LT3470ITS8#PBF - LINEAR TECHNOLOGY

LT3470

3470fd

Typical applicaTion

FeaTures

applicaTions

DescripTion

Micropower Buck Regulator

with Integrated Boost and

Catch Diodes

The LT

3470 is a micropower step-down DC/DC con-

verter that integrates a 300mA power switch, catch diode

and boost diode into low profile 3mm × 2mm DD and

ThinSOT™ packages. The LT3470 combines Burst Mode

and continuous operation to allow the use of tiny induc-

tor and capacitors while providing a low ripple output to

loads of up to 200mA.

With its wide input range of 4V to 40V, the LT3470 can

regulate a wide variety of power sources, from 2-cell Li-Ion

batteries to unregulated wall transformers and lead-acid

batteries. Quiescent current in regulation is just 26µA in

a typical application while a zero current shutdown mode

disconnects the load from the input source, simplifying

power management in battery-powered systems. Fast

current limiting and hysteretic control protects the LT3470

and external components against shorted outputs, even

at 40V input.

L, LT, LTC, LTM, Linear Technology, the Linear logo and Burst Mode are registered trademarks

and ThinSOT is a trademark of Linear Technology Corporation. All other trademarks are the

property of their respective owners.

Efficiency and Power Loss vs Load Current

n Low Quiescent Current: 26µA at 12VIN to 3.3VOUT

Integrated Boost and Catch Diodes

Input Range: 4V to 40V

n Low Output Ripple: <10mV

n <1µA in Shutdown Mode

Output Voltage: 1.25V to 16V

200mA Output Current

Hysteretic Mode Control

– Low Ripple Burst Mode

Operation at Light Loads

– Continuous Operation at Higher Loads

Solution Size as Small as 50mm2

n Low Profile (0.75mm) 3mm × 2mm Thermally

Enhanced 8-Lead DD and 1mm ThinSOT Packages

n Automotive Battery Regulation

Power for Portable Products

Distributed Supply Regulation

Industrial Supplies

n Wall Transformer Regulation

LOAD CURRENT (mA)

EFFICIENCY (%)

POWER LOSS (mW)

0.1 10 100

3470 TA02

1000

100

0.1

VIN = 12V

VIN BOOST

LT3470

SWSHDN

0.22µF

22pF

22µF

2.2µF

3470 TA01a

VIN

7V TO 40V

VOUT

200mA

604k

200k

33µH

BIAS

GND

OFF ON

LT3470

3470fd

absoluTe MaxiMuM raTings

VIN, SHDN Voltage ................................................... 40V

BOOST Pin Voltage .................................................. 47V

BOOST Pin Above SW Pin ........................................ 25V

FB Voltage .................................................................. 5V

BIAS Voltage .............................................................25V

SW Voltage ................................................................VIN

Maximum Junction Temperature

LT3470E, LT3470I ............................................. 125°C

LT3470H ........................................................... 150°C

(Note 1)

orDer inForMaTion

Operating Temperature Range (Note 2)

LT3470E ............................................... – 40°C to 85°C

LT3470I ............................................. –40°C to 125°C

LT3470H ............................................ –40°C to 150°C

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

Lead Temperature (Soldering, 10 sec) ..................300°C

TOP VIEW

DDB8 PACKAGE

8-LEAD (3mm × 2mm) PLASTIC DFN

1FB

BIAS

BOOST

SHDN

VIN

GND

θJA = 180°C/W

EXPOSED PAD (PIN 9) IS GROUND (MUST BE SOLDERED TO PCB)

SHDN 1

NC 2

VIN 3

GND 4

8 FB

7 BIAS

6 BOOST

5 SW

TOP VIEW

TS8 PACKAGE

8-LEAD PLASTIC TSOT-23

θJA = 140°C/W

pin conFiguraTion

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

LT3470EDDB#PBF LT3470EDDB#TRPBF LBPN 8-Lead (3mm × 2mm) Plastic DFN –40°C to 85°C

LT3470IDDB#PBF LT3470IDDB#TRPBF LBPP 8-Lead (3mm × 2mm) Plastic DFN –40°C to 125°C

LT3470HDDB#PBF LT3470HDDB#TRPBF LCNR 8-Lead (3mm × 2mm) Plastic DFN –40°C to 150°C

LT3470ETS8#PBF LT3470ETS8#TRPBF LTBDM 8-Lead Plastic TSOT-23 –40°C to 85°C

LT3470ITS8#PBF LT3470ITS8#TRPBF LTBPW 8-Lead Plastic TSOT-23 –40°C to 125°C

LT3470HTS8#PBF LT3470HTS8#TRPBF LTCNQ 8-Lead Plastic TSOT-23 –40°C to 150°C

LEAD BASED FINISH TAPE AND REEL PART MARKING PACKAGE DESCRIPTION TEMPERATURE RANGE

LT3470EDDB LT3470EDDB#TR LBPN 8-Lead (3mm × 2mm) Plastic DFN –40°C to 85°C

LT3470IDDB LT3470IDDB#TR LBPP 8-Lead (3mm × 2mm) Plastic DFN –40°C to 125°C

LT3470HDDB LT3470HDDB#TR LCNR 8-Lead (3mm × 2mm) Plastic DFN –40°C to 150°C

LT3470ETS8 LT3470ETS8#TR LTBDM 8-Lead Plastic TSOT-23 –40°C to 85°C

LT3470ITS8 LT3470ITS8#TR LTBPW 8-Lead Plastic TSOT-23 –40°C to 125°C

LT3470HTS8 LT3470HTS8#TR LTCNQ 8-Lead Plastic TSOT-23 –40°C to 150°C

Consult LTC Marketing for parts specified with wider operating temperature ranges.

