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LTC3446

3446ff

Typical applicaTion

FeaTures

applicaTions

DescripTion

Monolithic Buck Regulator

with Dual VLDO Regulators

The LTC

3446 combines a synchronous buck DC/DC con-

verter with two very low dropout (VLDO) linear regulators

to provide up to three stepped-down output voltages from

a single input voltage. The input voltage range is ideally

suited for Li-Ion battery-powered applications, as well

as powering low voltage logic from 5V or 3.3V rails. The

output voltage range extends down to 0.4V for the VLDO

regulators and 0.8V for the buck.

The 1A synchronous buck converter provides the main

output with high efﬁciency, typically 85%. The two 300mA

VLDO regulators can run off the main output to provide

two additional lower voltage outputs. A built-in supply

monitor provides a power good indication.

The buck converter switches at 2.25MHz, allowing the use

of small surface mount inductors and capacitors. Constant

frequency current mode operation produces controlled

output spectrum and fast transient response. A mode-

select pin allows automatic Burst Mode operation to be

enabled for higher efﬁciency at light load, or disabled for

lower noise operation down to very light loads. The two

VLDO regulators are stable with ceramic output capacitors

as small as 1µF.

L, LT, LTC, LTM and Burst Mode are registered trademarks and VLDO, Hot Swap, PowerPath

and Bat-Track are trademarks of Linear Technology Corporation. All other trademarks are the

property of their respective owners. Protected by U.S. Patents including 5481178, 6611131,

6304066, 6498466, 6580258.

Buck Efﬁciency vs Buck Load Current

n High Efﬁciency Triple Step-Down Outputs from a

Single Input Supply

n 1A Synchronous Buck Regulator Provides Main

Step-Down Output and Powers Two 300mA VLDO™

Linear Regulators

n Output Voltages as Low as 400mV (VLDO Outputs)

n Power Good Output

n Input Voltage Range: 2.7V to 5.5V

n Independent Enable Pin for Each Supply

n Low (140µA Typ) No-Load Quiescent Current with

All Outputs Enabled

n Constant Frequency Current Mode Operation

n 2.25MHz Switching Frequency Uses Small Inductors

n Defeatable Automatic Burst Mode

Operation for

High Efﬁciency at Light Loads

n ±1.5% Reference Accuracy

n Overtemperature Protection

n Thermally Enhanced 4mm × 3mm 14-Pin DFN

Package

n Low Power Handheld Devices

n Low Voltage and Multivoltage Power for Digital

Logic, I/O, FPGAs, CPLDs, ASICs and CPUs

VIN

59k

1.8µH

47.5k

BUCKFB

LVIN

LTC3446

GND

22µF

X7R

1000pF

DIGITAL

CONTROL

22µF

X7R

VOUT

1.8V

400mA MAX

110k

40.2k

2.2µF

X7R

VOUT

1.5V

300mA MAX

VIN

2.9V TO 5.5V

LVOUT1

PGOOD

MODESEL

ENBUCK

ENLDO1

ENLDO2

LVFB1

80.6k

40.2k

2.2µF

X7R

L1: TOKO A960AW-1R8M

3446 TA01

VOUT

1.2V

300mA MAX

LVOUT2

3.3k

ITH

LVFB2

LOAD CURRENT (mA)

EFFICIENCY (%)

100

10 100

3446 TA01b

1000

VIN = 2.7V

VIN = 3.6V

VIN = 4.2V

Burst Mode

OPERATION

PWM

MODE

LTC3446

3446ff

pin conFiguraTionabsoluTe MaxiMuM raTings

VIN, LVIN to GND ......................................... –0.3V to 6V

MODESEL, ENBUCK, ENLDO1,

ENLDO2 to GND .......................................... –0.3V to 6V

BUCKFB to GND .......................................... –0.3V to 6V

LVFB1, LVFB2 to GND .................................... –0.3V to 6V

ITH to GND ..... –0.3V to the Lesser of (VIN + 0.3V) or 3V

SW to GND ......–0.3V to the Lesser of (VIN + 0.3V) or 6V

LVOUT1, LVOUT2

to GND .......... –0.3V to the Lesser of (LVIN + 0.3V) or 6V

PGOOD to GND ............................................ –0.3V to 6V

LVOUT1, LVOUT2 Short-Circuit

to GND Duration ............................................... Indeﬁnite

Operating Junction Temperature Range

(Note 2) .................................................. –40°C to 125°C

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

(Note 1)

ENBUCK

BUCKFB

ENLDO1

LVFB1

LVFB2

ENLDO2

MODESEL

VIN

ITH

PGOOD

LVOUT1

LVIN

LVOUT2

TOP VIEW

DE PACKAGE

14-LEAD (4mm × 3mm) PLASTIC DFN

TJMAX = 125°C, θJA = 43°C/W

EXPOSED PAD (PIN 15) IS GND, MUST BE SOLDERED TO PCB

orDer inForMaTion

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

LTC3446EDE#PBF LTC3446EDE#TRPBF 3446 14-Lead (4mm × 3mm) Plastic DFN –40°C to 125°C

LTC3446IDE#PBF LTC3446IDE#TRPBF 3446 14-Lead (4mm × 3mm) Plastic DFN –40°C to 125°C

Consult LTC Marketing for parts speciﬁed with wider operating temperature ranges. *The temperature grade is identiﬁed by a label on the shipping container.

Consult LTC Marketing for information on non-standard lead based ﬁnish parts.

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

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

SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS

VIN Input Voltage Range (Note 3) ●2.7 5.5 V

VUVLO VIN Undervoltage Lockout Threshold

VIN Undervoltage Lockout Hysteresis

VIN Rising ●2.37

2.5

IQVIN Quiescent Current (Note 4)

Buck Enabled Only, Not Sleeping

Buck Enabled Only, Sleeping

One LDO Enabled Only

All Three Outputs Enabled, Buck Not Sleeping

All Three Outputs Enabled, Buck Sleeping

Shutdown

VBUCKFB = 0V, ISW = 0mA

VBUCKFB = 1V, ISW = 0mA

VLVIN = 1.5V, 10µA LDO Output Load

VBUCKFB = 0V, ISW = 0mA, VLVIN = 1.5V,

10µA Output Load on Each LDO

VBUCKFB = 1V, ISW = 0mA, VLVIN = 1.5V,

10µA Load on Each LDO

VENBUCK = 0V, VENLDO1 = 0V, VENLDO2 = 0V

310

400

140

500

100

600

210

µA

elecTrical characTerisTics

The l denotes the speciﬁcations which apply over the full operating

temperature range, otherwise speciﬁcations are at TA = 25°C. VIN = 3.6V unless otherwise speciﬁed. (Note 2)

LTC3446

3446ff

elecTrical characTerisTics

The l denotes the speciﬁcations which apply over the full operating

temperature range, otherwise speciﬁcations are at TA = 25°C. VIN = 3.6V unless otherwise speciﬁed. (Note 2)

SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS

VPG(THRESH) PGOOD Threshold (Note 8) ●8 10 %

RPGOOD PGOOD Output Resistance PGOOD Low, Sinking 1mA ●87 180 Ω

IPGOOD PGOOD Hi-Z Leakage Current VPGOOD = 6V ●1 µA

Synchronous Buck Converter

IBUCKFB Feedback Current (Note 5) ●±30 nA

VBUCKFB Regulated Feedback Voltage (Note 5) ●0.788 0.800 0.812 V

∆VBUCKFB Feedback Voltage Line Regulation VIN = 2.7V to 5.5V (Note 5) ●0.3 0.5 mV/V

IMAXP Maximum Peak Inductor Current VBUCKFB = 0V, Duty Cycle < 35% 1.2 1.55 2.0 A

IMAXN NMOS Overcurrent Limit 1.8 A

Feedback Voltage Load Regulation

(with Respect to VITH)

VITH = 0.5V to 1V, VMODESEL = VIN (Note 5) 0.5 mV/V

fOSC Oscillator Frequency ●1.8 2.25 2.7 MHz

RPFET RDS(ON) of P-Channel FET ISW = 500mA 0.13 Ω

RNFET RDS(ON) of N-Channel FET ISW = –500mA 0.14 Ω

ILSW SW Leakage VENBUCK = 0V, VSW = 0V or 5.5V, VIN = 5.5V ±1 µA

VENBUCK Buck Enable Pin Threshold ●0.3 0.65 1 V

IENBUCK Buck Enable Pin Leakage Current VENBUCK = 5.5V, All Other Pins Grounded ●1 µA

VMODESEL Mode Select Pin Threshold ●0.3 0.65 1 V

IMODESEL Mode Select Pin Leakage Current VMODESEL = 5.5V, All Other Pins Grounded ●1 µA

gmError Ampliﬁer Transconductance VITH = 0.6V 450 700 950 µA/V

Each VLDO: VIN = 3.6V, VLVIN = 1.5V, VLVOUT = 1.2V, Unless Otherwise Speciﬁed

VLVIN LVIN Pin Operating Voltage (Note 6) ●0.9 5.5 V

ILVIN LVIN Pin Operating Current IOUT = 10µA ●3 20 µA

LVIN Shutdown Current VENLDO = 0V 1.5 2 µA

VLVFB Feedback Pin Regulation Voltage (Note 7) 1mA ≤ IOUT ≤ 300mA, 1.5V ≤ VLVIN ≤ 5.5V

●

0.395

0.392

0.400

0.405

0.408

ILVFB Feedback Pin Input Current VLVFB at Regulation ●2 ±10 nA

ILVOUT(MAX) Continuous Output Current ●300 mA

Short-Circuit Output Current 760 mA

VENLDOx LDO Enable Pin Threshold ●0.3 0.65 1 V

IENLDOx LDO Enable Pin Leakage Current VENLDOx = 5.5V, All Other Pins Grounded ●1 µA

Output Voltage Load Regulation

(Referred to the LVFB Pin)

∆IOUT = 1mA to 300mA –1 mV/A

LVFB Line Regulation (with Respect to

the LVIN Pin)

VLVIN = 1.5V to 5.5V, VIN = 3.6V, VLVOUT = 1.2V,

IOUT = 1mA

7.5 µV/V

LVFB Line Regulation (with Respect to

the VIN Pin)

VLVIN = 1.5V, VIN = 2.7V to 5.5V, VLVOUT = 1.2V,

IOUT = 1mA

0.44 mV/V

VDO LVIN – LV OUT Dropout Voltage VIN = 2.8V, VLVIN = 1.5V, VLVFB = 0.37V,

IOUT = 300mA (Note 9)

68 175 mV

VIN to LVOUT Headroom Required for

Regulation (Note 3)

ILVOUT = 300mA l1.4 V

LTC3446

3446ff

elecTrical characTerisTics

Note 1: Stresses beyond those listed under Absolute Maximum Ratings

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

Maximum Rating condition for extended periods may affect device

reliability and lifetime.

Note 2: The LTC3446 is tested under pulsed load conditions such that

TJ ≈ TA. The LTC3446E is guaranteed to meet performance speciﬁcations

from 0°C to 85°C operating junction temperature. Speciﬁcations over

the –40°C to 125°C operating junction temperature range are assured by

design characterization and correlation with statistical process controls.

The LTC3446I is guaranteed to meet performance speciﬁcations over

the –40°C to 125°C operating junction temperature range. Note that the

maximum ambient temperature consistent with these speciﬁcations is

determined by speciﬁc operating conditions in conjunction with board

layout, the rated package thermal impedance and other environmental

factors. The junction temperature (TJ, in °C) is calculated from the ambient

temperature (TA, in °C) and power dissipation (PD, in Watts) according to

the formula:

TJ = TA + (PD • θJA)

where θJA (in °C/W) is the package thermal impedance.

Note 3: Minimum operating VIN voltage required for the VLDO regulators

to stay in regulation is:

VIN ≥ LV OUT(MAX) + 1.4V and VIN ≥ 2.7V

Note 4: Dynamic supply current is higher due to the internal gate charge

being delivered at the switching frequency.

Note 5: The LTC3446 is tested in a feedback loop that connects the

BUCKFB pin to the output of the buck converter’s error ampliﬁer (i.e., the

ITH pin).

Note 6: Minimum operating LVIN voltage required for the VLDO regulators

to stay in regulation is:

LVIN ≥ LVOUT(MAX) + 100mV and LVIN ≥ 0.9V

Note 7: Operating conditions are limited by maximum junction

temperature. The regulated output voltage speciﬁcation will not apply

for all possible combinations of input voltage and output current. When

operating at maximum input voltage, the output current range must be

limited. When operating at maximum output current, the input voltage

range must be limited.

Note 8: PGOOD assertion indicates that the feedback voltages of all

enabled supplies are within the speciﬁed percentage of their target values.

Note 9: Dropout voltage in the DFN package is assured by design,

characterization and statistical process control.

Buck Regulated Feedback Voltage

vs Temperature

LDO1 Regulated Feedback

Voltage vs Temperature

LDO2 Regulated Feedback

Voltage vs Temperature

TEMPERATURE (°C)

–50

VBUCKFB (mV)

800

802

804

110

3446 G01

798

796

792 –10 30 70

–30 130

10 50 90

794

808

806

VIN = 2.7V

VIN = 3.6V

VIN = 4.2V

VIN = 5.5V

TEMPERATURE (°C)

–50

VLVFB1 (mV)

400

402

404

110

3446 G02

398

396

392 –10 30 70

–30 130

10 50 90

394

408

406

VIN = 2.7V

VIN = 3.6V

VIN = 4.2V

VIN = 5.5V

TEMPERATURE (°C)

–50

VLVFB2 (mV)

400

402

404

110

3446 G03

398

396

392 –10 30 70

–30 130

10 50 90

394

408

406

VIN = 2.7V

VIN = 3.6V

VIN = 4.2V

VIN = 5.5V

Typical perForMance characTerisTics

LTC3446

3446ff

Typical perForMance characTerisTics

Undervoltage Lockout Threshold

vs Temperature

Maximum Peak Inductor Current

Oscillator Frequency

vs Temperature

TEMPERATURE (°C)

