LT3435(RevA) 데이터 시트보기 (PDF) - Linear Technology

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LT3435
(Rev.:RevA)

High Voltage 3A, 500kHz Step-Down Switching Regulator with 100µA Quiescent Current

Linear Technology 

LT3435

BLOCK DIAGRA

VIN

4

BIAS

10

SYNC

14

SHDN

15

9 CSS

INTERNAL REF

2.4V

UNDERVOLTAGE

LOCKOUT

THERMAL

SHUTDOWN

+

SHDN

COMP

–

1.3V

SOFT-START

FOLDBACK

DETECT

SLOPE

COMP

Σ

500kHz

OSCILLATOR

ANTISLOPE

COMP

BURST MODE

DETECT

VC

CLAMP

CURRENT

COMP

R

SWITCH

LATCH

Q

S

DRIVER

CIRCUITRY

BOOST

6

SW

2

FB

12

VC

11

PGFB

13

CT

7

1.25V

1.12V

–

ERROR

AMP

+

+

PG

COMP

–

1.2V CT

CLAMP

Figure 1. LT3435 Block Diagram

PG

16

GND 17

PGND 8

3435 BD

The LT3435 is a constant frequency, current mode buck

converter. This means that there is an internal clock and two

feedback loops that control the duty cycle of the power

switch. In addition to the normal error amplifier, there is a

current sense amplifier that monitors switch current on a

cycle-by-cycle basis. A switch cycle starts with an oscilla-

tor pulse which sets the RS latch to turn the switch on. When

switch current reaches a level set by the current compara-

tor the latch is reset and the switch turns off. Output volt-

age control is obtained by using the output of the error

amplifier to set the switch current trip point. This technique

means that the error amplifier commands current to be

8

delivered to the output rather than voltage. A voltage fed

system will have low phase shift up to the resonant fre-

quency of the inductor and output capacitor, then an abrupt

180° shift will occur. The current fed system will have 90°

phase shift at a much lower frequency, but will not have the

additional 90° shift until well beyond the LC resonant fre-

quency. This makes it much easier to frequency compen-

sate the feedback loop and also gives much quicker tran-

sient response.

Most of the circuitry of the LT3435 operates from an

internal 2.4V bias line. The bias regulator normally draws

3435fa

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