LT3431_技术文档

LT1936

1.4A, 500kHz Step-Down

Switching Regulator

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FEATURES

DESCRIPTIO

The LT^®1936 is a current mode PWM step-down DC/DC

converter with an internal 1.9A power switch, packaged in

a tiny, thermally enhanced 8-lead MSOP. The wide input

range of 3.6V to 36V makes the LT1936 suitable for

regulating power from a wide variety of sources, including

automotive batteries, 24V industrial supplies and unregu-

lated wall adapters. Its high operating frequency allows

the use of small, low cost inductors and ceramic capaci-

tors, resulting in low, predictable output ripple.

■

Wide Input Range: 3.6V to 36V

■

Short-Circuit Protected Over Full Input Range

■

1.9A Guaranteed Minimum Switch Current

■

5V at 1.4A from 10V to 36V Input

■

3.3V at 1.4A from 7V to 36V Input

■

5V at 1.2A from 6.3V to 36V Input

■

3.3V at 1.2A from 4.5V to 36V Input

■

Output Adjustable Down to 1.20V

■

500kHz Fixed Frequency Operation

■

Soft-Start

Cycle-by-cycle current limit, frequency foldback and ther-

malshutdownprovideprotectionagainstshortedoutputs,

and soft-start eliminates input current surge during start-

up. Transient response can be optimized by using external

compensation components, or board space can be mini-

mized by using internal compensation. The low current

(<2µA)shutdownmodeenableseasypowermanagement

in battery-powered systems.

■

Uses Small Ceramic Capacitors

■

Internal or External Compensation

■

Low Shutdown Current: <2µA

■

^{Thermally Enhan}U^{ced 8-Lead MSOP Package}

APPLICATIO S

■

Automotive Battery Regulation

Industrial Control Supplies

■

, LTC and LT are registered trademarks of Linear Technology Corporation.

All other trademarks are the property of their respective owners.

■

Unregulated Wall Adapters

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TYPICAL APPLICATIO

3.3V Step-Down Converter

Efficiency

95

V

IN

V

= 12V

IN

4.5V TO 36V

V

= 5V

OUT

90

85

80

75

70

65

0.22µF

V

BOOST

SW

IN

10µH

SHDN

ON OFF

V

3.3V

1.2A

OUT

V

= 3.3V

OUT

LT1936

4.7µF

17.4k

COMP

FB

GND

V

C

10k

22µF

1936 TA01a

0

0.5

1

1.5

LOAD CURRENT (A)

1936 TA01b

1936fa

1

LT1936

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ABSOLUTE AXI U RATI GS

PACKAGE/ORDER I FOR ATIO

(Note 1)

V_INVoltage ............................................... –0.4V to 36V

BOOST Voltage ........................................................ 43V

BOOST Above SW Voltage....................................... 20V

SHDN Voltage ........................................... –0.4V to 36V

FB, V_C, COMP Voltage ............................................... 6V

Operating Temperature Range (Note 2)

LT1936E ............................................. –40°C to 85°C

LT1936I ............................................ –40°C to 125°C

LT1936H .......................................... –40°C to 150°C

Maximum Junction Temperature

ORDER PART

NUMBER

TOP VIEW

LT1936EMS8E

LT1936IMS8E

LT1936HMS8E

BOOST

1

2

3

4

8 COMP

7 V

6 FB

V

SW

IN

C

9

5 SHDN

GND

MS8E PACKAGE

8-LEAD PLASTIC MSOP

MS8E PART MARKING

θ

_JA= 40°C/W, θ_JC= 10°C/W

EXPOSED PAD (PIN 9) IS GND

MUST BE CONNECTED TO PCB

LTBMT

LTBRV

LTBWB

LT1936E, LT1936I ............................................ 125°C

LT1936H ......................................................... 150°C

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

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

Order Options Tape and Reel: Add #TR

Lead Free: Add #PBF Lead Free Tape and Reel: Add #TRPBF

Lead Free Part Marking: http://www.linear.com/leadfree/

Consult factory for parts specified with wider operating temperature ranges.

ELECTRICAL CHARACTERISTICS

The

IN

●

denotes specifications which apply over the full operating temperature range, otherwise specifications are at T = 25°C.

A

V

= 12V, V

= 17V, unless otherwise noted. (Note 2)

BOOST

PARAMETER

CONDITIONS

MIN

TYP

3.45

1.8

MAX

3.6

UNITS

V

Undervoltage Lockout

Quiescent Current

V

= 1.5V

2.5

mA

µA

FB

Quiescent Current in Shutdown

FB Voltage

V

= 0V

0.1

2

SHDN

●

1.175

1.200

1.215

V

FB Pin Bias Current (Note 4)

V

= 1.20V, E and I Grades

●

50

200

300

nA

FB

H Grade

FB Voltage Line Regulation

V

= 5V to 36V

0.01

250

150

1.8

%/V

IN

Error Amp g

V = 0.5V, I = ±5µA

µS

m

C

VC

Error Amp Voltage Gain

V Clamp

V = 0.8V, 1.2V

C

V

C

V Switch Threshold

C

0.7

Internal Compensation R

Internal Compensation C

COMP Pin Leakage

50

kΩ

pF

V

= 1V

150

COMP

= 1.8V, E and I Grades

●

1

2

µA

H Grade

Switching Frequency

V

= 1.1V

= 0V

400

500

40

600

kHz

FB

Maximum Duty Cycle

Switch Current Limit

●

87

92

2.2

410

%

A

1.9

2.6

520

2

Switch V

I

= 1.2A

mV

µA

V

CESAT

SW

Switch Leakage Current

Minimum BOOST Voltage Above SW

2

2.2

1936fa

2

LT1936

ELECTRICAL CHARACTERISTICS

The

IN

●

denotes specifications which apply over the full operating temperature range, otherwise specifications are at T = 25°C.

