values will cause the burst

frequency to decrease.

Inductor Core Selection

Once the value for L is known, the type of inductor must

be selected. High efﬁ ciency converters generally cannot

afford the core loss found in low cost powdered iron cores,

forcing the use of more expensive ferrite or molypermalloy

cores. Actual core loss is independent of core size for a

ﬁ xed inductor value, but it is very dependent on inductance

value selected. As inductance increases, core losses go

down. Unfortunately, increased inductance requires more

turns of wire and therefore copper losses will increase.

Ferrite designs have very low core loss and are preferred

for high switching frequencies, so design goals can

concentrate on copper loss and preventing saturation.

Ferrite core material saturates hard, which means that

inductance collapses abruptly when the peak design current

is exceeded. This results in an abrupt increase in inductor

ripple current and consequent output voltage ripple. Do

not allow the core to saturate!

Power MOSFET and Schottky Diode

(Optional) Selection

Two external power MOSFETs must be selected for each

controller in the LTC3868-1: one N-channel MOSFET for

the top (main) switch, and one N-channel MOSFET for the

bottom (synchronous) switch.

The peak-to-peak drive levels are set by the INTVCC voltage.

This voltage is typically 5.1V during start-up (see EXTVCC

Pin Connection). Consequently, logic-level threshold

MOSFETs must be used in most applications. The only

exception is if low input voltage is expected (VIN < 4V);

then, sub-logic level threshold MOSFETs (VGS(TH) < 3V)

should be used. Pay close attention to the BVDSS speci-

ﬁ cation for the MOSFETs as well; many of the logic-level

MOSFETs are limited to 30V or less.

LTC3868-1

38681fd

APPLICATIONS INFORMATION

Selection criteria for the power MOSFETs include the

on-resistance, RDS(ON), Miller capacitance, CMILLER, input

voltage and maximum output current. Miller capacitance,

CMILLER, can be approximated from the gate charge curve

usually provided on the MOSFET manufacturers’ data

sheet. CMILLER is equal to the increase in gate charge

along the horizontal axis while the curve is approximately

ﬂ at divided by the speciﬁ ed change in VDS. This result is

then multiplied by the ratio of the application applied VDS

to the gate charge curve speciﬁ ed VDS. When the IC is

operating in continuous mode the duty cycles for the top

and bottom MOSFETs are given by:

Main Switch Duty Cycle =VOUT

VIN

Synchronous Switch Duty Cycle =V

IN −VOUT

VIN

The MOSFET power dissipations at maximum output

current are given by:

MAIN =VOUT

VIN

IMAX

()

21+δ

()

RDS(ON) +

VIN

()

2IMAX

⎛

⎝

⎜⎞

⎠

⎟RDR

()

CMILLER

()

•

VINTVCC –V

THMIN

VTHMIN

⎡

⎣

⎢⎤

⎦

⎥f

()

SYNC =VIN –V

OUT

VIN

IMAX

()

21+δ

()

RDS(ON)

where δ is the temperature dependency of RDS(ON) and

RDR (approximately 2) is the effective driver resistance

at the MOSFET’s Miller threshold voltage. VTHMIN is the

typical MOSFET minimum threshold voltage.

Both MOSFETs have I2R losses while the topside N-channel

equation includes an additional term for transition losses,

which are highest at high input voltages. For VIN < 20V

the high current efﬁ ciency generally improves with larger

MOSFETs, while for VIN > 20V the transition losses rapidly

increase to the point that the use of a higher RDS(ON) device

with lower CMILLER actually provides higher efﬁ ciency. The

synchronous MOSFET losses are greatest at high input

voltage when the top switch duty factor is low or during

a short-circuit when the synchronous switch is on close

to 100% of the period.

The term (1+ δ) is generally given for a MOSFET in the

form of a normalized RDS(ON) vs Temperature curve, but

δ = 0.005/°C can be used as an approximation for low

voltage MOSFETs.

The optional Schottky diodes D1 and D2 shown in Figure 10

conduct during the dead-time between the conduction of

the two power MOSFETs. This prevents the body diode of

the bottom MOSFET from turning on, storing charge during

the dead-time and requiring a reverse recovery period that

could cost as much as 3% in efﬁ ciency at high VIN. A 1A

to 3A Schottky is generally a good compromise for both

regions of operation due to the relatively small average

current. Larger diodes result in additional transition losses

due to their larger junction capacitance.

CIN and COUT Selection

The selection of CIN is simpliﬁ ed by the 2-phase architec-

ture and its impact on the worst-case RMS current drawn

through the input network (battery/fuse/capacitor). It can be

shown that the worst-case capacitor RMS current occurs

when only one controller is operating. The controller with

the highest (VOUT)(IOUT) product needs to be used in the

formula shown in Equation 1 to determine the maximum

RMS capacitor current requirement. Increasing the out-

put current drawn from the other controller will actually

decrease the input RMS ripple current from its maximum

value. The out-of-phase technique typically reduces the

input capacitor’s RMS ripple current by a factor of 30%

to 70% when compared to a single phase power supply

solution.

In continuous mode, the source current of the top MOSFET

is a square wave of duty cycle (VOUT)/(VIN). To prevent

large voltage transients, a low ESR capacitor sized for the

maximum RMS current of one channel must be used. The

maximum RMS capacitor current is given by:

CIN Required IRMS ≈IMAX

VOUT

()

IN –V

OUT

()

⎡

⎣⎤

⎦1/ 2

(1)

LTC3868-1

38681fd

APPLICATIONS INFORMATION

Figure 6. Setting Output Voltage

Equation 1 has a maximum at VIN = 2VOUT

, where IRMS

= IOUT/2. This simple worst-case condition is commonly

used for design because even signiﬁ cant deviations do not

offer much relief. Note that capacitor manufacturers’ ripple

current ratings are often based on only 2000 hours of life.

This makes it advisable to further derate the capacitor, or

to choose a capacitor rated at a higher temperature than

required. Several capacitors may be paralleled to meet

size or height requirements in the design. Due to the high

operating frequency of the LTC3868-1, ceramic capacitors

can also be used for CIN. Always consult the manufacturer

if there is any question.

The beneﬁ t of the LTC3868-1 2-phase operation can be

calculated by using Equation 1 for the higher power control-

ler and then calculating the loss that would have resulted

if both controller channels switched on at the same time.