–50

2.00

VIN (V)

2.05

2.15

2.20

2.25

2.50

2.35

–10 30 50 130

3446 G04

2.10

2.40

2.45

2.30

–30 10 70 90 110

UVLO RISING

UVLO FALLING

TEMPERATURE (°C)

–50

CURRENT (A)

2.0

2.4

2.3

2.2

2.1

1.9

2.5

10 50 130

3446 G05

1.6

1.8

1.7

1.2

1.1

1.4

1.5

1.3

1.0 –30 –10 30 70 11090

VIN = 2.7V

VIN = 3.6V

VIN = 4.2V

VIN = 5.5V

TEMPERATURE (°C)

–50

2.00

OSCILLATOR FREQUENCY (MHz)

2.05

2.15

2.20

2.25

2.50

2.35

–10 30 50 130

3446 G06

2.10

2.40

2.45

2.30

–30 10 70 90 110

VIN = 2.7V

VIN = 3.6V

VIN = 4.2V

VIN = 5.5V

Peak Inductor Current

vs ITH Voltage

VLDO Current Limit

vs LVIN Voltage

VLDO Dropout Voltage

vs Load Current

ITH PIN VOLTAGE (V)

PEAK INDUCTOR CURRENT (A)

0.8

1.2

3446 G07

0.4

00.5 11.5

0.25 0.75 1.25 1.75

1.6 VIN = 3.6V

VMODESEL = 3.6V

0.6

1.0

0.2

1.4

LVIN PIN VOLTAGE (V)

0.00

VLDO CURRENT LIMIT (mA)

200

600

800

1000

4.00

1800

3446 G08

400

2.00

1.00 5.00

3.00 6.00

1200

1400

1600

VIN = 3.6V

LVOUT = 0V

PULSED MEASUREMENT,

TJ ≈ TA ≈ 27°C

LOAD CURRENT (mA)

DROPOUT VOLTAGE (mV)

50 100 150 200

3446 G09

250

100

300

–45°C

27°C

90°C

130°C

VIN = 2.8V

LVOUT = 1.2V

VIN Bias Current

vs VLDO Load Current

LVIN No-Load Operating Current

Enable/MODESEL Thresholds

VLDO OUTPUT CURRENT (mA)

VIN CURRENT (µA)

150

200

250

200

450

3446 G10

100

50 250

150 300

300

350

400

–45°C

27°C

90°C

130°C

VIN = 3.6V

LVIN = 1.5V

LVOUT = 1.2V

ONLY ONE VLDO

ENABLED

LVIN PIN VOLTAGE (V)

LVIN CURRENT (µA)

123 4

3446 G11

VIN = 5V

BOTH VLDOS ON AND

REGULATING 0.8V

–45°C

27°C

90°C

130°C

VIN (V)

2.5

ENABLE/MODESEL THRESHOLD (mV)

200

400

600

33.5 4 4.5

3446 G12

800

1000

100

300

500

700

900

5.5

–45°C

27°C

90°C

130°C

LTC3446

3446ff

Typical perForMance characTerisTics

Buck PMOS Switch On-Resistance

Buck NMOS Switch On-Resistance

Buck Transient Response with

Burst Mode Defeated

Buck Transient Response with

Burst Mode Enabled

VLDO Transient Response

VLDO Rejection of Buck DC/DC

Burst Mode Ripple

TEMPERATURE (°C)

–45

PMOS RESISTANCE (mΩ)

160

180

200

30 80

3446 G13

140

120

–20 5 55 105 130

100

VIN = 2.7V

VIN = 3.6V

VIN = 4.2V

VIN = 5.5V

TEMPERATURE (°C)

–45

NMOS RESISTANCE (mΩ)

160

180

200

30 80

3446 G14

140

120

–20 5 55 105 130

100

VIN = 2.7V

VIN = 3.6V

VIN = 4.2V

VIN = 5.5V

100µs/DIV 3446 G15

BUCK OUTPUT

VOLTAGE

50mV/DIV

AC-COUPLED

LOAD

CURRENT

500mA

50mA

FRONT PAGE APPLICATION CIRCUIT

100µs/DIV 3446 G16

BUCK OUTPUT

VOLTAGE

50mV/DIV

AC-COUPLED

LOAD

CURRENT

500mA

50mA

FRONT PAGE APPLICATION CIRCUIT

100µs/DIV

LVOUT = 1.5V OR 1.2V

FRONT PAGE APPLICATION CIRCUIT

3446 G17

VLDO OUTPUT

VOLTAGE

20mV/DIV

AC-COUPLED

LOAD

CURRENT

300mA

30mA

VLVIN =

BUCK VOUT

20mV/DIV

AC-COUPLED

VLVIN = BUCK VOUT = 1.8V

VIN = 4.2V

VLVOUT = 1.5V

ILVOUT = 50mA

CLVOUT = 2.2µF

VLVOUT

10mV/DIV

AC-COUPLED

2µs/DIV 3446 G18

LTC3446

3446ff

pin FuncTions

MODESEL (Pin 1): Chooses Between Burst Mode Operation

and Pulse-Skipping Operation at Light Loads. Forcing this

pin below 0.3V allows the buck converter to automatically

enter Burst Mode operation at light loads. Forcing this pin

above 1V disallows entering Burst Mode operation; the

buck converter will cycle skip at light loads. Do not leave

this pin ﬂoating. This is a MOS gate input.

VIN (Pin 2): Input Supply to the LTC3446. Must be

closely decoupled to GND with a 10µF or greater ceramic

capacitor.

ITH (Pin 3): Buck Error Ampliﬁer Output and Servo-Loop

Compensation Point.

PGOOD (Pin 4): Supply Monitor Output, Open-Drain

NMOS.

LVOUT1 (Pin 5): Output of the First VLDO Regulator.

LVIN (Pin 6): Input Supply to the LTC3446’s VLDO Circuits.

Bypass LVIN to GND with at least a 1µF low ESR ceramic

capacitor. Typical LTC3446 application circuits will connect

this pin to the output of the buck converter but this is not

required. The VLDO regulators may be used independently

of the buck converter.

LVOUT2 (Pin 7): Output of the Second VLDO Regulator.

LVFB2 (Pin 9): Feedback Pin for the Second VLDO Regulator.

An output divider should be connected from LVOUT2 to LVFB2

to set the desired LVOUT2 regulated output voltage.

LVFB1 (Pin 10): Feedback Pin for the First VLDO Regulator.

An output divider should be connected from LVOUT1 to LVFB1

to set the desired LVOUT1 regulated output voltage.

ENLDO1/ENLDO2 (Pin 11/Pin 8): Enable Pin for the First

and Second VLDO Regulators, Respectively. Forcing this

pin above 1V enables the corresponding VLDO regulator

and forcing this pin below 0.3V shuts it down. Each VLDO

regulator draws <1µA of supply current in shutdown. Do

not leave this pin ﬂoating. This is a MOS gate input.