A

V

= 12V, V

= 17V, unless otherwise noted. (Note 2)

BOOST

PARAMETER

CONDITIONS

= 1.2A

MIN

TYP

28

MAX

50

UNITS

mA

µA

BOOST Pin Current

I

SW

BOOST Pin Leakage

SHDN Input Voltage High

SHDN Input Voltage Low

SHDN Pin Current

V

= 0V

0.1

1

SW

2.3

V

0.3

V

= 2.3V (Note 5)

= 12V

= 0V

34

140

0.01

50

240

0.1

µA

SHDN

Note 1: Absolute Maximum Ratings are those values beyond which the life

of the device may be impaired.

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

High junction temperatures degrade operating lifetimes. Operating lifetime

at junction temperatures greater than 125°C is derated to 1000 hours.

Note 3: Current limit guaranteed by design and/or correlation to static test.

Slope compensation reduces current limit at higher duty cycle.

Note 4: Current flows out of pin.

Note 5: Current flows into pin.

Note 2: The LT1936E is guaranteed to meet performance specifications

from 0°C to 70°C. Specifications over the –40°C to 85°C operating

temperature range are assured by design, characterization and correlation

with statistical process controls. The LT1936I specifications are

guaranteed over the –40°C to 125°C temperature range. The LT1936H

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TYPICAL PERFOR A CE CHARACTERISTICS

Efficiency, V

= 5V

Switch Current Limit

Efficiency, V

= 3.3V

OUT

3.0

2.5

100

90

100

90

V

= 12V

= 24V

IN

V

= 5V

IN

TYP

2.0

1.5

V

IN

V

= 12V

= 24V

IN

MIN

80

V

IN

1.0

0.5

0

70

V

A

= 3.3V

OUT

V

A

= 5V

OUT

T

= 25°C

T

= 25°C

D1 = DFLS140L

L1 = 10µH, TOKO D63CB

L1 = 15µH, TOKO D63CB

60

0

20

40

60

80

100

0

0.5

1.0

1.5

0

0.5

1.0

1.5

DUTY CYCLE (%)

LOAD CURRENT (A)

1936 G03

1936 G02

1936 G01

1936fa

3

LT1936

TYPICAL PERFOR A CE CHARACTERISTICS

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Maximum Load Current

Switch Voltage Drop

600

500

400

300

200

100

0

1.8

1.6

1.4

1.2

1.8

1.6

1.4

1.2

V

= 5V

V

OUT

= 3.3V

OUT

T

= 85°C

A

T

= 25°C

L = 10µH

A

L = 15µH

T

= –45°C

A

L = 10µH

L = 6.8µH

1.0

0

0.5

1.0

1.5

0

5

10

15

20

25

30

0

5

10

15

20

25

30

SWITCH CURRENT (A)

INPUT VOLTAGE (V)

1936 G06

1936 G04

1936 G05

Feedback Voltage

Undervoltage Lockout

Switching Frequency

1.210

1.205

1.200

1.195

1.190

1.185

3.8

3.6

3.4

3.2

600

550

500

450

3.0

400

–50 –25

0

25 50 75 100 125 150

–50 –25

0

25 50 75 100 125 150

–50 –25

0

25 50 75 100 125 150

TEMPERATURE (°C)

1936 G07

1936 G08

1936 G09

Frequency Foldback

Soft-Start

SHDN Pin Current

3.0

2.5

200

150

100

50

700

600

500

400

300

200

100

0

T

= 25°C

T

A

= 25°C

A

T

= 25°C

A

DC = 30%

2.0

1.5

1.0

0.5

0

1

2

3

4

0

4

8

12

16

1.0

0.5

FB PIN VOLTAGE (V)

1.5

0

SHDN PIN VOLTAGE (V)

1936 G11

1936 G12

1936 G10

1936fa

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LT1936

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TYPICAL PERFOR A CE CHARACTERISTICS

Minimum Input Voltage

Switch Current Limit

8

7

6

5

4

6.0

5.5

3.0

2.5

2.0

1.5

V

T

= 5V

V

T

= 3.3V

OUT

A

OUT

A

= 25°C

L = 15µH

L = 10µH

TO START

5.0

4.5

4.0

3.5

3.0

TO RUN

1.0

0.5

0

TO RUN

100

10

LOAD CURRENT (mA)

1000

1

0

10

100

1000

50

100 125 150

–50 –25

0

25

75

LOAD CURRENT (mA)

TEMPERATURE (°C)

1936 G13

1936 G14

1936 G15

Switching Waveforms,

Discontinuous Mode

Switching Waveforms

V

SW

10V/DIV

SW

10V/DIV

I

L

500mA/DIV

V

OUT

20mV/DIV

OUT

20mV/DIV

1936 G16

1936 G17

V

= 12V

1µs/DIV

V

= 12V

IN

1µs/DIV

IN

= 3.3V

OUT

= 1A

OUT

= 3.3V

OUT

I

= 50mA

L = 10µH

C

= 22µF

OUT

V Voltages

Error Amp Output Current

C

2.5

2.0

1.5

1.0

0.5

0

60

40

T

= 25°C

= 0.5V

A

C

V

CURRENT LIMIT CLAMP

SWITCHING THRESHOLD

20

0

–20

–40

–60

0

2

1

–50 –25

0

25 50 75 100 125 150

FB PIN VOLTAGE (V)

TEMPERATURE (°C)

1936 G19

1936 G18

1936fa

5

LT1936

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PI FU CTIO S

BOOST (Pin 1): The BOOST pin is used to provide a drive

voltage, higher than the input voltage, to the internal

bipolar NPN power switch.

FB (Pin 6): The LT1936 regulates its feedback pin to

1.200V. Connect the feedback resistor divider tap to this

pin. Set the output voltage according to V_OUT= 1.200V

(1 + R1/R2). A good value for R2 is 10k.

V_IN(Pin 2): The V_INpin supplies current to the LT1936’s

internal regulator and to the internal power switch. This

pin must be locally bypassed.

V_C(Pin 7): The V_Cpin is used to compensate the LT1936

control loop by tying an external RC network from this pin

to ground. The COMP pin provides access to an internal

RC network that can be used instead of the external

components.