The total RMS power lost is lower when both controllers

are operating due to the reduced overlap of current pulses

required through the input capacitor’s ESR. This is why

the input capacitor’s requirement calculated above for the

worst-case controller is adequate for the dual controller

design. Also, the input protection fuse resistance, battery

resistance, and PC board trace resistance losses are also

reduced due to the reduced peak currents in a 2-phase

system. The overall beneﬁ t of a multiphase design will

only be fully realized when the source impedance of the

power supply/battery is included in the efﬁ ciency testing.

The sources of the top MOSFETs should be placed within

1cm of each other and share a common CIN(s). Separating

the sources and CIN may produce undesirable voltage and

current resonances at VIN.

A small (0.1µF to 1µF) bypass capacitor between the chip

VIN pin and ground, placed close to the LTC3868-1, is

also suggested. A 10 resistor placed between CIN (C1)

and the VIN pin provides further isolation between the

two channels.

The selection of COUT is driven by the effective series

resistance (ESR). Typically, once the ESR requirement

is satisﬁ ed, the capacitance is adequate for ﬁ ltering. The

output ripple (∆VOUT) is approximated by:

ΔVOUT ≈ΔILESR+1

8•f•C

OUT

⎛

⎝

⎜⎞

⎠

⎟

where f is the operating frequency, COUT is the output

capacitance and ∆IL is the ripple current in the inductor.

The output ripple is highest at maximum input voltage

since ∆IL increases with input voltage.

Setting Output Voltage

The LTC3868-1 output voltages are each set by an exter-

nal feedback resistor divider carefully placed across the

output, as shown in Figure 6. The regulated output voltage

is determined by:

VOUT =0.8V 1+RB

⎛

⎝

⎜⎞

⎠

⎟

To improve the frequency response, a feedforward ca-

pacitor, CFF

, may be used. Great care should be taken to

route the VFB line away from noise sources, such as the

inductor or the SW line.

1/2 LTC3868-1

VFB

VOUT

RBCFF

38681 F05

Soft-Start (SS Pins)

The start-up of each VOUT is controlled by the voltage on

the respective SS pin. When the voltage on the SS pin

is less than the internal 0.8V reference, the LTC3868-1

regulates the VFB pin voltage to the voltage on the SS pin

instead of 0.8V. The SS pin can be used to program an

external soft-start function.

Soft-start is enabled by simply connecting a capacitor from

the SS pin to ground, as shown in Figure 7. An internal

1µA current source charges the capacitor, providing a

1/2 LTC3868-1

CSS

SGND

38681 F06

Figure 7. Using the SS Pin to Program Soft-Start

LTC3868-1

38681fd

APPLICATIONS INFORMATION

linear ramping voltage at the SS pin. The LTC3868-1 will

regulate the VFB pin (and hence VOUT) according to the

voltage on the SS pin, allowing VOUT to rise smoothly from

0V to its ﬁ nal regulated value. The total soft-start time will

be approximately:

tSS =CSS •0.8V

1µ A

INTVCC Regulators

The LTC3868-1 features two separate internal P-channel

low dropout linear regulators (LDO) that supply power

at the INTVCC pin from either the VIN supply pin or the

EXTVCC pin depending on the connection of the EXTVCC

pin. INTVCC powers the gate drivers and much of the

LTC3868-1’s internal circuitry. The VIN LDO and the EXTVCC

LDO regulate INTVCC to 5.1V. Each of these can supply a

peak current of 50mA and must be bypassed to ground

with a minimum of 4.7µF low ESR capacitor. No matter

what type of bulk capacitor is used, an additional 1µF ce-

ramic capacitor placed directly adjacent to the INTVCC and

PGND IC pins is highly recommended. Good bypassing

is needed to supply the high transient currents required

by the MOSFET gate drivers and to prevent interaction

between the channels.

High input voltage applications in which large MOSFETs

are being driven at high frequencies may cause the

maximum junction temperature rating for the LTC3868-1

to be exceeded. The INTVCC current, which is dominated

by the gate charge current, may be supplied by either

the VIN LDO or the EXTVCC LDO. When the voltage on

the EXTVCC pin is less than 4.7V, the VIN LDO is enabled.

Power dissipation for the IC in this case is highest and is

equal to VIN • IINTVCC. The gate charge current is depen-

dent on operating frequency as discussed in the Efﬁ ciency

Considerations section. The junction temperature can be

estimated by using the equations given in Note 2 of the

Electrical Characteristics. For example, the LTC3868-1

INTVCC current is limited to less than 22mA from a 28V

supply when not using the EXTVCC supply at 70°C ambient

temperature in the SSOP package:

J = 70°C + (22mA)(28V)(90°C/W) = 125°C

To prevent the maximum junction temperature from be-

ing exceeded, the input supply current must be checked

while operating in forced continuous mode (PLLIN/MODE

= INTVCC) at maximum VIN.

When the voltage applied to EXTVCC rises above 4.7V, the

VIN LDO is turned off and the EXTVCC LDO is enabled. The

EXTVCC LDO remains on as long as the voltage applied to

EXTVCC remains above 4.5V. The EXTVCC LDO attempts

to regulate the INTVCC voltage to 5.1V, so while EXTVCC

is less than 5.1V, the LDO is in dropout and the INTVCC

voltage is approximately equal to EXTVCC. When EXTVCC

is greater than 5.1V, up to an absolute maximum of 14V,

INTVCC is regulated to 5.1V.

Using the EXTVCC LDO allows the MOSFET driver and

control power to be derived from one of the LTC3868-1’s

switching regulator outputs (4.7V ≤ VOUT ≤ 14V) during

normal operation and from the VIN LDO when the out-

put is out of regulation (e.g., start-up, short-circuit). If

more current is required through the EXTVCC LDO than

is speciﬁ ed, an external Schottky diode can be added

between the EXTVCC and INTVCC pins. In this case, do

not apply more than 6V to the EXTVCC pin and make sure

that EXTVCC ≤ VIN.

Signiﬁ cant efﬁ ciency and thermal gains can be realized

by powering INTVCC from the output, since the VIN cur-

rent resulting from the driver and control currents will be

scaled by a factor of (Duty Cycle)/(Switcher Efﬁ ciency).

For 5V to 14V regulator outputs, this means connecting

the EXTVCC pin directly to VOUT

. Tying the EXTVCC pin to

a 8.5V supply reduces the junction temperature in the

previous example from 125°C to:

J = 70°C + (45mA)(8.5V)(90°C/W) = 87°C

However, for 3.3V and other low voltage outputs, addi-

tional circuitry is required to derive INTVCC power from

the output.

LTC3868-1

38681fd

APPLICATIONS INFORMATION

The following list summarizes the four possible connec-

tions for EXTVCC:

1. EXTVCC Left Open (or Grounded). This will cause INTVCC

to be powered from the internal 5.1V regulator result-

ing in an efﬁ ciency penalty of up to 10% at high input

voltages.