BUCKFB (Pin 12): Buck Converter’s Feedback Pin. Receives

the feedback voltage from an external resistive divider

across the output. External resistance from this pin to

ground should be equal to or less than 50k.

ENBUCK (Pin 13): Enable Pin for the LTC3446’s Buck

Converter Circuit. Forcing this pin above 1V enables the

buck converter and forcing this pin below 0.3V shuts down

the converter. In shutdown, the buck converter draws <1µA

of supply current. Do not leave this pin ﬂoating. This is a

MOS gate input.

SW (Pin 14): Switch Node Connection to Inductor. This

pin connects to the drains of the internal main and syn-

chronous power MOSFET switches.

GND (Exposed Pad, Pin 15): Ground. The Exposed Pad is

the only ground and must connect to the PCB ground for

electrical contact and rated thermal performance.

LTC3446

3446ff

block DiagraM

–

PMOS CURRENT

COMPARATOR

BURST

COMPARATOR

NMOS

OVERCURRENT

COMPARATOR

REVERSE

CURRENT

COMPARATOR

BBCLAMP

VIN

SLOPE

COMPENSATION

ITH

LIMIT

OSCILLATOR

POWER

GOOD

ITH

PARK

ITH

0.8V

0.4V VOLTAGE

REFERENCE

ERROR

AMPLIFIER

LOGIC

GND

EXPOSED

PAD

LVIN

LVOUT1

LVFB1

6µA

VLDO1

SOFT-START

VIN

–

LVOUT2

3446 BD

LVFB2

6µA

VLDO2

SOFT-START

VIN

PGOOD

ENBUCK

MODESEL

11 ENLDO1

8ENLDO2

12 BUCKFB

LTC3446

3446ff

The LTC3446 combines a constant frequency, current mode

synchronous buck converter with two very low dropout

(VLDO) linear DC regulators to provide up to three high

efﬁciency, low voltage outputs from a single higher voltage

input source. Each output can be independently enabled

and disabled. A power good circuit monitors all three sup-

plies. The LTC3446 incorporates an undervoltage lockout

circuit that shuts down the IC when the input voltage drops

below about 2.4V to prevent unstable operation.

SYNCHRONOUS BUCK OPERATION

A buck converter takes power from a high input voltage,

VIN, and delivers it at a lower output voltage, VOUT. The buck

converter inside the LTC3446 achieves over 80% efﬁcient

power conversion under a wide range of VIN, VOUT and load

conditions, whereas a linear regulator is limited by physics

to a maximum efﬁciency of (VOUT/VIN) × 100%.

Main Control Loop

During normal operation, the internal oscillator produces a

constant frequency 2.25MHz clock. The top power switch

(P-channel MOSFET) turns on at the beginning of a clock

cycle. Inductor current increases to a peak value which is

set by the voltage on the ITH pin. Then the top switch turns

off and the energy stored in the inductor ﬂows through

the bottom switch (N-channel MOSFET) into the load until

the next clock cycle.

The peak inductor current is controlled by the voltage on

the ITH pin, which is the output of the error ampliﬁer. This

ampliﬁer compares the BUCKFB pin to the 0.8V reference.

When the load current increases, the BUCKFB voltage de-

creases slightly below the reference. This decrease causes

the error ampliﬁer to increase the ITH voltage until the

average inductor current matches the new load current.

The main control loop is shut down by pulling the ENBUCK

pin to ground.

Overcurrent Protection

To help avert inductor current runaway in case the buck

output is accidentally shorted to ground, the LTC3446

features a bottom switch NMOS overcurrent limit, which

works as follows.

When the buck output is shorted to ground, inductor

current will rise to its maximum peak level, IMAXP, such

that on every oscillator cycle the PMOS top switch will

turn on for only its minimum duty cycle, and the bottom

switch NMOS turns on for the remainder of the cycle.

Temporarily ignoring inductor, switch and parasitic resis-

tance drops, which in most applications are designed to

be small in order to maximize buck converter efﬁciency,

it is to ﬁrst order true that when the PMOS is on, the VIN

supply voltage is placed across the inductor, increasing

the inductor current, but when the NMOS is on, there is no

output voltage to be placed across the inductor to reduce

its current. Inductor current ratchets up each cycle and

could lead to the destruction of the buck IC.

The NMOS overcurrent limit helps prevent this by sensing

the current through the NMOS bottom switch, and for as

long as this current exceeds the overcurrent limit level,

IMAXN, it:

1. Keeps the NMOS on, allowing the tiny voltage drops from

parasitic resistances to reduce the inductor current.

2. Refuses to allow the PMOS to turn on, preventing any

additional energy from being fed into the system.

Low Current Operation

The MODESEL pin controls the buck converter’s behavior at

light load currents to help optimize efﬁciency, output ripple

and noise. When the load is relatively light and MODESEL

is grounded, the buck converter automatically switches

into Burst Mode operation, which operates the PMOS

operaTion

LTC3446

3446ff

operaTion

switch intermittently based on load demand rather than

at a constant frequency. Every switch cycle during Burst

Mode operation delivers more energy than would occur

in constant frequency operation, minimizing the switch-

ing loss per unit of energy delivered. Since the dominant

power loss at light loads is gate charge switching loss in

the power MOSFETs, operating in Burst Mode operation

can dramatically improve light load efﬁciency. The tradeoff

is higher output ripple than in constant frequency opera-

tion, as well as the presence of noise below the 2.25MHz

clock frequency.

If MODESEL were instead tied to VIN, pulse skipping mode

is selected. In this mode, the buck converter continues to

switch at a constant frequency down to very light loads

where it will eventually begin skipping pulses. Because

constant frequency operation is extended down to light

loads, low output ripple is maintained and any coupled

or radiated noise is at or higher than the clock frequency.

The tradeoff is lower efﬁciency compared to Burst Mode

operation.

Dropout Operation

When the input supply voltage decreases toward the

output voltage, the duty cycle increases to 100%, which

is known as the dropout condition. In dropout, the PMOS

switch is turned on continuously with the output voltage

equal to the input voltage minus any voltage drop across

the PMOS switch and the external inductor.

VLDO LINEAR REGULATOR OPERATION

The two micropower, VLDO (very low dropout) linear

regulators in the LTC3446 operate from input voltages as

low as 0.9V. Each VLDO regulator provides a high accuracy

output that is capable of supplying 300mA of output cur-

rent with a typical dropout voltage of only 70mV. A single

ceramic capacitor as small as 1µF is all that is required

for output bypassing. A low reference voltage of 400mV

allows the VLDO regulators to be programmed to much

lower voltages than available in common LDOs.

As shown in the Block Diagram, the VIN input supplies

the internal reference and biases the VLDO circuitry while

all output current comes directly from the LVIN input for

high efﬁciency regulation. The low per-VLDO quiescent

supply currents ILVIN = 4µA, IVIN = 80µA drop to ILVIN <

2µA, IVIN < 1µA in shutdown, are well-suited to battery-

powered systems.