SW (Pin 3): The SW pin is the output of the internal power

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

boost capacitor.

COMP (Pin 8): To use the internal compensation network,

tie the COMP pin to the V_Cpin. Otherwise, tie COMP to

ground or leave it floating.

GND(Pin4):TietheGNDpintoalocalgroundplanebelow

the LT1936 and the circuit components. Return the feed-

back divider to this pin.

Exposed Pad (Pin 9): The Exposed Pad must be soldered

to the PCB and electrically connected to ground. Use a

large ground plane and thermal vias to optimize thermal

performance.

SHDN (Pin 5): The SHDN pin is used to put the LT1936

in shutdown mode. Tie to ground to shut down the

LT1936. Tie to 2.3V or more for normal operation. If the

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

SHDN also provides a soft-start function; see the Appli-

cations Information. Do not drive SHDN more than 5V

above V_IN.

W

BLOCK DIAGRA

V

IN

V

2

IN

C2

INT REG

AND

UVLO

D2

BOOST

1

Σ

ON OFF

SLOPE

COMP

R

S

Q

R3

SHDN

C3

5

DRIVER

Q1

C4

L1

SW

FB

OSC

V

3

6

OUT

C1

D1

FREQUENCY

FOLDBACK

R1

R2

V

C

g

m

C

R

1.200V

GND

C

150pF

50k

V

C

COMP

1936 BD

7

8

4

R4

C5

1936fa

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LT1936

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OPERATIO

(Refer to Block Diagram)

The LT1936 is a constant frequency, current mode step-

down regulator. A 500kHz oscillator enables an RS flip-

flop, turning on the internal 1.9A power switch Q1. An

amplifier and comparator monitor the current flowing

between the V_INand SW pins, turning the switch off when

this current reaches a level determined by the voltage at

V_C.Anerroramplifiermeasurestheoutputvoltagethrough

anexternalresistordividertiedtotheFBpinandservosthe

V_Cpin. If the error amplifier’s output increases, more

current is delivered to the output; if it decreases, less

currentisdelivered.Anactiveclamp(notshown)ontheV_C

pin provides current limit. The V_Cpin is also clamped to

the voltage on the SHDN pin; soft-start is implemented by

generating a voltage ramp at the SHDN pin using an

external resistor and capacitor.

An internal regulator provides power to the control cir-

cuitry. This regulator includes an undervoltage lockout to

prevent switching when V_INis less than ~3.45V. The

SHDN pin is used to place the LT1936 in shutdown,

disconnectingtheoutputandreducingtheinputcurrentto

less than 2µA.

The switch driver operates from either the input or from

the BOOST pin. An external capacitor and diode are 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.

The oscillator reduces the LT1936’s operating frequency

when the voltage at the FB pin is low. This frequency

foldbackhelpstocontroltheoutputcurrentduringstartup

and overload.

1936fa

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LT1936

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APPLICATIO S I FOR ATIO

FB Resistor Network

Inductor Selection and Maximum Output Current

A good first choice for the inductor value is

L = 2.2 (V_OUT+ V_D)

The output voltage is programmed with a resistor divider

between the output and the FB pin. Choose the 1%

resistors according to:

whereV_Disthevoltagedropofthecatchdiode(~0.4V)and

Lisin µH. Withthisvaluethemaximumoutputcurrentwill

be above 1.2A at all duty cycles and greater than 1.4A for

duty cycles less than 50% (V_IN> 2 V_OUT). The inductor’s

RMS current rating must be greater than the maximum

load current and its saturation current should be about

30% higher. For robust operation in fault conditions

(start-up or short circuit) and high input voltage (>30V),

the saturation current should be above 2.6A. To keep the

efficiency high, the series resistance (DCR) should be less

than 0.1Ω, and the core material should be intended for

high frequency applications. Table 1 lists several vendors

and suitable types.

V

⎛

⎝

⎞

⎠

OUT

R1= R2

– 1

⎟

⎜

1.200

R2 should be 20k or less to avoid bias current errors.

Reference designators refer to the Block Diagram.

Input Voltage Range

The input voltage range for LT1936 applications depends

on the output voltage and the Absolute Maximum Ratings

of the V_INand BOOST pins.

The minimum input voltage is determined by either the

LT1936’s minimum operating voltage of ~3.45V or by its

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

thattheinternalswitchisonandisdeterminedbytheinput

and output voltages:

Table 1. Inductor Vendors

VENDOR URL

PART SERIES

TYPE

Murata

TDK

www.murata.com

LQH55D

Open

www.component.tdk.com SLF7045

Shielded

SLF10145

V_OUT+ V_D

DC =

Toko

www.toko.com

D62CB

D63CB

D75C

Shielded

Open

V – V_SW+ V_D

IN

where V_Dis the forward voltage drop of the catch diode

(~0.5V) and V_SWis the voltage drop of the internal switch

(~0.5V at maximum load). This leads to a minimum input

voltage of:

D75F

Sumida

www.sumida.com

CR54

Open

CDRH74

CDRH6D38

CR75

Shielded

Open

V_OUT+ V_D

DC_MAX

with DC_MAX= 0.87.

V

=

– V_D+ V_SW

IN(MIN)

Of course, such a simple design guide will not always re-

sult in the optimum inductor for your application. A larger

valueprovidesaslightlyhighermaximumloadcurrentand

will reduce the output voltage ripple. If your load is lower

than 1.2A, then you can decrease the value of the inductor

and operate with higher ripple current. This allows you to

use a physically smaller inductor, or one with a lower DCR

resultinginhigherefficiency.Beawarethatiftheinductance

differsfromthesimpleruleabove, thenthemaximumload

current will depend on input voltage. There are several

graphs in the Typical Performance Characteristics section

The maximum input voltage is determined by the absolute

maximum ratings of the V_INand BOOST pins and by the

minimum duty cycle DC_MIN= 0.08:

V_OUT+ V_D

DC_MIN

V

=

– V_D+ V_SW

IN(MAX)

Note that this is a restriction on the operating input

voltage; the circuit will tolerate transient inputs up to the

absolute maximum ratings of the V_INand BOOST pins.