2. EXTVCC Connected Directly to VOUT

. This is the normal

connection for a 5V to 14V regulator and provides the

highest efﬁ ciency.

3. EXTVCC Connected to an External Supply. If an external

supply is available in the 5V to 14V range, it may be

used to power EXTVCC. Ensure that EXTVCC < VIN.

4. EXTVCC Connected to an Output-Derived Boost Network.

For 3.3V and other low voltage regulators, efﬁ ciency

gains can still be realized by connecting EXTVCC to an

output-derived voltage that has been boosted to greater

than 4.7V. This can be done with the capacitive charge

pump shown in Figure 8. Ensure that EXTVCC < VIN.

and the BOOST pin follows. With the topside MOSFET

on, the boost voltage is above the input supply: VBOOST =

VIN + VINTVCC. The value of the boost capacitor, CB, needs

to be 100 times that of the total input capacitance of the

topside MOSFET(s). The reverse breakdown of the external

Schottky diode must be greater than VIN(MAX).

When adjusting the gate drive level, the ﬁ nal arbiter is the

total input current for the regulator. If a change is made

and the input current decreases, then the efﬁ ciency has

improved. If there is no change in input current, then there

is no change in efﬁ ciency.

Fault Conditions: Current Limit and Current Foldback

When the output current hits the current limit, the output

voltage begins to drop. If the output voltage falls below

70% of its nominal output level, then the maximum sense

voltage is progressively lowered to about one-half of its

maximum selected value. Under short-circuit conditions

with very low duty cycles, the LTC3868-1 will begin cycle

skipping in order to limit the short-circuit current. In this

situation the bottom MOSFET will be dissipating most of

the power but less than in normal operation. The short-

circuit ripple current is determined by the minimum on-

time, tON(MIN), of the LTC3868-1 (≈90ns), the input voltage

and inductor value:

ΔIL(SC) =tON(MIN)

VIN

⎛

⎝

⎜⎞

⎠

⎟

The resulting average short-circuit current is:

ISC =50% •ILIM(MAX)

RSENSE

–1

2ΔIL(SC)

Fault Conditions: Overvoltage Protection (Crowbar)

The overvoltage crowbar is designed to blow a system

input fuse when the output voltage of the regulator rises

much higher than nominal levels. The crowbar causes huge

currents to ﬂ ow, that blow the fuse to protect against a

shorted top MOSFET if the short occurs while the control-

ler is operating.

A comparator monitors the output for overvoltage condi-

tions. The comparator detects faults greater than 10%

Figure 8. Capacitive Charge Pump for EXTVCC

EXTVCC

VIN

TG1

BG1

PGND

1/2 LTC3868-1 RSENSE

VOUT

VN2222LL

COUT

38681 F08

MBOT

MTOP

CIN

BAT85 BAT85

BAT85

Topside MOSFET Driver Supply (CB, DB)

External bootstrap capacitors, CB, connected to the BOOST

pins supply the gate drive voltages for the topside MOSFETs.

Capacitor CB in the Functional Diagram is charged though

external diode DB from INTVCC when the SW pin is low.

When one of the topside MOSFETs is to be turned on, the

driver places the CB voltage across the gate-source of the

desired MOSFET. This enhances the top MOSFET switch

and turns it on. The switch node voltage, SW, rises to VIN

LTC3868-1

38681fd

APPLICATIONS INFORMATION

Figure 9. Relationship Between Oscillator Frequency

and Resistor Value at the FREQ Pin

above the nominal output voltage. When this condition

is sensed, the top MOSFET is turned off and the bottom

MOSFET is turned on until the overvoltage condition is

cleared. The bottom MOSFET remains on continuously

for as long as the overvoltage condition persists; if VOUT

returns to a safe level, normal operation automatically

resumes.

A shorted top MOSFET will result in a high current condition

which will open the system fuse. The switching regulator

will regulate properly with a leaky top MOSFET by altering

the duty cycle to accommodate the leakage.

Phase-Locked Loop and Frequency Synchronization

The LTC3868-1 has an internal phase-locked loop (PLL)

comprised of a phase frequency detector, a lowpass ﬁ lter,

and a voltage-controlled oscillator (VCO). This allows the

turn-on of the top MOSFET of controller 1 to be locked to

the rising edge of an external clock signal applied to the

PLLIN/MODE pin. The turn-on of controller 2’s top MOSFET

is thus 180 degrees out of phase with the external clock.

The phase detector is an edge sensitive digital type that

provides zero degrees phase shift between the external

and internal oscillators. This type of phase detector does

not exhibit false lock to harmonics of the external clock.

If the external clock frequency is greater than the internal

oscillator’s frequency, fOSC, then current is sourced continu-

ously from the phase detector output, pulling up the VCO

input. When the external clock frequency is less than fOSC,

current is sunk continuously, pulling down the VCO input.

If the external and internal frequencies are the same but

exhibit a phase difference, the current sources turn on for

an amount of time corresponding to the phase difference.

The voltage at the VCO input is adjusted until the phase

and frequency of the internal and external oscillators are

identical. At the stable operating point, the phase detector

output is high impedance and the internal ﬁ lter capacitor,

CLP

, holds the voltage at the VCO input.

Typically, the external clock (on the PLLIN/MODE pin)

input high threshold is 1.6V, while the input low threshold

is 1.1V.

Rapid phase locking can be achieved by using the FREQ

pin to set a free-running frequency near the desired

synchronization frequency. The VCO’s input voltage is

prebiased at a frequency corresponding to the frequency

set by the FREQ pin. Once prebiased, the PLL only needs

to adjust the frequency slightly to achieve phase lock

and synchronization. Although it is not required that the

free-running frequency be near external clock frequency,

doing so will prevent the operating frequency from passing

through a large range of frequencies as the PLL locks.

Note that the LTC3868-1 can only be synchronized to an

external clock whose frequency is within range of the

LTC3868-1’s internal VCO, which is nominally 55kHz

to 1MHz. This is guaranteed to be between 75kHz and

850kHz.

Table 2 summarizes the different states in which the FREQ

pin can be used.