Each VLDO includes current limit protection. The fast

transient response of the follower output stage overcomes

the traditional tradeoff between dropout voltage, quiescent

current and load transient response inherent in most LDO

regulator architectures. Overshoot detection circuitry is

included to bring the output back into regulation when

going from heavy to light output loads (“load-dump”

handling).

POWER GOOD CIRCUIT OPERATION

The LTC3446 has a built-in supply monitor. The feedback

voltage of each enabled supply is monitored by a window

comparator to determine whether it is within 8% of its

target value. If they all are, then the PGOOD pin becomes

high impedance. If no supply is enabled, or if any enabled

supply is more than 8% away from its target, then the

PGOOD pin is driven to ground by an internal open-drain

NMOS.

The PGOOD pin may be connected through a pull-up

resistor to a supply voltage of up to 5.5V, independent of

the VIN pin voltage.

LTC3446

3446ff

applicaTions inForMaTion

A general LTC3446 application circuit is shown in Figure 1.

External component selection is driven by output voltage

and load requirements. The following text is divided into

two sections: the ﬁrst covers Buck regulator design and

the second covers use of the linear VLDO regulators.

BUCK REGULATOR DESIGN

Buck regulator design begins with the selection of the

L1 inductor based on desired ripple current. Once L1 is

chosen, CIN and COUTB can be selected based on output

voltage ripple requirements. Output voltage is programmed

through R1 and R2, and loop response can be optimized

by choice of RITH and CITH.

Inductor Selection

Although the inductor does not inﬂuence the operat-

ing frequency, the inductor value has a direct effect on

ripple current. The inductor ripple current ∆IL decreases

with higher inductance and increases with higher VIN or

VOUTB:

∆IL=VOUTB

fO•L•1−VOUTB

VIN













Accepting larger values of ∆IL allows the use of low

inductances, but results in higher output voltage ripple,

greater core losses, and lower output current capability.

A reasonable starting point for setting ripple current is

∆IL = 0.3 • IMAXP, where IMAXP is the peak switch current

limit. The largest ripple current ∆IL occurs at the maximum

input voltage. To guarantee that the ripple current stays

below a speciﬁed maximum, the inductor value should be

chosen according to the following equation:

L=VOUTB

fO•∆IL

•1−VOUTB

VIN(MAX)













The inductor value will also have an effect on Burst Mode

operation. The transition from low current operation

begins when the peak inductor current falls below a level

set by the burst clamp. Lower inductor values result in

higher ripple current which causes this to occur at lower

load currents. This causes a dip in efﬁciency in the upper

range of low current operation. In Burst Mode operation,

lower inductance values will cause the burst frequency

to increase.

Figure 1. General LTC3446 Application Circuit

VIN

BUCKFB

LVIN

LTC3446

GND

CIN

CITH

DIGITAL

CONTROL

COUTB

OPT

VOUTB

COUT1

VOUT1

VIN

2.7V TO 5.5V

LVOUT1

PGOOD

MODESEL

ENBUCK

ENLDO1

ENLDO2

LVFB1

COUT2

3446 F01

VOUT2

LVOUT2

RTH

ITH

LVFB2

LTC3446

3446ff

applicaTions inForMaTion

Inductor Core Selection

Different core materials and shapes will change the

size/current and price/current relationship of an induc-

tor. Toroid or shielded pot cores in ferrite or permalloy

materials are small and don’t radiate much energy, but

generally cost more than powdered iron core inductors

with similar electrical characteristics. The choice of which

style inductor to use often depends more on the price vs

size requirements and any radiated ﬁeld/EMI requirements

than on what the LTC3446 requires to operate. Table 1

shows some typical surface mount inductors that work

well in LTC3446 applications.

Table 1. Representative Surface Mount Inductors

MANU-

FACTURER

PART NUMBER

VALUE

MAX DC

CURRENT

DCR

HEIGHT

Toko A914BYW-2R2M-

D52LC

2.2µH 2.05A 49mΩ 2mm

Toko A915AY-2ROM-

D53LC

2µH 3.3A 22mΩ 3mm

Coilcraft D01608C-222 2.2µH 2.3A 70mΩ 3mm

Coilcraft LP01704-222M 2.2µH 2.4A 120mΩ 1mm

Sumida CDRH4D282R2 2.2µH 2.04A 23mΩ 3mm

Sumida CDC5D232R2 2.2µH 2.16A 30mΩ 2.5mm

Taiyo

Yuden

N06DB2R2M 2.2µH 3.2A 29mΩ 3.2mm

Taiyo

Yuden

N05DB2R2M 2.2µH 2.9A 32mΩ 2.8mm

Murata LQN6C2R2M04 2.2µH 3.2A 24mΩ 5mm

Würth 744042001 1µH 2.6A 20mΩ 2mm

Catch Diode Selection

Although unnecessary in most applications, a small

improvement in efﬁciency can be obtained in a few ap-

plications by including the optional diode D1 shown in

Figure 1, which conducts when the synchronous switch

is off. When using Burst Mode operation or pulse skip

mode, the synchronous switch is turned off at a low current

and the remaining current will be carried by the optional

diode. It is important to adequately specify the diode peak

current and average power dissipation so as not to exceed

the diode ratings. The main problem with Schottky diodes

is that their parasitic capacitance reduces the efﬁciency,

usually negating the possible beneﬁts for LTC3446 circuits.

Another problem that a Schottky diode can introduce is

higher leakage current at high temperatures, which could

reduce the low current efﬁciency.

Remember to keep lead lengths short and observe proper

grounding to avoid ringing and increased dissipation when

using a catch diode.

Input Capacitor (CIN) Selection

In continuous mode, the input current of the converter is a

square wave with a duty cycle of approximately VOUTB/VIN.

To prevent large voltage transients, a low equivalent series

resistance (ESR) input capacitor sized for the maximum

RMS current must be used. The maximum RMS capacitor

current is given by:

IRMS ≈IMAX

VOUTB(V

IN −VOUTB)

where the maximum average output current IMAX equals

the peak current minus half the peak-to-peak ripple cur-

rent, IMAX = IMAXP – ∆IL/2.

This formula has a maximum at VIN = 2VOUTB, where IRMS

= IOUT/2. This simple worst case is commonly used to

design because even signiﬁcant deviations do not offer

much relief. Note that capacitor manufacturer’s ripple cur-

rent ratings are often based on only 2000 hours lifetime.

This makes it advisable to further derate the capacitor,

or choose a capacitor rated at a higher temperature than

required. Several capacitors may also be paralleled to meet

the size or height requirements of the design. An additional

0.1µF to 1µF ceramic capacitor is also recommended on

VIN for high frequency decoupling, when not using an all

ceramic capacitor solution.

