P ACKAGE ORDER INFORMA TION

T

Plastic DIP

8-pin

0 to 70

LX157xCM

LX157xCDM

—

-40 to 85

LX157xIM

LX157xIDM

—

-55 to 125

—

—

LX157xMY

M

Plastic SOIC

8-pin

DM

Ceramic DIP

8-pin

Y

DESCRIPTION

KEY FEA TURES

p REPLACES COSTLY MAG-AMP CORES WITH

A LOW ON-RESISTANCE MOSFET

SWITCH TURN ON BEFORE THE AC INPUT

TO ACHIEVE 100% ENERGY TRANSFER

p LOWER OVERALL SYSTEM COST

p LOWER PEAK CURRENT STRESS ON THE

PRIMARY SWITCH

p ALLOWS HIGHER OPERATING FREQUENCY

AND SMALLER OUTPUT INDUCTOR

p EASY SHORT-CIRCUIT PROTECTION

p CURRENT MODE APPROACH ACHIEVES

EXCELLENT DYNAMIC RESPONSE

The LX1570/71 series of controller ICs are

designed to provide all control functions in

a secondary-side regulator for isolated auxil-

iary or secondary power supplies. Auxiliary

or secondary-side controllers are used in a

variety of applications including multiple

output off-line power supplies, commonly

found in desktop computers, as well as tele-

communications applications. Although they

can be used in all secondary output applica-

tions requiring precision regulation, they are

mainly optimized for outputs delivering more

than 3A current where standard three-termi-

nal regulators lack the desired efficiency. For

these applications, the Mag Amp regulators

have traditionally been used. However, Mag

Amps have several disadvantages. First, be-

cause they have to withstand the maximum

input voltage during a short-circuit condition,

they are "over designed", typically by 2 times,

increasing the cost and size of the power

supply. Second, Mag Amps are inherently

leading edge modulators, so they can only

approach a certain maximum duty cycle, lim-

ited by the minimum delay and the mag-

netic BH loop characteristic of the Mag Amp

core. This forces an increase in the size of

the main transformer as well as the output

inductor, resulting in higher overall system

cost. The LX1570/71 eliminates all the

disadvantages of the Mag Amp approach

as well as improving system perfor-

mance and reducing overall system cost.

The LX1570/71 is a current mode control-

ler IC that controls the duty cycle of a switch

in series with the secondary AC output of

the power transformer in buck-derived ap-

plications, such as forward or bridge topolo-

gies. It offers features such as 100% duty

cycle operation for maximum energy trans-

fer, pulse-by-pulse and hiccup current limit-

ing with long off-time between the cycles

for reduced power dissipation, high-fre-

quency operation for smaller magnetics, soft-

start, and current mode control for excel-

lent dynamic response.

PRODUCT HIGHLIGHT

APPLICA TIONS

s SECONDARY-SIDE REGULATOR IN OFF-LINE

POWER SUPPLIES

s COMPUTER POWER SUPPLIES, 3.3V OUTPUT

FOR NEW LOW-VOLTAGE PROCESSORS

AND MEMORIES

s TELECOMMUNICATION AND MILITARY

DC/DC CONVERTERS

LX1571

V

CC

OUT

DRV

C.S.

V

FB

COMP

S.S.

GND

C

T

Aux Output

12V/8A

OF

Copyright © 1997

Rev. 0.9.3

1/97

FOR FUR THER INFORMA TION CALL (714) 898-8121

LX1570/1571

1

Note: All surface-mount packages are available in Tape & Reel.

Append the letter "T" to part number. (i.e. LX157xCDMT)

Part #

C.L.

C.S.

Application

Threshold

Option

Resistive

Output

LX1570

-0.2V

Sensing

Currents

< 4A

Current

Output

LX1571

1V

Transformer Currents

Sensing

> 4A