Table 2

FREQ PIN PLLIN/MODE PIN FREQUENCY

0V DC Voltage 350kHz

INTVCC DC Voltage 535kHz

Resistor DC Voltage 50kHz–900kHz

Any of the Above External Clock Phase–Locked to

External Clock

FREQ PIN RESISTOR (k)

FREQUENCY (kHz)

600

800

1000

35 45 5525

38681 F09

400

200

500

700

900

300

100

065 75 85 95 105 115 125

LTC3868-1

38681fd

APPLICATIONS INFORMATION

Minimum On-Time Considerations

Minimum on-time, tON(MIN), is the smallest time dura-

tion that the LTC3868-1 is capable of turning on the top

MOSFET. It is determined by internal timing delays and the

gate charge required to turn on the top MOSFET. Low duty

cycle applications may approach this minimum on-time

limit and care should be taken to ensure that:

tON(MIN) <VOUT

VIN f

()

If the duty cycle falls below what can be accommodated

by the minimum on-time, the controller will begin to skip

cycles. The output voltage will continue to be regulated,

but the ripple voltage and current will increase.

The minimum on-time for the LTC3868-1 is approximately

95ns. However, as the peak sense voltage decreases the

minimum on-time gradually increases up to about 130ns.

This is of particular concern in forced continuous applica-

tions with low ripple current at light loads. If the duty cycle

drops below the minimum on-time limit in this situation,

a signiﬁ cant amount of cycle skipping can occur with cor-

respondingly larger current and voltage ripple.

Efﬁ ciency Considerations

The percent efﬁ ciency of a switching regulator is equal to

the output power divided by the input power times 100%.

It is often useful to analyze individual losses to determine

what is limiting the efﬁ ciency and which change would

produce the most improvement. Percent efﬁ ciency can

be expressed as:

%Efﬁ ciency = 100% – (L1 + L2 + L3 + ...)

where L1, L2, etc. are the individual losses as a percent-

age of input power.

Although all dissipative elements in the circuit produce

losses, four main sources usually account for most of

the losses in LTC3868-1 circuits: 1) IC VIN current, 2)

INTVCC regulator current, 3) I2R losses, 4) topside MOSFET

transition losses.

1. The VIN current is the DC input supply current given

in the Electrical Characteristics table, which excludes

MOSFET driver and control currents. VIN current typi-

cally results in a small (<0.1%) loss.

2. INTVCC current is the sum of the MOSFET driver and

control currents. The MOSFET driver current results

from switching the gate capacitance of the power

MOSFETs. Each time a MOSFET gate is switched from

low to high to low again, a packet of charge, dQ, moves

from INTVCC to ground. The resulting dQ/dt is a current

out of INTVCC that is typically much larger than the

control circuit current. In continuous mode, IGATECHG

= f(QT + QB), where QT and QB are the gate charges of

the topside and bottom side MOSFETs.

Supplying INTVCC from an output-derived power source

through EXTVCC will scale the VIN current required

for the driver and control circuits by a factor of (Duty

Cycle)/(Efﬁ ciency). For example, in a 20V to 5V applica-

tion, 10mA of INTVCC current results in approximately

2.5mA of VIN current. This reduces the midcurrent loss

from 10% or more (if the driver was powered directly

from VIN) to only a few percent.

3. I2R losses are predicted from the DC resistances of the

fuse (if used), MOSFET, inductor, current sense resis-

tor, and input and output capacitor ESR. In continuous

mode the average output current ﬂ ows through L and

RSENSE, but is chopped between the topside MOSFET

and the synchronous MOSFET. If the two MOSFETs have

approximately the same RDS(ON), then the resistance

of one MOSFET can simply be summed with the resis-

tances of L, RSENSE and ESR to obtain I2R losses. For

example, if each RDS(ON) = 30m, RL = 50m, RSENSE

= 10m and RESR = 40m (sum of both input and

output capacitance losses), then the total resistance

is 130m. This results in losses ranging from 3% to

13% as the output current increases from 1A to 5A for

a 5V output, or a 4% to 20% loss for a 3.3V output.

Efﬁ ciency varies as the inverse square of VOUT for the

same external components and output power level. The

combined effects of increasingly lower output voltages

and higher currents required by high performance digital

systems is not doubling but quadrupling the importance

of loss terms in the switching regulator system!

LTC3868-1

38681fd

APPLICATIONS INFORMATION

4. Transition losses apply only to the topside MOSFET(s),

and become signiﬁ cant only when operating at high

input voltages (typically 15V or greater). Transition

losses can be estimated from:

Transition Loss = (1.7) • VIN • 2 • IO(MAX) • CRSS • f

Other hidden losses such as copper trace and internal

battery resistances can account for an additional 5%

to 10% efﬁ ciency degradation in portable systems. It

is very important to include these system level losses

during the design phase. The internal battery and fuse

resistance losses can be minimized by making sure that

CIN has adequate charge storage and very low ESR at

the switching frequency. A 25W supply will typically

require a minimum of 20µF to 40µF of capacitance

having a maximum of 20m to 50m of ESR. The

LTC3868-1 2-phase architecture typically halves this

input capacitance requirement over competing solu-

tions. Other losses including Schottky conduction losses

during dead-time and inductor core losses generally

account for less than 2% total additional loss.

Checking Transient Response

The regulator loop response can be checked by looking at

the load current transient response. Switching regulators

take several cycles to respond to a step in DC (resistive)

load current. When a load step occurs, VOUT shifts by

an amount equal to ∆ILOAD (ESR), where ESR is the ef-

fective series resistance of COUT

. ∆ILOAD also begins to

charge or discharge COUT generating the feedback error

signal that forces the regulator to adapt to the current

change and return VOUT to its steady-state value. During

this recovery time VOUT can be monitored for excessive

overshoot or ringing, which would indicate a stability

problem. OPTI-LOOP compensation allows the transient

response to be optimized over a wide range of output

capacitance and ESR values. The availability of the ITH pin

not only allows optimization of control loop behavior, but

it also provides a DC coupled and AC ﬁ ltered closed-loop

response test point. The DC step, rise time and settling

at this test point truly reﬂ ects the closed-loop response.

Assuming a predominantly second order system, phase

margin and/or damping factor can be estimated using the

percentage of overshoot seen at this pin. The bandwidth

can also be estimated by examining the rise time at the

pin. The ITH external components shown in Figure 12

circuit will provide an adequate starting point for most

applications.

The ITH series RC-CC ﬁ lter sets the dominant pole-zero

loop compensation. The values can be modiﬁ ed slightly

(from 0.5 to 2 times their suggested values) to optimize

transient response once the ﬁ nal PC layout is done and

the particular output capacitor type and value have been

determined. The output capacitors need to be selected

because the various types and values determine the loop

gain and phase. An output current pulse of 20% to 80%

of full-load current having a rise time of 1µs to 10µs will

produce output voltage and ITH pin waveforms that will

give a sense of the overall loop stability without breaking

the feedback loop.