LTC3446

3446ff

applicaTions inForMaTion

Output Capacitor (COUTB) Selection

The selection of COUTB is driven by the required ESR to

minimize voltage ripple and load step transients. Typically,

once the ESR requirement is satisﬁed, the capacitance

is adequate for ﬁltering. The output ripple (∆VOUTB) is

determined by:

∆VOUTB ≈ ∆ILESR +1

8fOCOUTB













where f = 2.25MHz, COUTB = output capacitance and

∆IL = ripple current in the inductor. The output ripple is

highest at maximum input voltage since ∆IL increases

with input voltage.

Once the ESR requirements for COUTB have been met, the

RMS current rating generally far exceeds the IRIPPLE(P-P)

requirement, except for an all ceramic solution.

In surface mount applications, multiple capacitors may

have to be paralleled to meet the capacitance, ESR or RMS

current handling requirement of the application. Aluminum

electrolytic, special polymer, ceramic and dry tantulum

capacitors are all available in surface mount packages. The

OS-CON semiconductor dielectric capacitor available from

Sanyo has the lowest ESR(size) product of any aluminum

electrolytic at a somewhat higher price. Special polymer

capacitors, such as Sanyo POSCAP, offer very low ESR,

but have a lower capacitance density than other types.

Tantalum capacitors have the highest capacitance density,

but have a larger ESR and it is critical that the capacitors

are surge tested for use in switching power supplies.

An excellent choice is the AVX TPS series of surface

mount tantalums, avalable in case heights ranging from

2mm to 4mm. Aluminum electrolytic capacitors have a

signiﬁcantly larger ESR, and are often used in extremely

cost-sensitive applications provided that consideration is

given to ripple current ratings and long-term reliability.

Ceramic capacitors have the lowest ESR and cost but also

have the lowest capacitance density, a high voltage and

temperature coefﬁcient and exhibit audible piezoelectric

effects. In addition, the high Q of ceramic capacitors along

with trace inductance can lead to signiﬁcant ringing. Other

capacitor types include the Panasonic specialty polymer

(SP) capacitors.

In most cases, 0.1µF to 1µF of ceramic capacitors should

also be placed close to the LTC3446 in parallel with the

main capacitors for high frequency decoupling.

Ceramic Input and Output Capacitors

Higher value, lower cost ceramic capacitors are now be-

coming available in smaller case sizes. These are tempting

for switching regulator use because of their very low ESR.

Unfortunately, the ESR is so low that it can cause loop

stability problems. Solid tantalum capacitor ESR gener-

ates a loop “zero” at 5kHz to 50kHz that is instrumental in

giving acceptable loop phase margin. Ceramic capacitors

remain capacitive to beyond 300kHz and usually resonate

with their ESL before ESR becomes effective. Also, ceramic

caps are prone to temperature effects which requires the

designer to check loop stability over the operating tem-

perature range. To minimize their large temperature and

voltage coefﬁcients, only X5R or X7R ceramic capacitors

should be used. A good selection of ceramic capacitors

is available from Taiyo Yuden, TDK and Murata.

Great care must be taken when using only ceramic input

and output capacitors. When a ceramic capacitor is used

at the input and the power is being supplied through long

wires, such as from a wall adapter, a load step at the output

can induce ringing at the VIN pin. At best, this ringing can

couple to the output and be mistaken as loop instability.

At worst, the ringing at the input can be large enough to

damage the part.

Since the ESR of a ceramic capacitor is so low, the input

and output capacitor must instead fulﬁll a charge storage

requirement. During a load step, the output capacitor must

instantaneously supply the current to support the load

until the feedback loop raises the switch current enough

LTC3446

3446ff

applicaTions inForMaTion

to support the load. The time required for the feedback

loop to respond is dependent on the compensation com-

ponents and the output capacitor size. Typically, 3 to 4

cycles are required to respond to a load step, but only in

the ﬁrst cycle does the output drop linearly. The output

droop, VDROOP, is usually about 2 to 3 times the linear

drop of the ﬁrst cycle. Thus, a good place to start is with

the output capacitor size of approximately:

COUTB ≈2.5

∆I

OUT

fO•VDROOP

More capacitance may be required depending on the duty

cycle and load step requirements.

In most applications, the input capacitor is merely required

to supply high frequency bypassing, since the impedance

to the supply is very low. A 10µF ceramic capacitor is

usually enough for these conditions.

Setting the Buck Converter’s Output Voltage

The buck develops a 0.8V reference voltage between the

feedback pin, BUCKFB, and the signal ground as shown

in Figure 1. The output voltage is set by a resistive divider

according to the following formula:

VOUTB ≈0.8V 1+R2













Keep R1 at or less than 50k. Great care should be taken

to route the BUCKFB line away from noise sources, such

as the inductor or the SW line.

To improve high frequency loop response, a feed forward

capacitor, CF, can be added as shown in Figure 1. Capacitor

CF provides phase lead by creating a high frequency zero

with R2, improving phase margin.

Buck Converter Shutdown

The ENBUCK pin enables and shuts down the LTC3446’s

buck converter. Do not leave this pin ﬂoating! Tying

ENBUCK to ground disables the buck converter. Bringing

ENBUCK more than 1V above ground enables the buck.

Checking Buck Converter Transient Response

The OPTI-LOOP compensation allows the transient re-

sponse to be optimized for a wide range of loads and

output capacitors. The availability of the ITH pin not only

allows optimization of the control loop behavior but also

provides a DC coupled and AC ﬁltered closed-loop response

test point. The DC step, rise time and settling at this test

point truly reﬂects 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 the front page

Typical Application circuit will provide an adequate starting

point for most applications. The series R-C ﬁlter sets the

dominant pole-zero loop compensation. The values can

be modiﬁed slightly (from 0.5 to 2 times their suggested

values) to optimize transient response once the ﬁnal 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 feedback factor gain and phase. An output current

pulse of 20% to 100% 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.

Switching regulators take several cycles to respond to a

step in load current. When a load step occurs, VOUTB im-

mediately shifts by an amount equal to ∆ILOAD • ESR, where

ESR is the effective series resistance of COUT. ∆ILOAD also

begins to charge or discharge COUTB generating a feedback

error signal used by the regulator to return VOUTB to its

steady-state value. During this recovery time, VOUTB can

be monitored for overshoot or ringing that would indicate

a stability problem.

The initial output voltage step may not be within the

bandwidth of the feedback loop, so the standard second

order overshoot/DC ratio cannot be used to determine

phase margin. The gain of the loop increases with R and

the bandwidth of the loop increases with decreasing C.

LTC3446

3446ff

applicaTions inForMaTion

If R is increased by the same factor that C is decreased,

the zero frequency will be kept the same, thereby keeping

the phase 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. For a detailed explanation of

optimizing the compensation components, including a

review of control loop theory, refer to Linear Technology

Application Note 76.

Although a buck regulator is capable of providing the full

output current in dropout, it should be noted that as the

input voltage VIN drops toward VOUT, the load step capability

does decrease due to the decreasing voltage across the

inductor. Applications that require large load step capabil-

ity near dropout should use a different topology such as

SEPIC, Zeta or single inductor, positive buck/boost.

In some applications, a more severe transient can be caused

by switching in loads with large (>1µF) input capacitors.