1936fa

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LT1936

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APPLICATIO S I FOR ATIO

voltage rating of the LT1936. A ceramic input capacitor

combined with trace or cable inductance forms a high

quality (under damped) tank circuit. If the LT1936 circuit

is plugged into a live supply, the input voltage can ring to

twice its nominal value, possibly exceeding the LT1936’s

voltage rating. This situation is easily avoided; see the Hot

Plugging Safety section.

of this data sheet that show the maximum load current as

a function of input voltage and inductor value for several

popular output voltages. Low inductance may result in

discontinuous mode operation, which is okay but further

reduces maximum load current. For details of maximum

output current and discontinuous mode operation, see

Linear Technology Application Note 44. Finally, for duty

cycles greater than 50% (V_OUT/V_IN> 0.5), there is a mini-

mum inductance required to avoid subharmonic oscilla-

tions.ChoosingLgreaterthan1.6(V_OUT+V_D)µHprevents

subharmonic oscillations at all duty cycles.

For space sensitive applications, a 2.2µF ceramic capaci-

tor can be used for local bypassing of the LT1936 input.

However, the lower input capacitance will result in in-

creased input current ripple and input voltage ripple, and

may couple noise into other circuitry. Also, the increased

voltage ripple will raise the minimum operating voltage of

the LT1936 to ~3.7V.

Catch Diode

A 1A Schottky diode is recommended for the catch diode,

D1. The diode must have a reverse voltage rating equal to

or greater than the maximum input voltage. The ON

Semiconductor MBRM140 is a good choice. It is rated for

1A DC at a case temperature of 110°C and 1.5A at a case

temperature of 95°C. Diode Incorporated’s DFLS140L is

rated for 1.1A average current; the DFLS240L is rated for

2A average current. The average diode current in an

LT1936 application is approximately I_OUT(1 – DC).

Output Capacitor

The output capacitor has two essential functions. Along

with the inductor, it filters the square wave generated by

the LT1936 to produce the DC output. In this role it

determines the output ripple, and low impedance at the

switching frequency is important. The second function is

to store energy in order to satisfy transient loads and

stabilize the LT1936’s control loop.

Input Capacitor

Ceramic capacitors have very low equivalent series resis-

tance (ESR) and provide the best ripple performance. A

good value is:

Bypass the input of the LT1936 circuit with a 4.7µF or

higher value ceramic capacitor of X7R or X5R type. Y5V

types have poor performance over temperature and ap-

plied voltage, and should not be used. A 4.7µF ceramic is

adequate to bypass the LT1936 and will easily handle the

ripplecurrent.However,iftheinputpowersourcehashigh

impedance, or there is significant inductance due to long

wires or cables, additional bulk capacitance may be nec-

essary. This can be provided with a low performance

electrolytic capacitor.

150

V_OUT

C_OUT

=

whereC_OUTisinµF.UseX5RorX7Rtypes.Thischoicewill

provide low output ripple and good transient response.

Transient performance can be improved with a high value

capacitor if the compensation network is also adjusted to

maintain the loop bandwidth.

Step-down regulators draw current from the input supply

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

capacitor is required to reduce the resulting voltage ripple

at the LT1936 and to force this very high frequency

switching current into a tight local loop, minimizing EMI.

A 4.7µF capacitor is capable of this task, but only if it is

placed close to the LT1936 and the catch diode; see the

PCB Layout section. A second precaution regarding the

ceramic input capacitor concerns the maximum input

A lower value of output capacitor can be used, but tran-

sient performance will suffer. With an external compensa-

tion network, the loop gain can be lowered to compensate

for the lower capacitor value. When using the internal

compensation network, the lowest value for stable opera-

tion is:

66

C_OUT

>

V_OUT

1936fa

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LT1936

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APPLICATIO S I FOR ATIO

Table 2. Capacitor Vendors

Vendor

Phone

URL

Part Series

Comments

Panasonic

(714) 373-7366

www.panasonic.com

Ceramic,

Polymer,

Tantalum

EEF Series

Kemet

Sanyo

(864) 963-6300

(408) 749-9714

www.kemet.com

Ceramic,

Tantalum

T494, T495

POSCAP

www.sanyovideo.com Ceramic,

Polymer,

Tantalum

Murata

AVX

(404) 436-1300

www.murata.com

www.avxcorp.com

Ceramic

Ceramic,

Tantalum

TPS Series

Taiyo Yuden (864) 963-6300

www.taiyo-yuden.com Ceramic

parallel. Thiscapacitor(C_F)isnotpartoftheloopcompen-

sationbutisusedtofilternoiseattheswitchingfrequency,

and is required only if a phase-lead capacitor is used or if

the output capacitor has high ESR. An alternative to using

external compensation components is to use the internal

RC network by tying the COMP pin to the V_Cpin. This

reduces component count but does not provide the opti-

mum transient response when the output capacitor value

is high, and the circuit may not be stable when the output

capacitor value is low. If the internal compensation net-

work is not used, tie COMP to ground or leave it floating.

This is the minimum output capacitance required, not the

nominal capacitor value. For example, a 3.3V output

requires 20µF of output capacitance. If a small 22µF, 6.3V

ceramic capacitor is used, the circuit may be unstable

because the effective capacitance is lower than the nomi-

nal capacitance when biased at 3.3V. Look carefully at the

capacitor’s data sheet to find out what the actual capaci-

tance is under operating conditions (applied voltage and

temperature). A physically larger capacitor, or one with a

higher voltage rating, may be required.

High performance electrolytic capacitors can be used for

theoutputcapacitor.LowESRisimportant,sochooseone

that is intended for use in switching regulators. The ESR

should be specified by the supplier, and should be 0.05Ω

or less. Such a capacitor will be larger than a ceramic

capacitor and will have a larger capacitance, because the

capacitor must be large to achieve low ESR. Table 2 lists

several capacitor vendors.