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/

LT3470

3470fd

The l denotes the specifications which apply over the full operating

temperature range, otherwise specifications are at TA = 25°C. VIN = 10V, VSHDN = 10V, VBOOST = 15V, VBIAS = 3V unless otherwise specified.

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 LT3470E is guaranteed to meet performance specifications

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

temperature range are assured by design, characterization and

correlation with statistical process controls. The LT3470I specifications

are guaranteed over the –40°C to 125°C temperature range. LT3470H

specifications are guaranteed over –40°C to 150°C temperature range.

Note 3: Bias current flows out of the FB pin.

Note 4: This is the minimum voltage across the boost capacitor needed to

guarantee full saturation of the switch.

Note 5: This parameter is assured by design and correlation with statistical

process controls.

elecTrical characTerisTics

PARAMETER CONDITIONS MIN TYP MAX UNITS

Minimum Input Voltage ●4 V

Quiescent Current from VIN VSHDN = 0.2V

VBIAS = 3V, Not Switching

VBIAS = 0V, Not Switching

●

0.1

0.5

µA

Quiescent Current from Bias VSHDN = 0.2V

VBIAS = 3V, Not Switching

VBIAS = 0V, Not Switching

●

0.1

0.5

1.5

µA

FB Comparator Trip Voltage VFB Falling ●1.228 1.250 1.265 V

FB Pin Bias Current (Note 3) VFB = 1V, E- and I-Grade

●

150

H-Grade ●35 225 nA

FB Voltage Line Regulation 4V < VIN < 40V 0.0006 0.01 %/V

Minimum Switch Off-Time (Note 5) 500 ns

Switch Leakage Current 0.7 1.5 µA

Switch VCESAT ISW = 100mA (TS8 Package)

ISW = 100mA (DD8 Package)

215

300 mV

Switch Top Current Limit VFB = 0V 250 325 435 mA

Switch Bottom Current Limit VFB = 0V 225 mA

Catch Schottky Drop ISH = 100mA (TS8 Package)

ISH = 100mA (DD8 Package)

630

775 mV

Catch Schottky Reverse Leakage VSW = 10V 0.2 2 µA

Boost Schottky Drop ISH = 30mA 650 775 mV

Boost Schottky Reverse Leakage VSW = 10V, VBIAS = 0V 0.2 2 µA

Minimum Boost Voltage (Note 4) ●1.7 2.2 V

BOOST Pin Current ISW = 100mA 7 12 mA

SHDN Pin Current VSHDN = 2.5V 1 5 µA

SHDN Input Voltage High 2.5 V

SHDN Input Voltage Low 0.2 V

LT3470

3470fd

Typical perForMance characTerisTics

Efficiency, VOUT = 3.3V

Efficiency, VOUT = 5V

VFB vs Temperature

Top and Bottom Switch Current

Limits (VFB = 0V) vs Temperature

VIN Quiescent Current

vs Temperature

BIAS Quiescent Current

(Bias > 3V) vs Temperature

SHDN Bias Current

vs Temperature

FB Bias Current (VFB = 1V)

vs Temperature

LOAD CURRENT (mA)

EFFICIENCY (%)

0.1 10 100

3470 G01

L = TOKO D52LC 47µH

TA = 25°C VIN = 7V

VIN = 12V

VIN = 36V

VIN = 24V

LOAD CURRENT (mA)

EFFICIENCY (%)

0.1 10 100

3470 G02

L = TOKO D52LC 47µH

TA = 25°C

VIN = 12V

VIN = 36V

VIN = 24V

TEMPERATURE (°C)

–50

1.240

VFB (V)

1.245

1.250

1.255

1.260

–25 0 25 50

3470 G03

75 100 125

TEMPERATURE (°C)

–50

CURRENT LIMIT (mA)

350

3470 G04

200

100

–25 0 50

400

300

250

150

75 100 150125

TEMPERATURE (°C)

–50 –25

VIN CURRENT (µA)

050 75

3470 G05

25 100 150125

BIAS < 3V

BIAS > 3V

TEMPERATURE (°C)

–50

BIAS CURRENT (µA)

25 75

3470 G06

–25 0 50 100 150125

TEMPERATURE (°C)

–50

SHDN CURRENT (µA)

3470 G07

–25 75 100

25 150125

8VSHDN = 36V

VSHDN = 2.5V

TEMPERATURE (°C)

–50 –25

FB CURRENT (nA)

050 75

3470 G08

25 100 150125

LT3470

3470fd

Typical perForMance characTerisTics

FB Bias Current (VFB = 0V)

vs Temperature

Switch VCESAT (ISW = 100mA)

vs Temperature

Boost Diode VF (IF = 50mA)

vs Temperature

Catch Diode VF (IF = 100mA)

vs Temperature

Diode Leakage (VR = 36V)

vs Temperature

Switch VCESAT

BOOST Pin Current

Catch Diode Forward Voltage

TEMPERATURE (°C)

–50

FB CURRENT (µA)

100

120

25 75

3470 G09

–25 0 50 100 150125

TEMPERATURE (°C)

–50

SWITCH VCESAT (mV)

200

250

300

25 75

3470 G10

150

100

–25 0 50 100 150125

TEMPERATURE (°C)

–50

SCHOTTKY VF (V)

0.7

3470 G11

0.4

0.2

–25 0 50

0.1

0.8

0.6

0.5

0.3

75 100 150125

TEMPERATURE (°C)