Placing a resistive load and a power MOSFET directly

across the output capacitor and driving the gate with an

appropriate signal generator is a practical way to produce

a realistic load step condition. The initial output voltage

step resulting from the step change in output current may

not be within the bandwidth of the feedback loop, so this

signal cannot be used to determine phase margin. This

is why it is better to look at the ITH pin signal which is in

the feedback loop and is the ﬁ ltered and compensated

control loop response.

The gain of the loop will be increased by increasing RC

and the bandwidth of the loop will be increased by de-

creasing CC. If RC is increased by the same factor that CC

is decreased, the zero frequency will be kept the same,

thereby keeping the phase shift the same in the most

critical frequency range of the feedback loop. The output

voltage settling behavior is related to the stability of the

closed-loop system and will demonstrate the actual overall

supply performance.

A second, more severe transient is caused by switching

in loads with large (>1µF) supply bypass capacitors. The

discharged bypass capacitors are effectively put in parallel

with COUT

, causing a rapid drop in VOUT

. No regulator can

alter its delivery of current quickly enough to prevent this

sudden step change in output voltage if the load switch

resistance is low and it is driven quickly. If the ratio of

LTC3868-1

38681fd

APPLICATIONS INFORMATION

CLOAD to COUT is greater than 1:50, the switch rise time

should be controlled so that the load rise time is limited

to approximately 25 • CLOAD. Thus a 10µF capacitor would

require a 250µs rise time, limiting the charging current

to about 200mA.

Design Example

As a design example for one channel, assume VIN =

12V(nominal), VIN = 22V (max), VOUT = 3.3V, IMAX = 6A,

VSENSE(MAX) = 50mV and f = 350kHz.

The inductance value is chosen ﬁ rst based on a 30% ripple

current assumption. The highest value of ripple current

occurs at the maximum input voltage. Tie the FREQ pin

to GND, generating 350kHz operation. The minimum

inductance for 30% ripple current is:

ΔIL(NOM) =VOUT

()

1– VOUT

VIN(NOM)

⎛

⎝

⎜

⎞

⎠

⎟

A 3.9µH inductor will produce 29% ripple current. The

peak inductor current will be the maximum DC value plus

one half the ripple current, or 6.88A. Increasing the ripple

current will also help ensure that the minimum on-time

of 95ns is not violated. The minimum on-time occurs at

maximum VIN:

tON(MIN) =VOUT

VIN(MAX) f

()

=3.3V

22V 350kHz

()

=429ns

The equivalent RSENSE resistor value can be calculated by

using the minimum value for the maximum current sense

threshold (43mV):

RSENSE ≤43mV

6.88A =0.006Ω

Choosing 1% resistors: RA = 25k and RB = 78.1k yields

an output voltage of 3.299V.

The power dissipation on the topside MOSFET can be easily

estimated. Choosing a Fairchild FDS6982S dual MOSFET

results in: RDS(ON) = 0.035/0.022, CMILLER = 215pF. At

maximum input voltage with T(estimated) = 50°C:

MAIN =3.3V

22V 6A

()

21+0.005

()

50°C–25°C

()

⎡

⎣⎤

⎦

0.035Ω

()

+22V

()

26A

22.5Ω

()

215pF

()

•

5V – 2.3V +1

2.3V

⎡

⎣

⎢⎤

⎦

⎥350kHz

()

=433mW

A short-circuit to ground will result in a folded back cur-

rent of:

ISC =25mV

0.006Ω–1

95ns 22V

()

3.9µH

⎛

⎝

⎜⎞

⎠

⎟=3.9A

with a typical value of RDS(ON) and δ = (0.005/°C)(25°C)

= 0.125. The resulting power dissipated in the bottom

MOSFET is:

SYNC =3.9A

()

21.125

()

0.022Ω

()

=376mW

which is less than under full-load conditions.

CIN is chosen for an RMS current rating of at least 3A at

temperature assuming only this channel is on. COUT is

chosen with an ESR of 0.02 for low output ripple. The

output ripple in continuous mode will be highest at the

maximum input voltage. The output voltage ripple due to

ESR is approximately:

ORIPPLE = RESR(∆IL) = 0.02(1.75A) = 35mVP-P

LTC3868-1

38681fd

APPLICATIONS INFORMATION

PC Board Layout Checklist

When laying out the printed circuit board, the following

checklist should be used to ensure proper operation of

the IC. These items are also illustrated graphically in the

layout diagram of Figure 10. Figure 11 illustrates the current

waveforms present in the various branches of the 2-phase

synchronous regulators operating in the continuous mode.

Check the following in your layout:

1. Are the top N-channel MOSFETs MTOP1 and MTOP2

located within 1cm of each other with a common drain

connection at CIN? Do not attempt to split the input

decoupling for the two channels as it can cause a large

resonant loop.

2. Are the signal and power grounds kept separate? The

combined IC signal ground pin and the ground return

of CINTVCC must return to the combined COUT (–) ter-

minals. The path formed by the top N-channel MOSFET,

Schottky diode and the CIN capacitor should have short

leads and PC trace lengths. The output capacitor (–)

terminals should be connected as close as possible

to the (–) terminals of the input capacitor by placing

the capacitors next to each other and away from the

Schottky loop described above.

3. Do the LTC3868-1 VFB pins’ resistive dividers connect

to the (+) terminals of COUT? The resistive divider

must be connected between the (+) terminal of COUT

and signal ground. The feedback resistor connections

should not be along the high current input feeds from

the input capacitor(s).

4. Are the SENSE– and SENSE+ leads routed together with

minimum PC trace spacing? The ﬁ lter capacitor between

SENSE+ and SENSE– should be as close as possible

to the IC. Ensure accurate current sensing with Kelvin

connections at the SENSE resistor.

5. Is the INTVCC decoupling capacitor connected close

to the IC, between the INTVCC and the power ground

pins? This capacitor carries the MOSFET drivers’ cur-

rent peaks. An additional 1µF ceramic capacitor placed

immediately next to the INTVCC and PGND pins can help

improve noise performance substantially.

6. Keep the switching nodes (SW1, SW2), top gate nodes

(TG1, TG2), and boost nodes (BOOST1, BOOST2) away

from sensitive small-signal nodes, especially from

the opposites channel’s voltage and current sensing

feedback pins. All of these nodes have very large and

fast moving signals and therefore should be kept on

the output side of the LTC3868-1 and occupy minimum

PC trace area.