The discharged input capacitors are effectively put in paral-

lel with COUTB, causing a rapid drop in VOUT. No regulator

can deliver enough current to prevent this problem, if the

switch connecting the load has low resistance and is driven

quickly. The solution is to limit the turn-on speed of the load

switch driver. A Hot Swap controller is designed speciﬁcally

for this purpose and usually incorporates current limiting,

short-circuit protection, and soft-starting.

Efﬁciency Considerations

The percent efﬁciency 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 efﬁciency and which change would

produce the most improvement. Percent efﬁciency can

be expressed as:

%Efﬁciency = 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 LTC3446 circuits: 1) LTC3446 VIN current,

2) switching losses, 3) I2R losses, 4) other losses.

1) The VIN current is the DC supply current given in the

electrical characteristics which excludes MOSFET driver

and control currents. VIN current results in a small

(<0.1%) loss that increases with VIN, even at no load.

2) The switching current is the sum of the MOSFET driver

and control currents. The MOSFET driver current re-

sults 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 VIN to ground. The resulting dQ/dt is a current

out of VIN that is typically much larger than the DC bias

current. In continuous mode, IGATECHG = fO(QT + QB),

where QT and QB are the gate charges of the internal

top and bottom MOSFET switches. The gate charge

losses are proportional to VIN and thus their effects

will be more pronounced at higher supply voltages.

3) I2R Losses are calculated from the DC resistances of

the internal switches, RSW, and external inductor, RL. In

continuous mode, the average output current ﬂowing

through inductor L is “chopped” between the internal

top and bottom switches. Thus, the series resistance

looking into the SW pin is a function of both top and

bottom MOSFET RDS(ON) and the duty cycle (DC) as

follows:

RSW = (RDS(ON)TOP)(DC) + (RDS(ON)BOT)(1 – DC)

4) Other “hidden” losses such as copper trace and internal

battery resistances can account for additional efﬁciency

degradations in portable systems. It is very important

to include these “system” level losses in the design of a

system. 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 fre-

quency. Other losses including diode conduction losses

during dead-time and inductor core losses generally

account for less than 2% total additional loss.

LTC3446

3446ff

applicaTions inForMaTion

VLDO LINEAR REGULATOR DESIGN

Adjustable Output Voltage

Each VLDO regulator’s output voltage is set by the ratio

of two external resistors as shown in Figure 2. The VLDO

regulator servos the output to maintain the LVFB pin voltage

at 0.4V (referenced to ground). Thus the current in R1 is

equal to 0.4V/R1. For good transient response, stability and

accuracy, the current in R1 should be at least 8µA, thus

the value of R1 should be no greater than 50k. The current

in R2 is the current in R1 plus the LVFB pin bias current.

Since the LVFB pin bias current is typically <10nA, it can

be ignored in the output voltage calculation. The output

voltage can be calculated using the formula in Figure 2.

Note that in shutdown, the output is turned off and the

divider current will be zero once COUT is discharged.

Each VLDO regulator operates at a relatively high gain of

–0.7µV/mA referred to its LVFB input. Thus a load current

change of 1mA to 300mA produces a –0.2mV drop at the

LVFB input. To calculate the change referred to the output,

simply multiply by the gain of the feedback network (i.e.,

1 + R2/R1). For example, to program the output for 1.2V,

choose R2/R1 = 2. In this example, an output current

change of 1mA to 300mA produces –0.2mV • (1 + 2) =

0.6mV drop at the output.

Because the LVFB pins are relatively high impedance (de-

pending on the resistor dividers used), stray capacitance

at these pins should be minimized (<10pF) to prevent

phase shift in the error ampliﬁer loop. Additionally, special

attention should be given to any stray capacitances that

can couple external signals onto the LVFB pins producing

undesirable output ripple. For optimum performance,

connect each LVFB pin to its resistor divider with a short

PCB trace and minimize all other stray capacitance to the

LVFB pin.

VLDO Regulator Output Capacitance and Transient

Response

The VLDO regulators are designed to be stable with a

wide range of ceramic output capacitors. The ESR of the

output capacitor affects stability, most notably with small

capacitors. A minimum output capacitor of 1µF with an

ESR of 0.05Ω or less is recommended to ensure stability.

The VLDO regulators are micropower devices and output

transient response will be a function of output capacitance.

Larger values of output capacitance decrease the peak

deviations and provide improved transient response for

larger load current changes. Note that bypass capacitors

used to decouple individual components powered by a

VLDO regulator will increase the effective output capaci-

tor value. High ESR tantalum and electrolytic capacitors

may be used, but a low ESR ceramic capacitor must be

in parallel at the output. There is no minimum ESR or

maximum capacitor size requirements.

Extra consideration must be given to the use of ceramic

capacitors. Ceramic capacitors are manufactured with a

variety of dielectrics, each with different behavior across

temperature and applied voltage. The most common di-

electrics used are Z5U, Y5V, X5R and X7R. The Z5U and

Y5V dielectrics are good for providing high capacitances

in a small package, but exhibit large voltage and tem-

perature coefﬁcients as shown in Figures 3 and 4. When

used with a 2V regulator, a 1µF Y5V capacitor can lose as

much as 75% of its intial capacitance over the operating

temperature range. The X5R and X7R dielectrics result

Figure 2. Programming a VLDO Regulator’s Output Voltage

COUT

3446 F02

VOUT = 0.4V 1+ R2

( )

LVOUT

LVFB

GND

LTC3446

3446ff

applicaTions inForMaTion

in more stable characteristics and are usually more suit-

able for use as the output capacitor. The X7R type has

better stability across temperature, while the X5R is less

expensive and is available in higher values. In all cases,

the output capacitance should never drop below 0.4µF, or

instability or degraded performance may occur.

VLDO Output Short-Circuit Protection

The VLDO regulators have built-in short-circuit limiting.

During short-circuit conditions, internal circuitry automati-

cally limits the output current to approximately 760mA.

VLDO Regulator Soft-Start

Each VLDO regulator includes a soft-start feature to

prevent excessive current ﬂow during start-up. When

the VLDO regulator is enabled, the soft-start circuitry

gradually increases the VLDO regulator reference voltage

from 0V to 0.4V over a period of about 600µs. There is a

short 700µs delay from the time the part is enabled until

the LDO output starts to rise. Figure 5 shows the start-up

output waveform.