–50

0.4

0.5

0.7

25 75

3470 G12

0.3

0.2

–25 0 50 100 150125

0.1

0.6

SCHOTTKY VF (V)

TEMPERATURE (°C)

–50 –25

SCHOTTKY DIODE LEAKAGE (µA)

050 75

3470 G13

25 100 150125

CATCH

BOOST

SWITCH CURRENT (mA)

400

500

700

300

3470 G14

300

200

100 200 400

100

600

SWITCH VCESAT (mV)

SWITCH CURRENT (mA)

300

3470 G15

100 200 400

BOOST PIN CURRENT (mA)

CATCH DIODE CURRENT (mA)

SCHOTTKY VF (V)

0.4

0.6

400

3470 G16

0.2

0100 200 300

1.0

0.8

LT3470

3470fd

SHDN (Pin 1/Pin 8): The SHDN pin is used to put the

LT3470 in shutdown mode. Tie to ground to shut down

the LT3470. Apply 2V or more for normal operation. If the

shutdown feature is not used, tie this pin to the VIN pin.

NC (Pin 2/Pin 7): This pin can be left floating or connected

to VIN.

VIN (Pin 3/Pin 6): The VIN pin supplies current to the

LT3470’s internal regulator and to the internal power

switch. This pin must be locally bypassed.

GND (Pin 4/Pin 5): Tie the GND pin to a local ground plane

below the LT3470 and the circuit components. Return the

feedback divider to this pin.

SW (Pin 5/Pin 4): The SW pin is the output of the internal

power switch. Connect this pin to the inductor, catch diode

and boost capacitor.

BOOST (Pin 6/Pin 3): The BOOST pin is used to provide

a drive voltage, which is higher than the input voltage, to

the internal bipolar NPN power switch.

BIAS (Pin 7

/Pin 2):

The BIAS pin connects to the internal

boost Schottky diode and to the internal regulator. Tie to

VOUT when VOUT > 2V or to VIN otherwise. When VBIAS >

3V the BIAS pin will supply current to the internal regulator.

FB (Pin 8

/Pin 1):

The LT3470 regulates its feedback pin

to 1.25V. Connect the feedback resistor divider tap to this

pin. Set the output voltage according to VOUT = 1.25V

(1 + R1/R2) or R1 = R2 (VOUT/1.25 – 1).

Exposed Pad (

DD, Pin 9):

Ground. Must be soldered to

PCB.

Boost Diode Forward Voltage

Minimum Input Voltage, VOUT = 3.3V

Minimum Input Voltage, VOUT = 5V

Typical perForMance characTerisTics

BOOST DIODE CURRENT (mA)

SCHOTTKY VF (V)

500

600

700

200

3470 G17

400

300

050 100 150

100

200

900

800

LOAD CURRENT (mA)

3.0

INPUT VOLTAGE (V)

3.5

4.0

4.5

5.0

5.5

6.0

50 100 150 200

3470 G18

TA = 25°C VIN TO START

VIN TO RUN

LOAD CURRENT (mA)

INPUT VOLTAGE (V)

200

3470 G19

450 100 150

8TA = 25°C

VIN TO START

VIN TO RUN

pin FuncTions

(ThinSOT/DD)

LT3470

3470fd

block DiagraM

–

R Q′

S Q

500ns

ONE SHOT

VREF

1.25V

Burst Mode

DETECT

GND

3470 BD

R2 R1

SHDN

ENABLE

VIN

BIAS

BOOST

VOUT

LT3470

3470fd

Figure 1. Operating Waveforms of the LT3470 Converting 12V to 5V Using a 33µH Inductor and 10µF Output Capacitor

The LT3470 uses a hysteretic control scheme in conjunction

with Burst Mode operation to provide low output ripple

and low quiescent current while using a tiny inductor and

capacitors.

Operation can best be understood by studying the Block

Diagram. An error amplifier measures the output voltage

through an external resistor divider tied to the FB pin. If

the FB voltage is higher than VREF, the error amplifier will

shut off all the high power circuitry, leaving the LT3470

in its micropower state. As the FB voltage falls, the error

amplifier will enable the power section, causing the chip

to begin switching, thus delivering charge to the output

capacitor. If the load is light the part will alternate between

micropower and switching states to keep the output in

regulation (See Figure 1a). At higher loads the part will

switch continuously while the error amp servos the top

and bottom current limits to regulate the FB pin voltage

to 1.25V (See Figure 1b).

The switching action is controlled by an RS latch and two

current comparators as follows: The switch turns on,

and the current through it ramps up until the top current

comparator trips and resets the latch causing the switch

to turn off. While the switch is off, the inductor current

ramps down through the catch diode. When both the bot-

tom current comparator trips and the minimum off-time

one-shot expires, the latch turns the switch back on thus

completing a full cycle. The hysteretic action of this control

scheme results in a switching frequency that depends

on inductor value, input and output voltage. Since the

switch only turns on when the catch diode current falls

below threshold, the part will automatically switch slower

to keep inductor current under control during start-up or

short-circuit conditions.

The switch driver operates from either the input or from

the BOOST pin. An external capacitor and internal diode

is used to generate a voltage at the BOOST pin that is

higher than the input supply. This allows the driver to

fully saturate the internal bipolar NPN power switch for

efficient operation.

If the SHDN pin is grounded, all internal circuits are turned

off and VIN current reduces to the device leakage current,

typically a few nA.