Loop compensation determines the stability and transient

performance. Designing the compensation network is a

LT1936

CURRENT MODE

POWER STAGE

SW

OUTPUT

ERROR

AMPLIFIER

g

= 2mho

m

C

PL

R1

FB

–

g

=

m

250µmho

Frequency Compensation

ESR

+

1.25V

C1

+

600k

The LT1936 uses current mode control to regulate the

output. This simplifies loop compensation. In particular,

the LT1936 does not require the ESR of the output capaci-

tor for stability, so you are free to use ceramic capacitors

to achieve low output ripple and small circuit size.

C1

150pF

50k

POLYMER

OR

TANTALUM

CERAMIC

V

COMP

GND

C

R

C

R2

C

F

C

Frequency compensation is provided by the components

tied to the V_Cpin, as shown in Figure 1. Generally a

capacitor (C_C) and a resistor (R_C) in series to ground are

used. In addition, there may be lower value capacitor in

1936 F01

Figure 1. Model for Loop Response

1936fa

10

LT1936

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APPLICATIO S I FOR ATIO

bit complicated and the best values depend on the appli-

cation and in particular the type of output capacitor. A

practical approach is to start with one of the circuits in this

data sheet that is similar to your application and tune the

compensation network to optimize the performance. Sta-

bility should then be checked across all operating condi-

tions,includingloadcurrent,inputvoltageandtemperature.

The LT1375 data sheet contains a more thorough discus-

sion of loop compensation and describes how to test the

stability using a transient load.

output current proportional to the voltage at the V_Cpin.

Note that the output capacitor integrates this current, and

that the capacitor on the V_Cpin (C_C) integrates the error

amplifier output current, resulting in two poles in the loop.

In most cases a zero is required and comes from either the

output capacitor ESR or from a resistor R_Cin series with

C_C. This simple model works well as long as the value of

the inductor is not too high and the loop crossover

frequency is much lower than the switching frequency. A

phase lead capacitor (C_PL) across the feedback divider

may improve the transient response.

Figure1showsanequivalentcircuitfortheLT1936control

loop. The error amplifier is a transconductance amplifier

with finite output impedance. The power section, consist-

ing of the modulator, power switch and inductor, is

modeled as a transconductance amplifier generating an

Figure 2 compares the transient response across several

output capacitor choices and compensation schemes. In

each case the load current is stepped from 200mA to

800mA and back to 200mA.

C

= 22µF

OUT

(AVX 1210ZD226MAT)

V

OUT

100mV/DIV

(2a)

(2b)

(2c)

COMP

V

C

= 22µF ×2

OUT

V

OUT

100mV/DIV

COMP

V

C

= 150µF

OUT

(4TPC150M)

V

OUT

100mV/DIV

COMP

V

C

= 150µF

OUT

(4TPC150M)

V

OUT

100mV/DIV

(2d)

COMP

V

C

800mA

OUT

I

220k

100pF

500mA/DIV

200mA

1936 F02

50µs/DIV

Figure 2. Transient Load Response of the LT1936 with Different Output

Capacitors as the Load Current is Stepped from 200mA to 800mA. V

= 3.3V

OUT

1936fa

11

LT1936

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APPLICATIO S I FOR ATIO

BOOST Pin Considerations

Schottkydiode(suchastheONSemiMBR0540). Because

the required boost voltage increases at low temperatures,

the circuit will supply only 1A of output current when the

ambient temperature is –45°C, increasing to 1.2A at 0°C.

Also, the minimum input voltage to start the boost circuit

is higher at low temperature. See the Typical Applications

section for a 2.5V schematic and performance curves.

Capacitor C3 and diode D2 are used to generate a boost

voltage that is higher than the input voltage. In most cases

a 0.22µF capacitor and fast switching diode (such as the

1N4148 or 1N914) will work well. Figure 3 shows two

ways to arrange the boost circuit. The BOOST pin must be

at least 2.3V above the SW pin for best efficiency. For

outputs of 3V and above, the standard circuit (Figure 3a)

is best. For outputs between 2.8V and 3V, use a 0.47µF

capacitor and a Schottky diode. For lower output voltages

the boost diode can be tied to the input (Figure 3b), or to

another supply greater than 2.8V. The circuit in Figure 3a

is more efficient because the BOOST pin current comes

from a lower voltage. You must also be sure that the

maximumvoltageratingoftheBOOSTpinisnotexceeded.

The minimum operating voltage of an LT1936 application

is limited by the undervoltage lockout (~3.45V) and by the

maximum duty cycle as outlined above. For proper start-

up, the minimum input voltage is also limited by the boost

circuit.Iftheinputvoltageisrampedslowly,ortheLT1936

is turned on with its SHDN pin when the output is already

in regulation, then the boost capacitor may not be fully

charged. Because the boost capacitor is charged with the

energy stored in the inductor, the circuit will rely on some

minimum load current to get the boost circuit running

properly. This minimum load will depend on input and

output voltages, and on the arrangement of the boost

circuit. The minimum load generally goes to zero once the

circuit has started. Figure 4 shows a plot of minimum load

to start and to run as a function of input voltage. In many

cases the discharged output capacitor will present a load

to the switcher, which will allow it to start. The plots show

the worst-case situation where V_INis ramping very slowly.

For lower start-up voltage, the boost diode can be tied to

V_IN; however, this restricts the input range to one-half of

the absolute maximum rating of the BOOST pin.

A 2.5V output presents a special case. This is a popular

output voltage, and the advantage of connecting the boost

circuit to the output is that the circuit will accept a 36V

maximum input voltage rather than 20V (due to the

BOOST pin rating). However, 2.5V is marginally adequate

to support the boosted drive stage at low ambient tem-

peratures. Therefore, special care and some restrictions

onoperationarenecessarywhenpoweringtheBOOSTpin

from a 2.5V output. Minimize the voltage loss in the boost

circuitbyusinga1µFboostcapacitorandagood,lowdrop

D2

C3

BOOST

LT1936

At light loads, the inductor current becomes discontinu-

ous and the effective duty cycle can be very high. This

reduces the minimum input voltage to approximately

300mV above V_OUT. At higher load currents, the inductor

current is continuous and the duty cycle is limited by the

maximum duty cycle of the LT1936, requiring a higher

input voltage to maintain regulation.