Figure 3. Ceramic Capacitor DC Bias Characteristics Figure 4. Ceramic Capacitor Temperature Characteristics

Figure 5. VLDO Regulator Output Start-Up

DC BIAS VOLTAGE (V)

CHANGE IN VALUE (%)

3446 F03

–20

–40

–60

–80

–100 04810

2 6

X5R

Y5V

BOTH CAPACITORS ARE 1µF,

10V, 0603 CASE SIZE

TEMPERATURE (°C)

–50

–100

CHANGE IN VALUE (%)

–80

–60

–40

–20

X5R

Y5V

–25 0 25 50

3446 F04

BOTH CAPACITORS ARE 1µF,

10V, 0603 CASE SIZE

1.5V VLDO OUTPUT

1.2V VLDO OUTPUT

0.5V/DIV

BOTH VLDO

ENABLES

5V/DIV

VLVIN = 1.8V

VIN = 3.6V

FRONT PAGE APPLICATION CIRCUIT

WITH 10mA RESISTOR LOADS ON

EACH VLDO OUTPUT

200µs/DIV 3446 F05

LTC3446

3446ff

package DescripTion

DE Package

14-Lead Plastic DFN (4mm × 3mm)

(Reference LTC DWG # 05-08-1708 Rev B)

3.00 ±0.10

(2 SIDES)

4.00 ±0.10

(2 SIDES)

NOTE:

1. DRAWING PROPOSED TO BE MADE VARIATION OF VERSION (WGED-3) IN JEDEC

PACKAGE OUTLINE MO-229

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

0.40 ± 0.10

BOTTOM VIEW—EXPOSED PAD

1.70 ± 0.10

0.75 ±0.05

R = 0.115

TYP

R = 0.05

TYP

3.00 REF

1.70 ± 0.05

148

PIN 1

TOP MARK

(SEE NOTE 6)

0.200 REF

0.00 – 0.05

(DE14) DFN 0806 REV B

PIN 1 NOTCH

R = 0.20 OR

0.35 × 45°

CHAMFER

3.00 REF

RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS

APPLY SOLDER MASK TO AREAS THAT ARE NOT SOLDERED

2.20 ±0.05

0.70 ±0.05

3.60 ±0.05

PACKAGE

OUTLINE

0.25 ± 0.05

0.50 BSC

3.30 ±0.05

3.30 ±0.10

0.50 BSC

LTC3446

3446ff

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

E 5/11 Updated E-grade Temperature Grade to 125°C, Storage Maximum Temperature to 150°C and θJA to 43°C/W.

Updated PGOOD Output Resistance Maximum Limit.

Added VIN to LVOUT Headroom Speciﬁcation.

Updated Note 2.

F 5/11 Updated Parameter on VIN to LVOUT speciﬁcation 3

(Revision history begins at Rev E)

LTC3446

3446ff

Linear Technology Corporation

1630 McCarthy Blvd., Milpitas, CA 95035-7417

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

 LINEAR TECHNOLOGY CORPORATION 2006

LT 0511 REV F • PRINTED IN USA

relaTeD parTs

Typical applicaTion

PART NUMBER DESCRIPTION COMMENTS

LT3023 Dual, 2x100mA,

Low Noise Micropower LDO

VIN: 1.8V to 20V, VOUT(MIN) = 1.22V, VDO = 0.30V, IQ = 40µA, ISD < 1µA,

VOUT = ADJ, DFN, MS Packages, Low Noise < 20µVRMS(P-P), Stable with

1µF Ceramic Capacitors

LT3024 Dual, 100mA/500mA,

Low Noise Micropower LDO

VIN: 1.8V to 20V, VOUT(MIN) = 1.22V, VDO = 0.30V, IQ = 60µA, ISD < 1µA,

VOUT = ADJ, DFN, TSSOP Packages, Low Noise < 20µVRMS(P-P), Stable with

1µF Ceramic Capacitors

LTC3025 300mA, Micropower VLDO

Linear Regulator

VIN: 0.9V to 5.5V, VOUT(MIN) = 0.4V, 2.7V to 5.5V Bias Voltage Required,

VDO = 45mV, IQ = 50µA, ISD < 1µA, VOUT = ADJ, DFN Packages, Stable with

1µF Ceramic Capacitors

LTC3407 Dual Synchronous 600mA

Synchronous Step-Down DC/DC Regulator

1.5MHz Constant Frequency Current Mode Operation, VIN from 2.5V to

5.5V, VOUT Down to 0.6V, DFN, MS Packages

LTC3407-2 Dual Synchronous 800mA Synchronous

Step-Down DC/DC Regulator, 2.25MHz

2.25MHz Constant Frequency Current Mode Operation, VIN from 2.5V to

5.5V, VOUT Down to 0.6V, DFN, MS Packages

LTC3445 I2C Controllable Buck Regulator

with Two LDOs and and

Backup Battery Input

600mA, 1.5MHz Current Mode Buck Regulator, I2C Programmable VOUT

from 0.85V to 1.55V, two 50mA LDOs, Backup Battery Input with

PowerPath™ Control, QFN Package

LTC3555 High Efﬁciency USB Power Manager Plus

Triple Step-Down DC/DC

Maximizes Available Power from USB Port, Bat-Track™, “Instant On” Operation,

1.5A Maximum Charge Current, 180mΩ Ideal Diode with <50Ω Option, 3.3V/25mA

Always-On LDO, Three Synchronous Buck Regulators

(400mA/400mA/1A), 4mm × 5mm QFN28 Package

LTC3557 USB Power Manager with Li-Ion/Polymer

Charger and Triple Synchrounous

Buck Converter

Complete Multifunction ASSP: Linear Power Manager and Three Buck Regulators,

Charge Current Programmable Up to 1.5A from Wall Adapter Input,

Thermal Regulation, Synchronous Buck Efﬁciency: >95%, ADJ Outputs: 0.8V to 3.6V

at 400mA/400mA/600mA, Bat-Track Adaptive Output Control,

200mΩ Ideal Diode, 4mm × 4mm QFN28 Package

LTC3559 Linear USB Li-Ion Battery Charger with

Dual Buck Regulators

Charge Current Programmable Up to 950mA, USB Compatible, Dual Synchronous

400mA Buck Regulators, Efﬁciency >90%, 3mm × 3mm QFN16 Package

LTC3672B-1/

LTC3672B-2

Fixed-Output Monolithic 400mA Buck

Regulator with Dual 150mA LDOs in

a 2mm × 2mm DFN

>90% Efﬁciency, VIN: 2.9V to 5.5V, IQ = 260µA,

LTC3672B-1: Buck = 1.8V, LDO1 = 1.2V, LDO2 = 2.8V

LTC3672B-2: Buck = 1.2V, LDO1 = 2.8V, LDO2 = 1.8V

Consult LTC Factory for Other Voltage Combinations

LTC3700 Step-Down DC/DC Controller with

LDO Regulator

VIN from 2.65V to 9.8V, Constant Frequency 550kHz Operation

VIN

59k

1.8µH

47.5k

BUCKFB

LVIN

LTC3446

GND

22µF

X7R

1000pF

DIGITAL

CONTROL

22µF

X7R

VOUT

1.8V

400mA MAX

110k

40.2k

2.2µF

X7R

VOUT

1.5V

300mA MAX

VIN

2.9V TO 5.5V

LVOUT1

PGOOD

MODESEL

ENBUCK

ENLDO1

ENLDO2

LVFB1

80.6k

40.2k

2.2µF

X7R

L1: TOKO A960AW-1R8M

3446 TA02

VOUT

1.2V

300mA MAX

LVOUT2

3.3k

ITH

LVFB2