(1a) Burst Mode Operation (1b) Continuous Operation

VOUT

20mV/DIV

100mA/DIV

1ms/DIV

VOUT

20mV/DIV

100mA/DIV

5µs/DIV 3470 F01a

NO LOAD

10mA LOAD

VOUT

20mV/DIV

100mA/DIV

1µs/DIV

VOUT

20mV/DIV

100mA/DIV

1µs/DIV 3470 F1b

200mA LOAD

150mA LOAD

operaTion

LT3470

3470fd

applicaTions inForMaTion

Input Voltage Range

The minimum input voltage required to generate a par-

ticular output voltage in an LT3470 application is limited

by either its 4V undervoltage lockout or by its maximum

duty cycle. The duty cycle is the fraction of time that the

internal switch is on and is determined by the input and

output voltages:

DC =VOUT

VIN – VSW +VD

where VD is the forward voltage drop of the catch diode

(~0.6V) and VSW is the voltage drop of the internal switch

at maximum load (~0.4V). Given DCMAX = 0.90, this leads

to a minimum input voltage of:

VIN(MIN) =VOUT +VD

DCMAX









+VSW – VD

This analysis assumes the part has started up such that the

capacitor tied between the BOOST and SW pins is charged

to more than 2V. For proper start-up, the minimum input

voltage is limited by the boost circuit as detailed in the

section BOOST Pin Considerations.

The maximum input voltage is limited by the absolute

maximum VIN rating of 40V, provided an inductor of suf-

ficient value is used.

Inductor Selection

The switching action of the LT3470 during continuous

operation produces a square wave at the SW pin that

results in a triangle wave of current in the inductor. The

hysteretic mode control regulates the top and bottom

current limits (see Electrical Characteristics) such that

the average inductor current equals the load current. For

safe operation, it must be noted that the LT3470 cannot

turn the switch on for less than ~150ns. If the inductor is

small and the input voltage is high, the current through the

switch may exceed safe operating limit before the LT3470

is able to turn off. To prevent this from happening, the

following equation provides a minimum inductor value:

LMIN =VIN(MAX) •tON-TIME(MIN)

MAX

where VIN(MAX) is the maximum input voltage for the ap-

plication, tON-TIME(MIN)

is ~150ns and IMAX is the maximum

allowable increase in switch current during a minimum

switch on-time (150mA). While this equation provides a

safe inductor value, the resulting application circuit may

switch at too high a frequency to yield good efficiency.

It is advised that switching frequency be below 1.2MHz

during normal operation:

f=1– DC

( )

VD+VOUT

( )

L•∆I

where f is the switching frequency, ∆IL is the ripple current

in the inductor (~150mA), VD is the forward voltage drop

of the catch diode, and VOUT is the desired output voltage.

If the application circuit is intended to operate at high duty

cycles (VIN close to VOUT), it is important to look at the

calculated value of the switch off-time:

tOFF-TIME =1– DC

The calculated tOFF-TIME should be more than LT3470’s

minimum tOFF-TIME (See Electrical Characteristics), so

the application circuit is capable of delivering full rated

output current. If the full output current of 200mA is not

required, the calculated tOFF-TIME can be made less than

minimum tOFF-TIME possibly allowing the use of a smaller

inductor. See Table 1 for an inductor value selection guide.

Table 1. Recommended Inductors for Loads up to 200mA

VOUT VIN UP TO 16V VIN UP TO 40V

2.5V 10µH 33µH

3.3V 10µH 33µH

5V 15µH 33µH

12V 33µH 47µH

Choose an inductor that is intended for power applications.

Table 2 lists several manufacturers and inductor series.

For robust output short-circuit protection at high VIN (up

to 40V) use at least a 33µH inductor with a minimum

450mA saturation current. If short-circuit performance is

not required, inductors with ISAT of 300mA or more may

LT3470

3470fd

Table 2. Inductor Vendors

VENDOR URL PART SERIES INDUCTANCE RANGE (µH) SIZE (mm)

Coilcraft www.coilcraft.com DO1605

ME3220

DO3314

10 to 47

1.8 × 5.4 × 4.2

2.0 × 3.2 × 2.5

1.4 × 3.3 × 3.3

Sumida www.sumida.com CR32

CDRH3D16/HP

CDRH3D28

CDRH2D18/HP

10 to 47

10 to 33

10 to 47

10 to 15

3.0 × 3.8 × 4.1

1.8 × 4.0 × 4.0

3.0 × 4.0 × 4.0

2.0 × 3.2 × 3.2

Toko www.tokoam.com DB320C

D52LC

10 to 27

10 to 47

2.0 × 3.8 × 3.8

2.0 × 5.0 × 5.0

Würth Elektronik www.we-online.com WE-PD2 Typ S

WE-TPC Typ S

10 to 47

10 to 22

3.2 × 4.0 × 4.5

1.6 × 3.8 × 3.8

Coiltronics www.cooperet.com SD10 10 to 47 1.0 × 5.0 × 5.0

Murata www.murata.com LQH43C

LQH32C

10 to 47

10 to 15

2.6 × 3.2 × 4.5

1.6 × 2.5 × 3.2

applicaTions inForMaTion

be used. It is important to note that inductor saturation

current is reduced at high temperatures—see inductor

vendors for more information.

Input Capacitor

Step-down regulators draw current from the input sup-

ply in pulses with very fast rise and fall times. The input

capacitor is required to reduce the resulting voltage ripple

at the VIN pin of the LT3470 and to force this switching

current into a tight local loop, minimizing EMI. The input

capacitor must have low impedance at the switching

frequency to do this effectively. A 1µF to 2.2µF ceramic

capacitor satisfies these requirements.

