LTC1735C-1 View Datasheet(PDF) - Linear Technology

Part Name

Description

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LTC1735C-1

High Efficiency Synchronous Step-Down Switching Regulator

Linear Technology 

LTC1735-1

W

FU CTIO AL DIAGRA

VIN

INTVCC

100k

COSC

INTVCC

13 VIN

0.8V UVL

REF

PGOOD 4

COSC 1 SGND 8

C

SYNC

OSC

0.17µA

1.2V

–

0.8V +

FC

F

BOOST

15

FORCE BOT

TG

TOP

16

0.74V

0.86V

OV

+

–

DROP

OUT

SW

DET

BOT

SWITCH

14

S

Q

R

TOP ON

0.55V

+B

LOGIC

VOSENSE

7

R1

R2

VFB

0.8V

– gm =1.3m

EA

+

SD

1.2µA

6V

RUN

SOFT-

START

+

OVER-

CURRENT

LATCHOFF

0.86V

2.4V

2k

ICMP

I1 – +

BURST

DISABLE

FC

4(VFB)

A

BUFFERED

ITH

SLOPE COMP

45k

+–

3mV

30k

–

SD

45k

– IREV

+ I2

INTVCC

4.8V

30k

BOT

VIN

5.2V

LDO

REG

+

–

RUN/SS 2

RC

3 ITH

SENSE+ 6

5 SENSE–

EXTVCC 9

INTVCC

DB

CB

D1

INTVCC

12

+

CINTVCC

BG

11

PGND

10

CSS

CC

+

CIN

L

RSENSE

+

VOUT

COUT

1735-1 FD

U

OPERATIO (Refer to Functional Diagram)

Main Control Loop:

The LTC1735-1 uses a constant frequency, current mode

step-down architecture. During normal operation, the top

MOSFET is turned on each cycle when the oscillator sets

the RS latch, and turned off when the main current

comparator I1 resets the RS latch. The peak inductor

current at which I1 resets the RS latch is controlled by the

voltage on Pin ITH, which is the output of error amplifier

EA. Pin VOSENSE, described in the Pin Functions, allows EA

to receive an output feedback voltage VFB from the external

resistive divider. When the load current increases, it

causes a slight decrease in VFB relative to the 0.8V refer-

ence, which in turn causes the ITH voltage to increase until

the average inductor current matches the new load cur-

rent. While the top MOSFET is off, the bottom MOSFET is

turned on until either the inductor current starts to reverse,

as indicated by current comparator I2, or the beginning of

the next cycle.

The top MOSFET driver is powered from a floating boot-

strap capacitor CB. This capacitor is normally recharged

from INTVCC through an external Schottky diode when the

top MOSFET is turned off. As VIN decreases towards VOUT,

the converter will attempt to turn on the top MOSFET con-

tinuously (“dropout’’). A dropout counter detects this con-

dition and forces the top MOSFET to turn off for about 500ns

every tenth cycle to recharge the bootstrap capacitor.

8

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