V

IN

V

OUT

V

SW

IN

GND

V

– V ≅ V

SW OUT

BOOST

MAX V

≅ V + V

IN OUT

(3a)

D2

C3

BOOST

LT1936

Soft-Start

V

OUT

V

SW

IN

The SHDN pin can be used to soft-start the LT1936,

reducing the maximum input current during start-up. The

SHDN pin is driven through an external RC filter to create

a voltage ramp at this pin. Figure 5 shows the start-up

waveforms with and without the soft-start circuit. By

choosing a large RC time constant, the peak start-up

GND

1933 F03

V

– V ≅ V

BOOST

SW

IN

MAX V

≅ 2V

BOOST

(3b)

Figure 3. Two Circuits for Generating the Boost Voltage

1936fa

12

LT1936

U

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APPLICATIO S I FOR ATIO

Minimum Input Voltage V

= 5V

Minimum Input Voltage V

= 3.3V

OUT

8

7

6

5

4

6.0

5.5

V

T

= 5V

V

T

= 3.3V

OUT

A

OUT

A

= 25°C

L = 15µH

L = 10µH

TO START

5.0

4.5

4.0

3.5

3.0

TO RUN

100

10

LOAD CURRENT (mA)

1000

0

10

100

1000

1

LOAD CURRENT (mA)

1936 G14

1936 G13

Figure 4. The Minimum Input Voltage Depends on Output Voltage, Load Current and Boost Circuit

RUN

5V/DIV

I

RUN

SHDN

GND

IN

500mA/DIV

V

OUT

5V/DIV

1936 F05a

50µs/DIV

RUN

15k

RUN

5V/DIV

I

SHDN

GND

IN

500mA/DIV

0.22µF

V

OUT

5V/DIV

1936 F05b

0.5ms/DIV

Figure 5. To Soft-Start the LT1936, Add a Resistor and Capacitor to the SHDN Pin.

= 12V, V = 3.3V, C = 2 × 22µF, R = 3.3Ω

V

IN

OUT

LOAD

current can be reduced to the current that is required to where the output will be held high when the input to the

regulate the output, with no overshoot. Choose the value LT1936 is absent. This may occur in battery charging

of the resistor so that it can supply 60µA when the SHDN applications or in battery backup systems where a battery

pin reaches 2.3V.

or some other supply is diode OR-ed with the LT1936’s

output. If the V_INpin is allowed to float and the SHDN pin

is held high (either by a logic signal or because it is tied to

V_IN), then the LT1936’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

1936fa

Shorted and Reversed Input Protection

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

sively, an LT1936 buck regulator will tolerate a shorted

output. There is another situation to consider in systems

13

LT1936

U

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APPLICATIO S I FOR ATIO

the SHDN pin, the SW pin current will drop to essentially

zero. However, if the V_INpin is grounded while the output

is held high, then parasitic diodes inside the LT1936 can

pulllargecurrentsfromtheoutputthroughtheSWpinand

the V_INpin. Figure 6 shows a circuit that will run only when

the input voltage is present and that protects against a

shorted or reversed input.

IN

C2

GND

MINIMIZE

LT1936

C2, D1 LOOP

R4

D2

C3

R2

R1

D1

D4

MBRS140

V

IN

V

BOOST

SW

IN

L1

C1

LT1936

V

OUT

SHDN

GND

V

C

OUT

VIAS

COMP GND FB

1936 F07

BACKUP

Figure 7. A Good PCB Layout Ensures Low EMI Operation

1936 F06

High Temperature Considerations

Figure 6. Diode D4 Prevents a Shorted Input from Discharging

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

from a Reversed Input. The LT1936 Runs Only When the Input

is Present

The die temperature of the LT1936 must be lower than the

maximum rating of 125°C (150°C for the H grade). This is

generally not a concern unless the ambient 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 LT1936. The maximum load current should

be derated as the ambient temperature approaches 125°C

(150°C for the H grade).

PCB Layout

For proper operation and minimum EMI, care must be

taken during printed circuit board layout. Figure 7 shows

therecommendedcomponentplacementwithtrace,ground

plane and via locations. Note that large, switched currents

flowintheLT1936’sV_INandSWpins, thecatchdiode(D1)

and the input capacitor (C2). The loop formed by these

components should be as small as possible. These com-

ponents, along with the inductor and output capacitor,

shouldbeplacedonthesamesideofthecircuitboard, and

their connections should be made on that layer. Place a

local, unbroken ground plane below these components.

The SW and BOOST nodes should be as small as possible.

Finally, keep the FB and V_Cnodes small so that the ground

traces will shield them from the SW and BOOST nodes.

The Exposed Pad on the bottom of the package must be

soldered to ground so that the pad acts as a heat sink. To

keep thermal resistance low, extend the ground plane as

muchaspossible,andaddthermalviasunderandnearthe

LT1936 to additional ground planes within the circuit

board and on the bottom side.

ThedietemperatureiscalculatedbymultiplyingtheLT1936

power dissipation by the thermal resistance from junction

to ambient. Power dissipation within the LT1936 can be

estimated by calculating the total power loss from an

efficiency measurement and subtracting the catch diode

loss. The resulting temperature rise at full load is nearly

independentofinputvoltage. Thermalresistancedepends

on the layout of the circuit board, but values from 40°C/W

to 60°C/W are typical.

Die temperature rise was measured on a 4-layer, 5cm ×

6.5cm circuit board in still air at a load current of 1.4A. For

12V input to 3.3V output the die temperature elevation

above ambient was 26°C; for 24V in to 3.3V out the rise

was 31°C; for 12V in to 5V the rise was 31°C and for 24V

in to 5V the rise was 34°C.