If the input source impedance is high, a larger value ca-

pacitor may be required to keep input ripple low. In this

case, an electrolytic of 10µF or more in parallel with a 1µF

ceramic is a good combination. Be aware that the input

capacitor is subject to large surge currents if the LT3470

circuit is connected to a low impedance supply, and that

some electrolytic capacitors (in particular tantalum) must

be specified for such use.

Output Capacitor and Output Ripple

The output capacitor filters the inductor’s ripple current

and stores energy to satisfy the load current when the

LT3470 is quiescent. In order to keep output voltage ripple

low, the impedance of the capacitor must be low at the

LT3470’s switching frequency. The capacitor’s equivalent

series resistance (ESR) determines this impedance. Choose

one with low ESR intended for use in switching regulators.

The contribution to ripple voltage due to the ESR is ap-

proximately ILIM • ESR. ESR should be less than ~150mΩ.

The value of the output capacitor must be large enough to

accept the energy stored in the inductor without a large

change in output voltage. Setting this voltage step equal

to 1% of the output voltage, the output capacitor must be:

COUT >50 •L•ILIM

OUT











where ILIM is the top current limit with VFB = 0V (see Elec-

trical Characteristics). For example, an LT3470 producing

3.3V with L = 33µH requires 22µF. The calculated value

can be relaxed if small circuit size is more important than

low output ripple.

Sanyo’s POSCAP series in B-case and provides very good

performance in a small package for the LT3470. Similar

performance in traditional tantalum capacitors requires

a larger package (C-case). With a high quality capacitor

filtering the ripple current from the inductor, the output

voltage ripple is determined by the delay in the LT3470’s

feedback comparator. This ripple can be reduced further

by adding a small (typically 22pF) phase lead capacitor

between the output and the feedback pin.

LT3470

3470fd

applicaTions inForMaTion

Ceramic Capacitors

Ceramic capacitors are small, robust and have very low

ESR. However, ceramic capacitors can cause problems

when used with the LT3470. Not all ceramic capacitors are

suitable. X5R and X7R types are stable over temperature

and applied voltage and give dependable service. Other

types, including Y5V and Z5U have very large temperature

and voltage coefficients of capacitance. In an application

circuit they may have only a small fraction of their nominal

capacitance resulting in much higher output voltage ripple

than expected.

Ceramic capacitors are piezoelectric. The LT3470’s switch-

ing frequency depends on the load current, and at light

loads the LT3470 can excite the ceramic capacitor at audio

frequencies, generating audible noise. Since the LT3470

operates at a lower current limit during Burst Mode opera-

tion, the noise is typically very quiet to a casual ear. If this

audible noise is unacceptable, use a high performance

electrolytic capacitor at the output. The input capacitor

can be a parallel combination of a 2.2µF ceramic capacitor

and a low cost electrolytic capacitor.

A final precaution regarding ceramic capacitors concerns

the maximum input voltage rating of the LT3470. A ceramic

input capacitor combined with trace or cable inductance

forms a high quality (under damped) tank circuit. If the

LT3470 circuit is plugged into a live supply, the input volt-

age can ring to twice its nominal value, possibly exceeding

the LT3470’s rating. This situation is easily avoided; see

the Hot-Plugging Safely section.

BOOST and BIAS Pin Considerations

Capacitor C3 and the internal boost Schottky diode (see

Block Diagram) are used to generate a boost voltage that

is higher than the input voltage. In most cases a 0.22µF

capacitor will work well. Figure 2 shows two ways to ar-

range the boost circuit. The BOOST pin must be more than

2.5V above the SW pin for best efficiency. For outputs of

3.3V and above, the standard circuit (Figure 2a) is best.

For outputs between 2.5V and 3V, use a 0.47µF. For lower

output voltages the boost diode can be tied to the input

Figure 2. Two Circuits for Generating the Boost Voltage

Table 3. Capacitor Vendors

VENDOR PHONE URL PART SERIES COMMENTS

Panasonic (714) 373-7366 www.panasonic.com Ceramic,

Polymer,

Tantalum

EEF Series

Kemet (864) 963-6300 www.kemet.com Ceramic,

Tantalum

T494, T495

Sanyo (408) 749-9714 www.sanyovideo.com Ceramic,

Polymer,

Tantalum

POSCAP

Murata (404) 436-1300 www.murata.com Ceramic

AVX www.avxcorp.com Ceramic,

Tantalum

TPS Series

Taiyo Yuden (864) 963-6300 www.taiyo-yuden.com Ceramic

VIN BOOST

LT3470

(2a)

(2b)

0.22µF

VIN

VOUT

VBOOST – VSW ≅ VOUT

MAX VBOOST ≅ VIN + VOUT

BIAS

GND

VIN BOOST

LT3470

SWBIAS

0.22µF

VIN

VOUT

3470 F02

VBOOST – VSW ≅ VIN

MAX VBOOST ≅ 2VIN

GND

LT3470

3470fd

Figure 3. The Minimum Input Voltage Depends on Output

Voltage, Load Current and Boost Circuit

Minimum Input Voltage, VOUT = 3.3V

Minimum Input Voltage, VOUT = 5V

Figure 4. Diode D1 Prevents a Shorted Input from Discharging a

Backup Battery Tied to the Output; It Also Protects the Circuit

from a Reversed Input. The LT3470 Runs Only When the Input Is

Present Hot-Plugging Safely

applicaTions inForMaTion

(Figure 2b). The circuit in Figure 2a is more efficient

because the BOOST pin current and BIAS pin quiescent

current comes from a lower voltage source. You must also

be sure that the maximum voltage ratings of the BOOST

and BIAS pins are not exceeded.

