P
A T E N T
P
E N D I N G
LIN D
O C
#:
1570
LX1570/1571
P
HASE
M
ODULATED
AC S
YNCHRONOUS
S
ECONDARY
-S
IDE
C
ONTROLLER
T
H E
I
N F I N I T E
P
O W E R
O F
I
N N O VA T I O N
P
R E L I M I N A R Y
D
A T A
S
H E E T
DESCRIPTION
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.
K E Y F E AT U R E S
p
REPLACES COSTLY MAG-AMP CORES WITH
A LOW ON-RESISTANCE MOSFET
p
LOOK-AHEAD SWITCHING
TM
ENSURES
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
A P P L I C AT I O N S
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
PRODUCT HIGHLIGHT
Aux Output
12V/8A
A
VA I L A B L E
O
P T I O N S
Part #
LX1570
C.L.
Threshold
-0.2V
PER
P
ART
#
Application
Output
Currents
< 4A
C.S.
Option
Resistive
Sensing
LX1571
1V
Current
Output
Transformer Currents
Sensing
> 4A
V
CC
OUT
DRV
C.S.
V
FB
LX1571
COMP
C
T
GND
S.S.
PA C K A G E O R D E R I N F O R M AT I O N
T
A
(°C)
0 to 70
-40 to 85
-55 to 125
M
Plastic DIP
8-pin
LX157xCM
LX157xIM
—
DM
Plastic SOIC
8-pin
LX157xCDM
LX157xIDM
—
Y
Ceramic DIP
8-pin
—
—
LX157xMY
Note: All surface-mount packages are available in Tape & Reel.
Append the letter "T" to part number. (i.e. LX157xCDMT)
F O R F U R T H E R I N F O R M AT I O N C A L L ( 7 1 4 ) 8 9 8 - 8 1 2 1
Copyright © 1997
Rev. 0.9.3 1/97
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ESTERN
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VENUE
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ARDEN
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ROVE
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1
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