1936fa

14

LT1936

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APPLICATIO S I FOR ATIO

Hot Plugging Safely

nominal input voltage, possibly exceeding the LT1936’s

rating and damaging the part. If the input supply is poorly

controlled or the user will be plugging the LT1936 into an

energizedsupply, theinputnetworkshouldbedesignedto

prevent this overshoot.

The small size, robustness and low impedance of ceramic

capacitors make them an attractive option for the input

bypass capacitor of LT1936 circuits. However, these ca-

pacitors can cause problems if the LT1936 is plugged into

a live supply (see Linear Technology Application Note 88

for a complete discussion). The low loss ceramic capaci-

tor combined with stray inductance in series with the

powersourceformsanunderdampedtankcircuit, andthe

voltage at the V_INpin of the LT1936 can ring to twice the

Figure8showsthewaveformsthatresultwhenanLT1936

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

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

a 4.7µF ceramic capacitor at the input. The input voltage

ringsashighas50Vandtheinputcurrentpeaksat26A.One

CLOSING SWITCH

DANGER

SIMULATES HOT PLUG

V

IN

I

IN

20V/DIV

V

IN

RINGING V MAY EXCEED

IN

ABSOLUTE MAXIMUM

RATING OF THE LT1936

LT1936

4.7µF

+

I

IN

LOW

STRAY

10A/DIV

IMPEDANCE

ENERGIZED

24V SUPPLY

INDUCTANCE

DUE TO 6 FEET

(2 METERS) OF

TWISTED PAIR

20µs/DIV

(8a)

V

IN

20V/DIV

LT1936

4.7µF

+

22µF

35V

AI.EI.

I

IN

10A/DIV

(8b)

20µs/DIV

0.7Ω

V

IN

20V/DIV

LT1936

4.7µF

+

0.1µF

I

IN

10A/DIV

1936 F08

20µs/DIV

(8c)

Figure 8. A Well Chosen Input Network Prevents Input Voltage Overshoot and

Ensures Reliable Operation When the LT1936 is Connected to a Live Supply

1936fa

15

LT1936

U

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APPLICATIO S I FOR ATIO

method of damping the tank circuit is to add another ca-

pacitor with a series resistor to the circuit. In Figure 8b 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

slightlyimprovetheefficiencyofthecircuit,thoughitislikely

to be the largest component in the circuit. An alternative

solution is shown in Figure 8c. A 0.7Ω 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

andlessexpensivethantheelectrolyticcapacitor. Forhigh

input voltages its impact on efficiency is minor, reducing

efficiency by one percent for a 5V output at full load oper-

ating from 24V.

Other Linear Technology Publications

Application Notes 19, 35 and 44 contain more detailed

descriptions and design information for buck regulators

and other switching regulators. The LT1376 data sheet

has a more extensive discussion of output ripple, loop

compensationandstabilitytesting.DesignNote100shows

how to generate a bipolar output supply using a buck

regulator.

U

TYPICAL APPLICATIO S

3.3V Step-Down Converter

D2

V

IN

4.5V TO 36V

C3

0.22µF

L1

10µH

V

BOOST

SW

IN

SHDN

ON OFF

V

3.3V

1.2A

OUT

C1

4.7µF

D1

R1

17.4k

LT1936

COMP

FB

GND

V

C

R2

10k

C2

47µF

1936 TA03

5V Step-Down Converter

D2

V

IN

6.3V TO 36V

C3

0.22µF

L1

15µH

V

BOOST

SW

IN

SHDN

ON OFF

V

OUT

C1

4.7µF

D1

R1

31.6k

LT1936

5V

1.2A

COMP

FB

GND

V

C

R2

10k

C2

22µF

1936 TA04

1936fa

16

LT1936

U

TYPICAL APPLICATIO S

1.8V Step-Down Converter

Efficiency, 1.8V Output

90

80

70

60

2.0

D2

V

A

= 1.8V

OUT

V

IN

T

= 25°C

3.6V TO 20V

C3

L1

V

BOOST

SW

IN

0.22µF

V

= 5V

1.5

1.0

0.5

IN

4.7µH

V

1.8V

1.3A

OUT

SHDN

ON OFF

C1

4.7µF

V

= 12V

D1

IN

R1

10k

LT1936

COMP

FB

GND

C2

47µF

×2

V

C

R2

20k

POWER LOSS

1

D1: DFLS140L

D2: 1N4148

L1: TOKO D63CB

1936 TA05a

50

0

0.5

1.5

LOAD CURRENT (A)

1936 TA05b

1.2V Step-Down Converter

Efficiency, 1.2V Output

2.0

1.5

1.0

0.5

0

80

75

70

65

60

55

50

D2

V

A

= 1.2V

OUT

V

IN

T

= 25°C

3.6V TO 20V

C3

L1

3.3µH

V

BOOST

SW

IN

0.22µF

V

= 5V

IN

V

1.2V

1.3A

OUT

SHDN

ON OFF

C1

D1

LT1936

V

= 12V

4.7µF

IN

COMP

FB

GND

C2

47µF

×2

V

C

100k

POWER LOSS

D1: DFLS140L

D2: 1N4148

1936 TA06a

L1: TOKO D63CB

0

0.5

1

1.5

LOAD CURRENT (A)

1936 TA06b

1936fa

17

LT1936

TYPICAL APPLICATIO S

U

2.5V Step-Down Converter

D2

V

IN

3.6V TO 36V

C3

1µF

L1

6.2µH

V

BOOST

SW

IN

SHDN

ON OFF

V

OUT

2.5V

1.2A

C1

4.7µF

D1

R1

11k

LT1936

T

> 0°C

A

COMP

FB

GND

V

C

R2

10k

C2

47µF

D1: DFLS140L

D2: MBRO540

L1: TOKO D63CB

1936 TA07a

Efficiency, 2.5V Output

Minimum Input Voltage

5.5

100

90

V

= 2.5V

OUT

V

A

= 2.5V

OUT

T

= 25°C

5.0

4.5

4.0

3.5

3.0

TO START

T

A

= –45°C

V

= 5V

IN

80

V

= 12V

IN

TO START

= 25°C

T

A

TO RUN

= –45°C

T

A

70

TO RUN

= 25°C

T

A

60

100

10

LOAD CURRENT (mA)

