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
Not Recommended for New Design
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. 0.9.3a
9/15/2004
11861 W
ESTERN
A
VENUE
, G
ARDEN
G
ROVE
, CA. 92841
1
PRODUCT DATABOOK 1996/1997
LX1570/1571
P
HASE
M
ODULATED
AC S
YNCHRONOUS
S
ECONDARY
-S
IDE
C
ONTROLLER
P
R E L I M I N A R Y
D
A T A
S
H E E T
A B S O L U T E M A X I M U M R AT I N G S
(Note 1)
PACKAGE PIN OUTS
S.S.
V
FB
COMP
C.S.
1
2
3
4
8
7
6
5
Supply Voltage (V
CC
) .................................................................................................... 40V
Digital Inputs ....................................................................................................... -0.3 to 7V
Output Peak Current Source (500nS) ........................................................................... 1A
Output Peak Current Sink (500nS) ................................................................................ 1A
Note 1. Exceeding these ratings could cause damage to the device. All voltages are with respect
to Ground. Currents are positive into, negative out of the specified terminal.
C
T
V
CC
OUT DRV
GND
M & Y PACKAGE
(Top View)
T H ER MAL DATA
M PACKAGE:
THERMAL RESISTANCE-JUNCTION TO AMBIENT,
θ
JA
DM PACKAGE:
THERMAL RESISTANCE-JUNCTION TO AMBIENT,
θ
JA
Y PACKAGE:
THERMAL RESISTANCE-JUNCTION TO AMBIENT,
θ
JA
130°C/W
165°C/W
95°C/W
S.S.
V
FB
COMP
C.S.
1
2
3
4
8
7
6
5
C
T
V
CC
OUT DRV
GND
DM PACKAGE
(Top View)
Junction Temperature Calculation: T
J
= T
A
+ (P
D
x
θ
JA
).
The
θ
JA
numbers are guidelines for the thermal performance of the device/pc-board system.
All of the above assume no ambient airflow.
LX1571 BLOCK DIAGRAM
2.5V
0.25V
Minimum
Current Comp
PWM Latch
R
S.S.
1
V
FB
2
COMP
3
C.S.
4
0.5V
Voltage Hiccup
Comp.
Voltage Mode Hiccup
5V
Internal
Bias
CHG
CONTROL
QUICK
CHG
CONTROL
Q
Error Amp
2R
1V
R
Current Mode
Hiccup Comp
C.S. Comp
S
6
OUT DRV
1.5V
2.5V
2.5V
REF
LATCH RESET
CONTROL
7
V
CC
C
T
8
Timing / Duty Cycle
Control
DISCH
CONTROL
VALLEY
THRESHOLD
CONTROL
LATCH
SET CONTROL
6V
16V
5
GND
Q
Q
R
S
Hiccup
Latch
2
Copyright © 1997
Rev. 0.9.3a
9/15/2004
PRODUCT DATABOOK 1996/1997
LX1570/1571
P
HASE
M
ODULATED
AC S
YNCHRONOUS
S
ECONDARY
-S
IDE
C
ONTROLLER
P
R E L I M I N A R Y
D
A T A
S
H E E T
ELECTRICAL CHARACTERISTICS
(Unless otherwise specified, these specifications apply over the ranges T
A
= -55 to 125ºC for the LX1570M/1571M, T
A
= -40 to 85ºC for the
LX1570I/1571I, and T
A
= 0 to 70ºC for LX1570C/1571C. V
CC
= 15V. Typ. number represents T
A
= 25ºC value.)
Parameter
Reference Section
Initial Accuracy
Line Regulation
Temp Stability
Symbol
Test Conditions
LX1570/1571
Min. Typ.
Max.
2.475
2.500
2.525
±1
±1.5
110
115
±1
Units
V
RI
∆V
RL
∆V
RT
f
O
∆f
OL
I
CHG
I
DISCH
I
LK
V
RPP
T
A
= 25ºC, measured at F.B pin
11V < V
CC
< 25
Note 2
C
T
= , T
J
= 25°C, measured at pin 6
Over Temp, measured at pin 6
V
%
%
kHz
kHz
%
mA
mA
µA
V
V
µΩ
µA
dB
µA
µA
V
V
V/µSec
ms/µF
mA
V
V
µA
µA
V/V
V/V
mV
mV
ns
V
V
V
V
V
Timing Section
Initial Accuracy
Line Voltage Stability
Charging Current
Discharging Current
Leakage Current
Ramp PK to PK
90
85
100
100
3
3.5
4
0.6
6
0.005
0.1
70
400
400
5.1
1
35
50
TBD
65
C.S.
INPUT
= 1.5V
C.S.
INPUT
= 0V
C.S.
INPUT
= 1.5V (1571), C.S.