The minimum operating voltage of an LT3470 application

is limited by the undervoltage lockout (4V) and by the

maximum duty cycle as outlined in a previous section. For

proper start-up, the minimum input voltage is also limited

by the boost circuit. If the input voltage is ramped slowly,

or the LT3470 is turned on with its SHDN pin when the

output is already in regulation, then the boost capacitor may

not be fully charged. The plots in Figure 3 show minimum

VIN to start and to run. At light loads, the inductor current

becomes discontinuous and the effective duty cycle can

be very high. This reduces the minimum input voltage to

approximately 300mV above VOUT. At higher load currents,

the inductor current is continuous and the duty cycle is

limited by the maximum duty cycle of the LT3470, requiring

a higher input voltage to maintain regulation.

Shorted Input Protection

If the inductor is chosen so that it won’t saturate exces-

sively at the top switch current limit maximum of 450mA,

an LT3470 buck regulator will tolerate a shorted output

even if VIN = 40V. There is another situation to consider

in systems where the output will be held high when the

input to the LT3470 is absent. This may occur in battery

charging applications or in battery backup systems where

a battery or some other supply is diode OR-ed with the

LT3470’s output. If the VIN pin is allowed to float and the

SHDN pin is held high (either by a logic signal or because

it is tied to VIN), then the LT3470’s internal circuitry will

pull its quiescent current through its SW pin. This is fine

if your system can tolerate a few mA in this state. If you

ground the SHDN pin, the SW pin current will drop to es-

sentially zero. However, if the VIN pin is grounded while

the output is held high, then parasitic diodes inside the

LT3470 can pull large currents from the output through

the SW pin and the VIN pin. Figure 4 shows a circuit that

will run only when the input voltage is present and that

protects against a shorted or reversed input.

LOAD CURRENT (mA)

3.0

INPUT VOLTAGE (V)

3.5

4.0

4.5

5.0

5.5

6.0

50 100 150 200

3470 G18

TA = 25°C VIN TO START

VIN TO RUN

LOAD CURRENT (mA)

INPUT VOLTAGE (V)

200

3470 G19

450 100 150

8TA = 25°C

VIN TO START

VIN TO RUN

VIN BOOST

LT3470 SOT-23

SWSHDN

3470 F04

VIN

100k

VOUT

BACKUP

BIAS

GND

LT3470

3470fd

applicaTions inForMaTion

PCB Layout

For proper operation and minimum EMI, care must be

taken during printed circuit board layout. Note that large,

switched currents flow in the power switch, the internal

catch diode and the input capacitor. The loop formed by

these components should be as small as possible. Further-

more, the system ground should be tied to the regulator

ground in only one place; this prevents the switched cur-

rent from injecting noise into the system ground. These

components, along with the inductor and output capacitor,

should be placed on the same side of the circuit board,

and their connections should be made on that layer. Place

a local, unbroken ground plane below these components,

and tie this ground plane to system ground at one location,

ideally at the ground terminal of the output capacitor C2.

Additionally, the SW and BOOST nodes should be kept as

small as possible. Unshielded inductors can induce noise

in the feedback path resulting in instability and increased

output ripple. To avoid this problem, use vias to route the

VOUT trace under the ground plane to the feedback divider

(as shown in Figure 5). Finally, keep the FB node as small

as possible so that the ground pin and ground traces

will shield it from the SW and BOOST nodes. Figure 5

shows component placement with trace, ground plane

and via locations. Include vias near the GND pin, or pad,

of the LT3470 to help remove heat from the LT3470 to

the ground plane.

Figure 5. A Good PCB Layout Ensures Proper, Low EMI Operation

SHDN

VIN

VOUT

(5a) (5b)

VOUT

3470 F05

GND

SHDN

VIN

GND

VIAS TO FEEDBACK DIVIDER

VIAS TO LOCAL GROUND PLANE

OUTLINE OF LOCAL GROUND PLANE

LT3470

3470fd

applicaTions inForMaTion

Hot-Plugging Safely

The small size, robustness and low impedance of ceramic

capacitors make them an attractive option for the input

bypass capacitor of LT3470. However, these capacitors can

cause problems if the LT3470 is plugged into a live supply

(see Linear Technology Application Note 88 for a complete

discussion). The low loss ceramic capacitor combined with

stray inductance in series with the power source forms an

under damped tank circuit, and the voltage at the VIN pin

of the LT3470 can ring to twice the nominal input voltage,

possibly exceeding the LT3470’s rating and damaging the

part. If the input supply is poorly controlled or the user will

be plugging the LT3470 into an energized supply, the input

network should be designed to prevent this overshoot.

Figure 6 shows the waveforms that result when an LT3470

circuit is connected to a 24V supply through six feet of

24-gauge twisted pair. The first plot is the response with

a 2.2µF ceramic capacitor at the input. The input voltage

rings as high as 35V and the input current peaks at 20A.

One method of damping the tank circuit is to add another

capacitor with a series resistor to the circuit. In Figure 6b

an aluminum electrolytic capacitor has been added. This

capacitor’s high equivalent series resistance damps the

circuit and eliminates the voltage overshoot. The extra

capacitor improves low frequency ripple filtering and can

slightly improve the efficiency of the circuit, though it is

likely to be the largest component in the circuit. An alterna-

tive solution is shown in Figure 6c. A 1Ω resistor is added

in series with the input to eliminate the voltage overshoot

(it also reduces the peak input current). A 0.1µF capacitor

improves high frequency filtering. This solution is smaller

and less expensive than the electrolytic capacitor. For high

input voltages its impact on efficiency is minor, reducing

efficiency less than one half percent for a 5V output at full

load operating from 24V.