7. Use a modiﬁ ed star ground technique: a low impedance,

large copper area central grounding point on the same

side of the PC board as the input and output capacitors

with tie-ins for the bottom of the INTVCC decoupling

capacitor, the bottom of the voltage feedback resistive

divider and the SGND pin of the IC.

PC Board Layout Debugging

Start with one controller on at a time. It is helpful to use

a DC-50MHz current probe to monitor the current in the

inductor while testing the circuit. Monitor the output

switching node (SW pin) to synchronize the oscilloscope

to the internal oscillator and probe the actual output voltage

as well. Check for proper performance over the operating

voltage and current range expected in the application. The

frequency of operation should be maintained over the input

voltage range down to dropout and until the output load

drops below the low current operation threshold—typi-

cally 10% of the maximum designed current level in Burst

Mode operation.

The duty cycle percentage should be maintained from cycle

to cycle in a well-designed, low noise PCB implementation.

Variation in the duty cycle at a subharmonic rate can sug-

gest noise pickup at the current or voltage sensing inputs

or inadequate loop compensation. Overcompensation of

the loop can be used to tame a poor PC layout if regula-

tor bandwidth optimization is not required. Only after

each controller is checked for its individual performance

should both controllers be turned on at the same time.

A particularly difﬁ cult region of operation is when one

controller channel is nearing its current comparator trip

point when the other channel is turning on its top MOSFET.

This occurs around 50% duty cycle on either channel due

to the phasing of the internal clocks and may cause minor

duty cycle jitter.

LTC3868-1

38681fd

APPLICATIONS INFORMATION

Figure 10. Recommended Printed Circuit Layout Diagram

CB2

CB1

VOUT1

RPU1

PGOOD1

VPULL-UP

(<6V)

CINTVCC CIN

1µF

CERAMIC

M1 M2

M3 M4 D2

CVIN

VIN

RIN

COUT1

VOUT1

GND

VOUT2

38681 F10

COUT2

RSENSE

fIN

1µF

CERAMIC

ITH1

VFB1

SENSE1+

SENSE1–

FREQ

SENSE2–

SENSE2+

VFB2

ITH2

SS2

SS1

PGOOD1

TG1

SW1

BOOST1

BG1

VIN

PGND

EXTVCC

INTVCC

BG2

BOOST2

SW2

TG2

PLLIN/MODE

RUN1

RUN2

SGND

LTC3868-1

Reduce VIN from its nominal level to verify operation

of the regulator in dropout. Check the operation of the

undervoltage lockout circuit by further lowering VIN while

monitoring the outputs to verify operation.

Investigate whether any problems exist only at higher out-

put currents or only at higher input voltages. If problems

coincide with high input voltavges and low output currents,

look for capacitive coupling between the BOOST, SW, TG,

and possibly BG connections and the sensitive voltage

and current pins. The capacitor placed across the current

sensing pins needs to be placed immediately adjacent to

the pins of the IC. This capacitor helps to minimize the

effects of differential noise injection due to high frequency

capacitive coupling. If problems are encountered with

high current output loading at lower input voltages, look

for inductive coupling between CIN, Schottky and the top

MOSFET components to the sensitive current and voltage

sensing traces. In addition, investigate common ground

path voltage pickup between these components and the

SGND pin of the IC.

An embarrassing problem, which can be missed in an

otherwise properly working switching regulator, results

when the current sensing leads are hooked up backwards.

The output voltage under this improper hookup will still

be maintained but the advantages of current mode control

will not be realized. Compensation of the voltage loop will

be much more sensitive to component selection. This

behavior can be investigated by temporarily shorting out

the current sensing resistor—don’t worry, the regulator

will still maintain control of the output voltage.

LTC3868-1

38681fd

APPLICATIONS INFORMATION

Figure 11. Branch Current Waveforms

RL1

SW1 RSENSE1 VOUT1

COUT1

VIN

CIN

RIN

RL2

BOLD LINES INDICATE

HIGH SWITCHING

CURRENT. KEEP LINES

TO A MINIMUM LENGTH.

SW2

38681 F11

RSENSE2 VOUT2

COUT2

LTC3868-1

38681fd

TYPICAL APPLICATIONS

Start-Up SW Node Waveforms

Figure 12. High Efﬁ ciency Dual 8.5V/3.3V Step-Down Converter

SENSE1+

SENSE1–

SENSE2–

SENSE2+

VFB1

ITH1

SGND

EXTVCC

RUN1

RUN2

FREQ

SS2

VFB2

SS1

PLLIN/MODE

PGOOD1

BG1

SW1

BOOST1

TG1

VIN

INTVCC

PGND

TG2

BG2

BOOST2

SW2

1nF

RB1

215k

COUT1, COUT2: SANYO 10TPD150M

L1: SUMIDA CDEP105-3R2M

L2: SUMIDA CDEP105-7R2M

MTOP1, MTOP2, MBOT1, MBOT2: VISHAY Si7848DP

CF1

15pF

CITH1A 150pF

CSS1 0.1µF

CSS2 0.1µF

CITH1 820pF

CINT

4.7µF

CB1

0.47µF

CB2

0.47µF

VIN

9V TO 24V

LTC3868-1

3.3µH

7.2µH

RSENSE1

7m

RSENSE2

10m

MBOT1

MTOP2

MTOP1

MBOT2

38681 F12

100k

INTVCC

COUT1

150µF

VOUT1

3.3V

VOUT2

8.5V

CIN

22µF

COUT2

150µF

RA1

68.1k

RA2

44.2k

RB2

442k

CF2

39pF

RITH1 15k

1nF

ITH2

CITH2 680pF

CITH2A 100pF

RITH2 27k

Efﬁ ciency vs Output Current

OUTPUT CURRENT (A)

0.00001 0.0001

EFFICIENCY (%)

