LTC1698 데이터 시트보기 (PDF) - Linear Technology

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LTC1698

Isolated Secondary Synchronous Rectifier Controller

Linear Technology 

LTC1698

BLOCK DIAGRA

VAUX

14

SYNC

15

MARGIN

7

AUX GEN

SYNC IN

1

VDD

VCC GEN

VCC

RMARGIN

I-TO-V CONVERTER

FG

16

CG

2

VREF

BANDGAP

OPTODRV

5

OPTO

20k

100k

PWRGD MPWRGD

10

PWRGD

–+

VFB 0.94VREF

MILIM

RILIM

3k

ROVP

3k

±5% VREF

ERR

VFB

8

VCOMP

6

ISNSGND

11

ILIM

25mV

+

ISNS

12

ICOMP

13

VREF

OVP

OVPIN

9

1698 BD

U

OPERATIO (Refer to Block Diagram)

The LTC1698 is a secondary-side synchronous rectifier

controller designed to work with the LT3781 primary-side

synchronous controller chip to form an isolated synchro-

nous forward converter. This chip set uses a dual transis-

tor forward topology that is predominantly used in distrib-

uted power supply systems where isolated low voltages

are needed to power complex electronic equipment. The

primary stage is a current mode, fixed frequency forward

converter and provides the typical PWM operation. A

power transformer is used to provide the functions of

input/output isolation and voltage step-down to achieve

the required low output voltage. Instead of using typical

8

Schottky diodes, synchronous rectification on the sec-

ondary offers isolation with high efficiency. It supplies

high power without the need of bulky heat sinks, which is

often a problem in any space constrained application.

The LTC1698 not only provides synchronous drivers for

the external MOSFETs, it comes with other housekeeping

functions performed on the secondary side of the power

supply, all within a single integrated controller. Figure 1

shows the typical chip-set application. Upon power up, the

LTC1698’s VDD input is low, the gate drivers TG and BG are

both at the ground potential. The secondary forward and

1698f

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