LTC1736IG データシートの表示（PDF） - Linear Technology

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LTC1736IG

5-Bit Adjustable High Efficiency Synchronous Step-Down Switching Regulator

Linear Technology 

LTC1736

W

FU CTIO AL DIAGRA

47pF

VOSENSE

10

VFB

R2

10k

9

R1

SGND

5

VIDVCC

INTVCC

16

40k

VID4 15

VID3 14

VID2 13

VID1 12

VID0 11

6 PGOOD

COSC

COSC 1

0.74V

0.86V

OV

+

–

VFB

0.8V

– gm =1.3mΩ

EA

+

SD

1.2µA

6V

VID

DECODER

RUN/SS 2

CSS

RUN

SOFT

START

+

OVER-

CURRENT

LATCH-OFF

RC

CC

VIN

R4

21 VIN

SYNC

OSC

0.17µA

C

1.2V

R3

0.8V UVL

REF

4 FCB

FC

–F

0.8V +

FORCE BOT

TOP

DROP

OUT

DET

BOT

S

Q

R

TOP ON

0.55V

+B

SWITCH

LOGIC

0.86V

2.4V

2k

ICMP

I1 – +

BURST

DISABLE

FC

4(VFB)

A

BUFFERED

ITH

SLOPE COMP

–

SD

45k

45k

– IREV

+ – I2

BOT

+

3mV

VIN

5.2V

INTVCC LDO

REG

4.8V +

30k

30k

–

INTVCC

BOOST

DB

23

CB

TG

24

SW

22

D1

INTVCC

20

+

CINTVCC

BG

19

3 ITH

SENSE+ 8

7 SENSE–

EXTVCC 17

PGND

18

+

CIN

VSEC

+

•

•

+

CSEC

VOUT

COUT

RSENSE

1736 FD

U

OPERATIO (Refer to Functional Diagram)

Main Control Loop

The LTC1736 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 the error

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

allows EA to receive an output feedback voltage VFB from

the internal resistive divider. When the load current

increases, it causes a slight decrease in VFB relative to the

0.8V reference, which in turn causes the ITH voltage to

increase until the average inductor current matches the

new load current. While the top MOSFET is off, the

bottom MOSFET is turned on until either the inductor

current starts to reverse, as indicated by current com-

parator I2, or the beginning of the next cycle.

The top MOSFET driver is powered from a floating

bootstrap capacitor CB. This capacitor is normally re-

charged 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 continuously (‘’dropout’’). A dropout counter

detects this condition and forces the top MOSFET to turn

off for about 500ns every tenth cycle to recharge the

bootstrap capacitor.

8

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