0.001 0.01 0.1 1 10

38681 F12b

100

VIN = 12V

Burst Mode OPERATION

VOUT = 8.5V VOUT = 3.3V

38681 F12c

VOUT2

2V/DIV

VOUT1

2V/DIV

20ms/DIV 38681 F12d

SW1

5V/DIV

SW2

5V/DIV

1µs/DIV

LTC3868-1

38681fd

TYPICAL APPLICATIONS

SENSE1+

SENSE1–

SENSE2–

SENSE2+

VFB1

ITH1

SGND

EXTVCC

RUN1

RUN2

FREQ

SS2

VFB2

SS1

PLLIN/MODE

PGOOD1

BG1

SW1

BOOST1

TG1

VIN

INTVCC

PGND

TG2

BG2

BOOST2

SW2

1nF

RB1

143k

COUT1, COUT2: SANYO 10TPD150M

L1: SUMIDA CDEP105-2R5

L2: SUMIDA CDEP105-3R2M

MTOP1, MTOP2, MBOT1, MBOT2: VISHAY Si7848DP

CF1

22pF

CITH1A 100pF

CSS1 0.01µF

CSS2 0.01µF

CITH1 820pF

CINT

4.7µF

CB1

0.47µF

CB2

0.47µF

VIN

4V TO 24V

LTC3868-1

2.4µH

3.2µH

RSENSE1

7m

RSENSE2

7m

MBOT1

MTOP2

MTOP1

MBOT2

38681 F13

100k

INTVCC

COUT1

150µF

VOUT1

2.5V

VOUT2

3.3V

CIN

22µF

COUT2

150µF

RA1

68.1k

RA2

68.1k

RB2

215k

CF2

15pF

RITH1 22k

1nF

ITH2

CITH2 820pF

CITH2A 150pF

RITH2 15k

High Efﬁ ciency Dual 2.5V/3.3V Step-Down Converter

LTC3868-1

38681fd

TYPICAL APPLICATIONS

High Efﬁ ciency Dual 12V/5V Step-Down Converter

SENSE1+

SENSE1–

SENSE2–

SENSE2+

VFB1

ITH1

SGND

EXTVCC

RUN1

RUN2

FREQ

SS2

VFB2

SS1

PLLIN/MODE

PGOOD1

BG1

SW1

BOOST1

TG1

VIN

INTVCC

PGND

TG2

BG2

BOOST2

SW2

1nF

RB1

422k

COUT1: KEMET T525D476M016E035

COUT2: SANYO 10TPD150M

L1: SUMIDA CDEP105-8R8M

L2: SUMIDA CDEP105-4R3M

MTOP1, MTOP2, MBOT1, MBOT2: VISHAY Si7848DP

CF1

33pF

CITH1A 100pF

CSS1 0.01µF

CSS2 0.01µF

CITH1 680pF

CINT

4.7µF

CB1

0.47µF

CB2

0.47µF

VIN

12.5V TO 24V

LTC3868-1

8.8µH

4.3µH

RSENSE1

10m

RSENSE2

7m

MBOT1

MTOP2

MTOP1

MBOT2

38681 TA02a

100k

INTVCC

COUT1

47µF

VOUT1

12V

VOUT2

5.5A

CIN

22µF

COUT2

150µF

RA1

34k

RA2

75k

RB2

393k

CF2

15pF

RFREQ

60k

RITH1 33k

1nF

ITH2

CITH2 680pF

CITH2A 100pF

RITH2 17k

LTC3868-1

38681fd

TYPICAL APPLICATIONS

SENSE1+

SENSE1–

SENSE2–

SENSE2+

VFB1

ITH1

SGND

EXTVCC

RUN1

RUN2

FREQ

SS2

VFB2

SS1

PLLIN/MODE

PGOOD1

BG1

SW1

BOOST1

TG1

VIN

INTVCC

PGND

TG2

BG2

BOOST2

SW2

1nF

RB1

28.7k

COUT1, COUT2: SANYO 2RSTPE220M

L1: SUMIDA CDEP105-3R2M

L2: SUMIDA CDEP105-7R2M

MTOP1, MTOP2: RENESAS RJK0305

MBOT1, MBOT2: RENESAS RJK0328

CF1

56pF

CITH1A 220pF

CSS1 0.01µF

CSS2 0.01µF

CITH1 1000pF

CINT

4.7µF

CB1

0.47µF

CB2

0.47µF

VIN

12V

LTC3868-1

0.47µH

RSENSE1

4m

RSENSE2

4m

MBOT1

MTOP2

MTOP1

MBOT2

38681 TA03a

100k

INTVCC

COUT1

220µF

×2

VOUT1

VOUT2

1.2V

CIN

22µF

COUT2

220µF

×2

RA1

115k

RA2

115k

RB2

57.6k

CF2

56pF

RFREQ

60k

RITH1 3.93k

1nF

ITH2

CITH2 1000pF

CITH2A 220pF

RITH2 3.43k

High Efﬁ ciency Dual 1V/1.2V Step-Down Converter

LTC3868-1

38681fd

SENSE1+

SENSE1–

SENSE2–

SENSE2+

VFB1

ITH1

SGND

EXTVCC

RUN1

RUN2

FREQ

SS2

VFB2

SS1

PLLIN/MODE

PGOOD1

BG1

SW1

BOOST1

TG1

VIN

INTVCC

PGND

TG2

BG2

BOOST2

SW2

0.1µF

RB1

28.7k

COUT1, COUT2: SANYO 2R5TPE220M

L1, L2: SUMIDA IHL P2525CZERR47M06

MTOP1, MTOP2: RENESAS RJK0305

MBOT1, MBOT2: RENESAS RJK0328

CF1

56pF

CITH1A 200pF

CSS1 0.01µF

CSS2 0.01µF

CITH1 1000pF

CINT

4.7µF

CB1

0.47µF

CB2

0.47µF

VIN

12V

LTC3868-1

0.47µH

MBOT1

MTOP2

MTOP1

MBOT2

38681 TA05

100k

RS1 1.18k

RS2 1.18k

INTVCC

COUT1

220µF

×2

VOUT1

VOUT2

1.2V

CIN

22µF

COUT2

220µF

×2

RA1

115k

RA2

115k

RB2

57.6k

CF2

56pF

RFREQ

65k

RITH1 3.93k

0.1µF

ITH2

CITH2 1000pF

CITH2A 220pF

RITH2 3.93k

High Efﬁ ciency Dual 1V/1.2V Step-Down Converter with Inductor DCR Current Sensing

TYPICAL APPLICATIONS

LTC3868-1

38681fd

PACKAGE DESCRIPTION

UFD Package

28-Lead Plastic QFN (4mm × 5mm)

(Reference LTC DWG # 05-08-1712 Rev B)

4.00 p 0.10

(2 SIDES)

2.50 REF

5.00 p 0.10

(2 SIDES)

NOTE:

1. DRAWING PROPOSED TO BE MADE A JEDEC PACKAGE OUTLINE MO-220 VARIATION (WXXX-X).

2. DRAWING NOT TO SCALE

3. ALL DIMENSIONS ARE IN MILLIMETERS

4. DIMENSIONS OF EXPOSED PAD ON BOTTOM OF PACKAGE DO NOT INCLUDE

MOLD FLASH. MOLD FLASH, IF PRESENT, SHALL NOT EXCEED 0.15mm ON ANY SIDE

5. EXPOSED PAD SHALL BE SOLDER PLATED

6. SHADED AREA IS ONLY A REFERENCE FOR PIN 1 LOCATION

ON THE TOP AND BOTTOM OF PACKAGE

PIN 1

TOP MARK

(NOTE 6)