High Temperature Considerations

The die junction temperature of the LT3470 must be

lower than the maximum rating of 125°C (150°C for the

H-grade). This is generally not a concern unless the ambi-

ent temperature is above 85°C. For higher temperatures,

care should be taken in the layout of the circuit to ensure

good heat sinking of the LT3470. The maximum load

current should be derated as the ambient temperature

approaches the maximum junction rating. The die tem-

perature is calculated by multiplying the LT3470 power

dissipation by the thermal resistance from junction to

ambient. Power dissipation within the LT3470 can be

estimated by calculating the total power loss from an

efficiency measurement. Thermal resistance depends

on the layout of the circuit board and choice of package.

The DD package with the exposed pad has a thermal

resistance of approximately 80°C/W while the ThinSOT

is approximately 150°C/W. Finally, be aware that at high

ambient temperatures the internal Schottky diode will

have significant leakage current (see Typical Performance

Characteristics) increasing the quiescent current of the

LT3470 converter.

LT3470

3470fd

applicaTions inForMaTion

Figure 6. A Well Chosen Input Network Prevents Input Voltage Overshoot and

Ensures Reliable Operation When the LT3470 Is Connected to a Live Supply

LT3470

2.2µF

VIN

10V/DIV

IIN

10A/DIV

10µs/DIV

VIN

CLOSING SWITCH

SIMULATES HOT PLUG

IIN

(6a)

(6b)

(6c)

LOW

IMPEDANCE

ENERGIZED

24V SUPPLY

STRAY

INDUCTANCE

DUE TO 6 FEET

(2 METERS) OF

TWISTED PAIR

LT3470

2.2µF

10µF

35V

AI.EI.

LT3470

2.2µF0.1µF

1Ω

3470 F06

LT3470

3470fd

3.3V Step-Down Converter 5V Step-Down Converter

2.5V Step-Down Converter

VIN BOOST

LT3470

SWSHDN

0.22µF, 6.3V

22pF C2

22µF

3470 TA03

1µF

VIN

5.5V TO 40V

VOUT

3.3V

200mA

324k

200k

C1: TDK C3216JB1H105M

C2: CE JMK316 BJ226ML-T

L1: TOKO A993AS-270M=P3

33µH

BIAS

GND

OFF ON

VIN BOOST

LT3470

SWSHDN

0.22µF, 6.3V

22pF C2

22µF

3470 TA04

1µF

VIN

7V TO 40V

VOUT

200mA

604k

200k

33µH

BIAS

GND

OFF ON

C1: TDK C3216JB1H105M

C2: CE JMK316 BJ226ML-T

L1: TOKO A914BYW-330M=P3

Typical applicaTions

1.8V Step-Down Converter

12V Step-Down Converter

VIN BOOST

LT3470

SWSHDN

0.47µF, 6.3V

22pF C2

22µF

3470 TA07

1µF

VIN

4.7V TO 40V

VOUT

2.5V

200mA

200k

C1: TDK C3216JB1H105M

C2: TDK C2012JB0J226M

L1: SUMIDA CDRH3D28

33µH

BIAS

GND

OFF ON

VIN BOOST

LT3470

SWSHDN

BIAS

0.22µF, 25V

22pF

22µF

3470 TA05

1µF

VIN

4V TO 23V

VOUT

1.8V

200mA

147k

332k

22µH

GND

OFF ON

C1: TDK C3216JB1H105M

C2: TDK C2012JB0J226M

L1: MURATA LQH32CN150K53

VIN BOOST

LT3470

SWSHDN

0.22µF, 16V

22pF C2

10µF

3470 TA06

1µF

VIN

15V TO 34V

VOUT

12V

200mA

866k

100k

C1: TDK C3216JB1H105M

C2: TDK C3216JB1C106M

L1: MURATA LQH32CN150K53

33µH

BIAS

GND

OFF ON

LT3470

3470fd

package DescripTion

TS8 Package

8-Lead Plastic TSOT-23

(Reference LTC DWG # 05-08-1637 Rev A)

1.50 – 1.75

(NOTE 4)

2.80 BSC

0.22 – 0.36

8 PLCS (NOTE 3)

DATUM ‘A’

0.09 – 0.20

(NOTE 3)

TS8 TSOT-23 0710 REV A

2.90 BSC

(NOTE 4)

0.65 BSC

1.95 BSC

0.80 – 0.90

1.00 MAX 0.01 – 0.10

0.20 BSC

0.30 – 0.50 REF

PIN ONE ID

NOTE:

1. DIMENSIONS ARE IN MILLIMETERS

2. DRAWING NOT TO SCALE

3. DIMENSIONS ARE INCLUSIVE OF PLATING

4. DIMENSIONS ARE EXCLUSIVE OF MOLD FLASH AND METAL BURR

5. MOLD FLASH SHALL NOT EXCEED 0.254mm

6. JEDEC PACKAGE REFERENCE IS MO-193

3.85 MAX

0.40

MAX

0.65

REF

RECOMMENDED SOLDER PAD LAYOUT

PER IPC CALCULATOR

1.4 MIN

2.62 REF

1.22 REF

Please refer to http://www.linear.com/designtools/packaging/ for the most recent package drawings.