1000

1

0

0.5

1.0

1.5

LOAD CURRENT (A)

1936 TA07c

1936 TA07b

1936fa

18

LT1936

U

PACKAGE DESCRIPTION

MS8E Package

8-Lead Plastic MSOP

(Reference LTC DWG # 05-08-1662)

BOTTOM VIEW OF

EXPOSED PAD OPTION

2.06 ± 0.102

(.081 ± .004)

1

1.83 ± 0.102

(.072 ± .004)

0.889 ± 0.127

(.035 ± .005)

2.794 ± 0.102

(.110 ± .004)

5.23

(.206)

MIN

3.20 – 3.45

(.126 – .136)

2.083 ± 0.102

(.082 ± .004)

8

3.00 ± 0.102

(.118 ± .004)

(NOTE 3)

0.52

(.0205)

REF

0.65

(.0256)

BSC

0.42 ± 0.038

(.0165 ± .0015)

TYP

8

7 6

5

RECOMMENDED SOLDER PAD LAYOUT

3.00 ± 0.102

(.118 ± .004)

(NOTE 4)

4.90 ± 0.152

(.193 ± .006)

DETAIL “A”

0.254

(.010)

0° – 6° TYP

GAUGE PLANE

1

2

3

4

0.53 ± 0.152

(.021 ± .006)

1.10

(.043)

MAX

0.86

(.034)

REF

DETAIL “A”

0.18

(.007)

SEATING

PLANE

0.22 – 0.38

(.009 – .015)

TYP

0.127 ± 0.076

(.005 ± .003)

0.65

(.0256)

BSC

MSOP (MS8E) 0603

NOTE:

1. DIMENSIONS IN MILLIMETER/(INCH)

2. DRAWING NOT TO SCALE

3. DIMENSION DOES NOT INCLUDE MOLD FLASH, PROTRUSIONS OR GATE BURRS.

MOLD FLASH, PROTRUSIONS OR GATE BURRS SHALL NOT EXCEED 0.152mm (.006") PER SIDE

4. DIMENSION DOES NOT INCLUDE INTERLEAD FLASH OR PROTRUSIONS.

INTERLEAD FLASH OR PROTRUSIONS SHALL NOT EXCEED 0.152mm (.006") PER SIDE

5. LEAD COPLANARITY (BOTTOM OF LEADS AFTER FORMING) SHALL BE 0.102mm (.004") MAX

1936fa

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 represen-

tationthattheinterconnectionofitscircuitsasdescribedhereinwillnotinfringeonexistingpatentrights.

19

LT1936

U

TYPICAL APPLICATIO

2.5V Step-Down Converter

Minimum Input Voltage

D2

5.5

5.0

4.5

4.0

3.5

3.0

V

= 2.5V

V

OUT

IN

3.6V TO 20V

C3

L1

CONNECTING THE BOOST CIRCUIT TO THE

INPUT LOWERS THE MINIMUM INPUT

VOLTAGE TO RUN AND TO START TO LESS

THAN 3.7V AT ALL LOADS

V

BOOST

SW

IN

0.22µF

8.2µH

V

2.5V

1.3A

OUT

SHDN

ON OFF

C1

D1

R1

11k

LT1936

4.7µF

COMP

FB

GND

V

C

R2

10k

C2

47µF

D1: DFLS140L

D2: 1N4148

1936 TA08a

L1: TOKO D63CB

100

10

LOAD CURRENT (mA)

1000

1

1936 TA08b

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LT1940

25V, Dual 1.4A (I ), 1.1MHz, High Efficiency Step-Down

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V : 3V to 25V, V

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V : 5.5V to 60V, V = 1.20V, I = 2.5mA, I = 25µA,

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60V, 1.2A (I ), 500kHz, High Efficiency Step-Down

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60V, 1.2A (I ), 200kHz, High Efficiency Step-Down

V : 3.3V to 60V, V = 1.20V, I = 100µA, I < 1µA,

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LT3010

V : 1.5V to 80V, V

= 1.28V, I = 30µA, I < 1µA,

OUT(MIN) Q SD

IN

MS8E Package

LTC^®3407

LTC3412

LTC3414

LT3430/LT3431

Dual 600mA (I ), 1.5MHz, Synchronous Step-Down

DC/DC Converter

V : 2.5V to 5.5V, V

10-Lead MSE Package

= 0.6V, I = 40µA, I < 1µA,

OUT(MIN) Q SD

OUT

IN

2.5A (I ), 4MHz, Synchronous Step-Down

V : 2.5V to 5.5V, V

= 0.8V, I = 60µA, I < 1µA,

Q SD

OUT

IN

OUT(MIN)

DC/DC Converter

16-Lead TSSOPE Package

4A (I ), 4MHz, Synchronous Step-Down

V : 2.3V to 5.5V, V

= 0.8V, I = 64µA, I < 1µA,

Q SD

OUT

IN

OUT(MIN)

DC/DC Converter

20-Lead TSSOPE Package

60V, 2.75A (I ), 200kHz/500kHz, High Efficiency

V : 5.5V to 60V, V

IN

= 1.20V, I = 2.5mA, I = 30µA,

Q SD

OUT

OUT(MIN)

Step-Down DC/DC Converters

16-Lead TSSOPE Package

Burst Mode is a registered trademark of Linear Technology Corporation. ThinSOT is a trademark of Linear Technology Corporation.

1936fa

LT/LT 0705 REV A • PRINTED IN USA

LinearTechnology Corporation

1630 McCarthy Blvd., Milpitas, CA 95035-7417

20

●

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


型号：	LT3431
厂家：	Linear
描述：	1.4A, 500kHz Step-Down Switching Regulator 1.4A ， 500kHz的降压型开关稳压器稳压器开关
文件：	总20页 (文件大小：321K)
中文：	中文翻译
下载：	下载PDF数据表文档文件

LT3431 [Linear]

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