0.40 p 0.10

27 28

BOTTOM VIEW—EXPOSED PAD

3.50 REF

0.75 p 0.05 R = 0.115

TYP

R = 0.05

TYP

PIN 1 NOTCH

R = 0.20 OR 0.35

s 45o CHAMFER

0.25 p 0.05

0.50 BSC

0.200 REF

0.00 – 0.05

(UFD28) QFN 0506 REV B

RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS

APPLY SOLDER MASK TO AREAS THAT ARE NOT SOLDERED

0.70 p0.05

0.25 p0.05

0.50 BSC

2.50 REF

3.50 REF

4.10 p 0.05

5.50 p 0.05

2.65 p 0.05

3.10 p 0.05

4.50 p 0.05

PACKAGE

OUTLINE

2.65 p 0.10

3.65 p 0.10

3.65 p 0.05

LTC3868-1

38681fd

PACKAGE DESCRIPTION

GN Package

28-Lead Plastic SSOP (Narrow .150 Inch)

(Reference LTC DWG # 05-08-1641)

.386 – .393*

(9.804 – 9.982)

GN28 (SSOP) 0204

345678 9 10 11 12

.229 – .244

(5.817 – 6.198)

.150 – .157**

(3.810 – 3.988)

202122232425262728 19 18 17

13 14

1615

.016 – .050

(0.406 – 1.270)

.015 p .004

(0.38 p 0.10) s 45o

0o – 8o TYP

.0075 – .0098

(0.19 – 0.25)

.0532 – .0688

(1.35 – 1.75)

.008 – .012

(0.203 – 0.305)

TYP

.004 – .0098

(0.102 – 0.249)

.0250

(0.635)

BSC

.033

(0.838)

REF

.254 MIN

RECOMMENDED SOLDER PAD LAYOUT

.150 – .165

.0250 BSC.0165 p.0015

.045 p.005

* DIMENSION DOES NOT INCLUDE MOLD FLASH. MOLD FLASH

SHALL NOT EXCEED 0.006" (0.152mm) PER SIDE

** DIMENSION DOES NOT INCLUDE INTERLEAD FLASH. INTERLEAD

FLASH SHALL NOT EXCEED 0.010" (0.254mm) PER SIDE

INCHES

(MILLIMETERS)

NOTE:

1. CONTROLLING DIMENSION: INCHES

2. DIMENSIONS ARE IN

3. DRAWING NOT TO SCALE

LTC3868-1

38681fd

Information furnished by Linear Technology Corporation is believed to be accurate and reliable.

However, no responsibility is assumed for its use. Linear Technology Corporation makes no representa-

tion that the interconnection of its circuits as described herein will not infringe on existing patent rights.

REVISION HISTORY

REV DATE DESCRIPTION PAGE NUMBER

B 12/09 Change to Absolute Maximum Ratings

Change to Electrical Characteristics

Change to Typical Performance Characteristics

Change to Pin Functions

Text Changes to Operation Section

Text Changes to Applications Information Section

Change to Figure 10

Changes to Related Parts

3, 4

8, 9

11, 12, 13

21, 22, 23, 24, 26

C 08/10 Change to Electrical Characteristics

Added Typical Application to Back Page and Updated Related Parts

D 12/10 Change to Absolute Maximum Ratings

Changes to Electrical Characteristics

Changes to Pin Functions

Changes to Functional Diagram

Changes to Typical Applications

3, 4

8, 9

30 to 34, 38

(Revision history begins at Rev B)

LTC3868-1

38681fd

Linear Technology Corporation

1630 McCarthy Blvd., Milpitas, CA 95035-7417

(408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com

LT 1210 REV D • PRINTED IN USA

TYPICAL APPLICATION

High Efﬁ ciency 2-Phase 12V/150W Step-Down Converter

SENSE1+

SENSE1–

SENSE2–

SENSE2+

VFB1

ITH1

SGND

EXTVCC

VOUT

RUN1

RUN2

FREQ

SS2SS1

ITH1

VFB1 VFB2

PLLIN/MODE

PGOOD1

BG1

SW1

BOOST1

TG1

VIN

INTVCC

PGND

TG2

BG2

BOOST2

SW2

1nF

RB1

698k

COUT1, COUT2: SANYO 16TQC22M

L1, L2: SUMIDA CDEP106-6ROM

MTOP1, MTOP2: INFINEON BSZ097N04LS

MBOT1, MBOT2: INFINEON BSZ097N04LS

CF1

10pF

CITH1A 68pF

CSS1 0.1µF

CITH1 3300pF

CINT

4.7µF

CB1

0.47µF

CB2

0.47µF

VIN

19V TO 24V

LTC3868-1

6µH

RSENSE1

5m

RSENSE2

5m

MBOT1

MTOP2

MTOP1

MBOT2

38681 TA06

100k

INTVCC

COUT1

22µF

16V

VOUT1

12V

12.5A

10µF

16V

CIN

10µF

50V

10µF

50V

COUT2

22µF

16V

10µF

16V

RA1

49.9k

RITH1 2.94k

1nF

ITH2

RELATED PARTS

PART NUMBER DESCRIPTION COMMENTS

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Outputs (≥5V) Remain in Regulation Through Cold Crank 2.5V ≤ VIN ≤ 38V,

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LTC3857/LTC3857-1 Low IQ, Dual Output 2-Phase Synchronous Step-

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Phase-Lockable Fixed Operating Frequency 50kHz to 900kHz,

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Phase-Lockable Fixed Operating Frequency 140kHz to 650kHz,

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LTC3835/LTC3835-1 Low IQ, Synchronous Step-Down DC/DC

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Phase-Lockable Fixed Operating Frequency 140kHz to 650kHz,

4V ≤ VIN ≤ 36V, 0.8V ≤ VOUT ≤ 10V, IQ = 80µA

LT3845A Low IQ, High Voltage Synchronous Step-Down

DC/DC Controller

Adjustable Fixed Operating Frequency 100kHz to 500kHz, 4V ≤ VIN ≤ 60V,

1.23V ≤ VOUT ≤ 36V, IQ = 120µA, TSSOP-16

LTC3824 Low IQ, High Voltage DC/DC Controller, 100%

Duty Cycle

Selectable Fixed 200kHz to 600kHz Operating Frequency 4V ≤ VIN ≤ 60V,

0.8V ≤ VOUT ≤ VIN, IQ = 40µA, MSOP-10E