LT3971AIMSE-TRPBF 데이터 시트보기 (PDF) - Linear Technology

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LT3971AIMSE-TRPBF

38V, 1.3A, 2MHz Step-Down Regulator with 2.2µA Quiescent Current

Linear Technology 

BLOCK DIAGRAM

LT3971A/LT3971A-5

VIN

C1

RT

VIN

INTERNAL 1.19V REF

1V +

– SHDN

EN

RT

–

+

Σ

SLOPE COMP

OSCILLATOR

200kHz TO 2MHz

SYNC

PG

1.09V

ERROR AMP

+

–

Burst Mode

DETECT

VC CLAMP

VC

C5

GND

FB

R2

R1

R2 R1

LT3971A-5

ONLY

VOUT

SWITCH

LATCH

R

Q

S

BD

BOOST

SW

1μA

SS

SHDN

LT3971A

ONLY C5

LT3971A-5: R1 = 7.62M, R2 = 2.38M

C3

L1

D1

VOUT

C2

R3

C4

3971A BD

OPERATION

The LT3971A is a constant frequency, current mode step-

down regulator. An oscillator, with frequency set by RT,

sets an RS flip-flop, turning on the internal power switch.

An amplifier and comparator monitor the current flowing

between the VIN and SW pins, turning the switch off when

this current reaches a level determined by the voltage at

VC (see Block Diagram). An error amplifier measures the

output voltage through an external resistor divider tied to

the FB pin and servos the VC node. If the error amplifier’s

output increases, more current is delivered to the output;

if it decreases, less current is delivered. An active clamp

on the VC node provides current limit. The VC node is

also clamped by the voltage on the SS pin; soft-start is

implemented by generating a voltage ramp at the SS pin

using an external capacitor.

If the EN pin is low, the LT3971A is shut down and draws

700nA from the input. When the EN pin exceeds 1V, the

switching regulator will become active.

The switch driver operates from either VIN or from the

BOOST pin. An external capacitor 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.

To further optimize efficiency, the LT3971A automatically

switches to Burst Mode operation in light load situations.

Between bursts, all circuitry associated with controlling

the output switch is shut down, reducing the input supply

current to 1.7μA. In a typical application, 2.8μA will be

consumed from the supply when regulating with no load.

The oscillator reduces the LT3971A’s operating frequency

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

back helps to control the output current during start-up

and overload.

The LT3971A contains a power good comparator which

trips when the FB pin is at 91% of its regulated value. The

PG output is an open-drain transistor that is off when the

output is in regulation, allowing an external resistor to pull

the PG pin high. Power good is valid when the LT3971A

is enabled and VIN is above 4.3V.

3971af

9

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