LT3470

3470fd

package DescripTion

DDB Package

8-Lead Plastic DFN (3mm × 2mm)

(Reference LTC DWG # 05-08-1702 Rev B)

2.00 ±0.10

(2 SIDES)

NOTE:

1. DRAWING CONFORMS TO VERSION (WECD-1) IN JEDEC PACKAGE OUTLINE M0-229

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

0.56 ± 0.05

(2 SIDES)

0.75 ±0.05

R = 0.115

TYP

R = 0.05

TYP

2.15 ±0.05

(2 SIDES)

3.00 ±0.10

(2 SIDES)

PIN 1 BAR

TOP MARK

(SEE NOTE 6)

0.200 REF

0 – 0.05

(DDB8) DFN 0905 REV B

0.25 ± 0.05

2.20 ±0.05

(2 SIDES)

RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS

0.61 ±0.05

(2 SIDES)

1.15 ±0.05

0.70 ±0.05

2.55 ±0.05

PACKAGE

OUTLINE

0.25 ± 0.05

0.50 BSC

PIN 1

R = 0.20 OR

0.25 × 45°

CHAMFER

0.50 BSC

Please refer to http://www.linear.com/designtools/packaging/ for the most recent package drawings.

LT3470

3470fd

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

D 09/11 Corrected lead-based tape and reel part numbers in the Order Information section. 2

(Revision history begins at Rev D)

LT3470

3470fd

Linear Technology Corporation

1630 McCarthy Blvd., Milpitas, CA 95035-7417

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

 LINEAR TECHNOLOGY CORPORATION 2004

LT 0911 REV D • PRINTED IN USA

relaTeD parTs

PART NUMBER DESCRIPTION COMMENTS

LT1616 25V, 500mA (IOUT), 1.4MHz, High Efficiency

Step-Down DC/DC Converter

VIN = 3.6V to 25V, VOUT = 1.25V, IQ = 1.9mA, ISD < 1µA,

ThinSOT Package

LT1676 60V, 440mA (IOUT), 100kHz, High Efficiency

Step-Down DC/DC Converter

VIN = 7.4V to 60V, VOUT = 1.24V, IQ = 3.2mA, ISD = 2.5µA,

S8 Package

LT1765 25V, 2.75A (IOUT), 1.25MHz, High Efficiency

Step-Down DC/DC Converter

VIN = 3V to 25V, VOUT = 1.2V, IQ = 1mA, ISD = 15µA,

S8, TSSOP16E Packages

LT1766 60V, 1.2A (IOUT), 200kHz, High Efficiency

Step-Down DC/DC Converter

VIN = 5.5V to 60V, VOUT = 1.2V, IQ = 2.5mA, ISD = 25µA,

TSSOP16/E Package

LT1767 25V, 1.2A (IOUT), 1.25MHz, High Efficiency

Step-Down DC/DC Converter

VIN = 3V to 25V; VOUT = 1.2V, IQ = 1mA, ISD = 6µA,

MS8/E Packages

LT1776 40V, 550mA (IOUT), 200kHz, High Efficiency

Step-Down DC/DC Converter

VIN = 7.4V to 40V; VOUT = 1.24V, IQ = 3.2mA, ISD = 30µA,

N8, S8 Packages

LT C

1877 600mA (IOUT), 550kHz, Synchronous

Step-Down DC/DC Converter

VIN = 2.7V to 10V; VOUT = 0.8V, IQ = 10µA, ISD ≤ 1µA,

MS8 Package

LTC1879 1.2A (IOUT), 550kHz, Synchronous

Step-Down DC/DC Converter

VIN = 2.7V to 10V; VOUT = 0.8V, IQ = 15µA, ISD ≤ 1µA,

TSSOP16 Package

LT1933 36V, 600mA, 500kHz, High Efficiency

Step-Down DC/DC Converter

VIN = 3.6V to 36V; VOUT = 1.25V, IQ = 2.5µA, ISD ≤ 1µA,

??? Package

LT1934 34V, 250mA (IOUT), Micropower, Step-Down

DC/DC Converter

VIN = 3.2V to 34V; VOUT = 1.25V, IQ = 12µA, ISD ≤ 1µA,

??? Package

LT1956 60V, 1.2A (IOUT), 500kHz, High Efficiency

Step-Down DC/DC Converter

VIN = 5.5V to 60V, VOUT = 1.2V, IQ = 2.5mA, ISD = 25µA,

TSSOP16/E Package

LTC3405/LTC3405A 300mA (IOUT), 1.5MHz, Synchronous

Step-Down DC/DC Converter

VIN = 2.7V to 6V, VOUT = 0.8V, IQ = 20µA, ISD ≤ 1µA,

ThinSOT Package

LTC3406/LTC3406B 600mA (IOUT), 1.5MHz, Synchronous

Step-Down DC/DC Converter

VIN = 2.5V to 5.5V, VOUT = 0.6V, IQ = 20µA, ISD ≤ 1µA,

ThinSOT Package

LTC3411 1.25A (IOUT), 4MHz, Synchronous

Step-Down DC/DC Converter

VIN = 2.5V to 5.5V, VOUT = 0.8V, IQ = 60µA, ISD ≤ 1µA,

MS Package

LTC3412 2.5A (IOUT), 4MHz, Synchronous

Step-Down DC/DC Converter

VIN = 2.5V to 5.5V, VOUT = 0.8V, IQ = 60µA, ISD ≤ 1µA,

TSSOP16E Package

LTC3430 60V, 2.75A (IOUT), 200kHz, High Efficiency

Step-Down DC/DC Converter

VIN = 5.5V to 60V, VOUT = 1.2V, IQ = 2.5mA, ISD = 30µA,

TSSOP16E Package