INPUT
= -0.4V (1570)
Error Amp / Soft Start Comp Section
Transconductance
Input Bias Current
Open Loop Gain
Output Sink Current
Output Source Current
Output HI Voltage
Output LO Voltage
Slew Rate
g
m
I
B
A
VOL
I
EA(SINK)
I
EA(SOURCE)
V
COMP-HI
V
COMP-LO
S
K
SS
I
SS-DIS
LX1570
LX1571
LX1570
LX1571
LX1570
LX1571
LX1570
LX1571
LX1570
LX1571
LX1570
LX1571
V
CSI
-0.3
I
CSB
A
CS
V
CSMIN
10% Overdrive
V
CLP
V
CLH
V
HCCP
-0.18
0.9
-13.5
2.7
-15
3
-50
250
100
-0.2
1
-0.3
1.5
2
V
FB
= 2.6V
V
FB
= 2.4V
1
60
200
200
0.8
Soft-Start Section
Soft Start Timing Factor
Soft Start Discharge Current
Current Sense Section
Input Range
Input Current
C.S. Amplifier Gain
Minimum Current Threshold Voltage
C.S. Delay to Driver Output
C.L. Pulse-By-Pulse Threshold Voltage
C.L. Hiccup Threshold Voltage
Voltage Hiccup Threshold
-0.8
6
25
1
-16.5
3.3
200
-0.22
1.1
Note 2. Although this parameter is guaranteed, it is not 100% tested in production.
Copyright © 1997
Rev. 0.9.3a
9/15/2004
3
PRODUCT DATABOOK 1996/1997
LX1570/1571
P
HASE
M
ODULATED
AC S
YNCHRONOUS
S
ECONDARY
-S
IDE
C
ONTROLLER
P
R E L I M I N A R Y
D
A T A
S
H E E T
ELECTRICAL CHARACTERISTICS
Parameter
PWM Section
E.A. Output to PWM Drive Offset
Fixed Duty Cycle
V
OFS
D
t
R
/ t
F
V
DH
V
DL
V
DPD
V
ST
V
OFF
V
H
I
Qd
I
ST
Out Freq = 100kHz, C
L
= 0
C
L
= 1000pF
I
SOURCE
= 200mA, V
CS
= 0V, V
FB
= 2.3V
I
SINK
= 200mA, V
CS
= 1.2V, V
FB
= 2.3V
V
CC
= 0V, I
PULL UP
= 2mA
(Con't.)
Symbol
Test Conditions
LX1570/1571
Min. Typ.
Max.
1.7
52
2.0
54
50
13.5
0.8
1
15
9
5.5
16
10
6
18
150
17
11
6.5
30
250
2.4
56
Units
V
%
ns
V
V
V
V
V
V
mA
µA
Output Drive Section
Rise / Fall Time
Output HI
Output LO
Output Pull Down
UVLO Section
Start-Up Threshold
Turn Off Threshold
Hysterises
Supply Current Section
Dynamic Operating Current
Start-Up Current
FUNCTIONAL PIN DESCRIPTION
Pin
S.S.
#
1
Description
This pin acts as the soft-start pin. A capacitor connected from this pin to GND allows slow ramp up of the NI input
resulting in output soft start during start up. This pin is clamped to the internal voltage reference during the normal
operation and sets the reference for the feedback regulator.
This pin is the inverting input of the Error Amplifier. It is normally connected to the switching power supply output
through a resistor divider to program the power supply voltage. This pin instead of the NI pin is internally trimed to
1% tolerance to include the offset voltage error of the error amp.
This pin is the Error Amplifier output and is made available for loop compensation. Typically a series R&C network
is connected from this pin to GND.
A voltage proportional to the inductor current is sensed by an external sense resistor (1570) or current transformer (1571)
in series with the return line and is connected to this pin. The output drive is terminated and latched off when this voltage
amplified by the internal gain (see option table) exceeds the voltage set by the E.A output voltage. The maximum
allowable voltage at this pin during normal operation is -0.8V typ for LX1570 and 6V typ for LX1571.
This pin is combined control circuitry and power GND. All other pins must be positive with respect to this pin, except
for C.S pin.
This pin drives a gate drive transformer which drives the power mosfet. A Schottky diode such as 1N5817 must be
connected from this pin to GND in order to prevent the substrate diode conduction.
This pin is the positive supply voltage for the control IC. A high frequency capacitor must be closely placed and
connected from this pin to GND to provide the turn-on and turn-off peak currents required for fast switching of the power
Mosfet.
The free running oscillator frequency is programmed by connecting a capacitor from this pin to GND.
V
FB
2
COMP
C.S.
3
4
GND
OUT
DRV
V
CC
5
6
7
C
T
8
4
Copyright © 1997
Rev. 0.9.3a
9/15/2004
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