LINFINITY LX1570, LX11571 DATA SHEET

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YNCHRONOUS SECONDARY-SIDE CONTROLLER

LIN DOC #:

1570

LX1570/1571

THE INFINITE POWER OF INNOVATION

DESCRIPTION KEY FEATURES

The LX1570/71 series of controller ICs are
designed to provide all control functions in
a secondary-side regulator for isolated auxil­iary 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 tele­communications applications. Although they
can be used in all secondary output applica­tions requiring precision regulation, they are
mainly optimized for outputs delivering more
than 3A current where standard three-termi­nal regulators lack the desired efficiency. For
these applications, the Mag Amp regulators
have traditionally been used. However, Mag
Amps have several disadvantages. First, be­cause 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, lim­ited by the minimum delay and the mag­netic 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 perfor­mance and reducing overall system cost.

The LX1570/71 is a current mode control­ler IC that controls the duty cycle of a switch
in series with the secondary AC output of
the power transformer in buck-derived ap­plications, such as forward or bridge topolo­gies. It offers features such as 100% duty
cycle operation for maximum energy trans­fer, pulse-by-pulse and hiccup current limit­ing with long off-time between the cycles
for reduced power dissipation, high-fre­quency operation for smaller magnetics, soft­start, and current mode control for excel­lent dynamic response.

P RELIMINARY DATA SHEET

p REPLACES COSTLY MAG-AMP CORES WITH

A LOW ON-RESISTANCE MOSFET

p LOOK-AHEAD SWITCHING

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

APPLICATIONS

■ SECONDARY-SIDE REGULATOR IN OFF-LINE

POWER SUPPLIES

■ COMPUTER POWER SUPPLIES, 3.3V OUTPUT

FOR NEW LOW-VOLTAGE PROCESSORS
AND MEMORIES

■ TELECOMMUNICATION AND MILITARY

DC/DC CONVERTERS

TM

ENSURES

PRODUCT HIGHLIGHT

Aux Outpu
12V/8A

A VAILABLE OPTIONS PER PART #

Part #

LX1570 -0.2V

LX1571 1V

OUT
DRV

V

CC

C.S.

V

FB

LX1571

C

COMP

GND

S.S.

T

PACKAGE ORDER INFORMATION

T

(°C)

A

Plastic DIP

M

8-pin

Plastic SOIC

DM

8-pin

Ceramic DIP

Y

8-pin

0 to 70 LX157xCM LX157xCDM —

-40 to 85 LX157xIM LX157xIDM —

-55 to 125 — — LX157xMY

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

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

C.L. C.S.

Threshold

Transformer

Option

Resistive

Sensing

Application

Output

Currents

< 4A

Current Output

Currents

Sensing > 4A

Copyright © 1997
Rev. 0.9.3 1/97

FOR FURTHER INFORMATION CALL [phone]

11861 WESTERN AVENUE, GARDEN GROVE, CA. 92841

1

LX1570/1571

C

PRODUCT DATABOOK 1996/1997

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YNCHRONOUS SECONDARY-SIDE CONTROLLER

RELIMINARY DAT A SHEET

ABSOLUTE MAXIMUM RATINGS (Note 1)

Supply Voltage (VCC) .................................................................................................... 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.

THERMAL DATA

M PACKAGE:

THERMAL RESISTANCE-JUNCTION TO AMBIENT,

θθ

θ

θθ

JA

95°C/W

DM PACKAGE:

THERMAL RESISTANCE-JUNCTION TO AMBIENT,

θθ

θ

θθ

JA

165°C/W

Y PACKAGE:

D

x θ

θθ

θ

θθ

JA

).

JA

THERMAL RESISTANCE-JUNCTION TO AMBIENT,

Junction Temperature Calculation: T

numbers are guidelines for the thermal performance of the device/pc-board system.

The θ

JA

All of the above assume no ambient airflow.

= TA + (P

J

130°C/W

LX1571 BLOCK DIAGRAM

Minimum
Current Comp

C.S. Comp

Current Mode
Hiccup Comp

S.S.

V

OMP

C.S.

2.5V

0.5V

Error Amp

Voltage Hiccup

Comp.

1

2

FB

3

4

0.25V

2R

1V

R

1.5V

PWM Latch

RSQ

PACKAGE PIN OUTS

S.S.

1 8

V

27

FB

COMP

36

C.S.

45

M & Y PACKAGE

(Top View)

1 8

S.S.

27

V

FB

COMP

C.S.

2.5V

36

45

DM PACKAGE

(Top View)

2.5V
REF

C
V
OUT DRV
GND

6

OUT DRV

T

CC

C

T

V

CC

OUT DRV
GND

Voltage Mode Hiccup

QUICK

CHG

CONTROL

DISCH

CONTROL

Timing / Duty Cycle

8

C

T

CHG

CONTROL

Control

VALLEY

THRESHOLD

CONTROL

LATCH RESET
CONTROL

LATCH
SET CONTROL

Hiccup

Latch

RSQQ

2

5V

Internal

Bias

V

7

6V

16V

CC

5

GND

Copyright © 1997
Rev. 0.9.3 1/97

PRODUCT DATABOOK 1996/1997

LX1570/1571

P

HASE MODULATED

P RELIMINARY DATA SHEET

ELECTRICAL CHARACTERISTICS

(Unless otherwise specified, these specifications apply over the ranges TA = -55 to 125ºC for the LX1570M/1571M, TA = -40 to 85ºC for the
LX1570I/1571I, and T

Reference Section

Initial Accuracy VRITA = 25ºC, measured at F.B pin

Line Regulation ∆V
Temp Stability ∆V

Timing Section

Initial Accuracy f

Line Voltage Stability ∆f

Charging Current I

Discharging Current I
Leakage Current I

Ramp PK to PK V

Error Amp / Soft Start Comp Section

Transconductance g

Input Bias Current I
Open Loop Gain A

Output Sink Current I

Output Source Current I
Output HI Voltage V

Output LO Voltage V

Slew Rate S

Soft-Start Section

Soft Start Timing Factor K

Soft Start Discharge Current I

Current Sense Section

Input Range LX1570 V

Input Current LX1570 I

C.S. Amplifier Gain LX1570 A

Minimum Current Threshold Voltage LX1570 V

C.S. Delay to Driver Output 10% Overdrive

C.L. Pulse-By-Pulse Threshold Voltage LX1570 V

C.L. Hiccup Threshold Voltage LX1570 V

Voltage Hiccup Threshold V

Note 2. Although this parameter is guaranteed, it is not 100% tested in production.

= 0 to 70ºC for LX1570C/1571C. VCC = 15V. Typ. number represents TA = 25ºC value.)

A

Parameter

LX1571

LX1571

LX1571

LX1571

LX1571

LX1571

AC S

Symbol

RL

RT

O

OL

CHG

DISCH

LK

RPP

m

B

VOL

EA(SINK)VFB

EA(SOURCE)VFB

COMP-HI

COMP-LO

SS

SS-DIS

CSI

CSB

CS

CSMIN

CLP

CLH

HCCP

YNCHRONOUS SECONDARY-SIDE CONTROLLER

Test Conditions

11V < VCC < 25

Note 2

CT = , TJ = 25°C, measured at pin 6

Over Temp, measured at pin 6

C.S.

= 1.5V

INPUT

C.S.

= 0V

INPUT

C.S.

= 1.5V (1571), C.S.

INPUT

= 2.6V

= 2.4V

= -0.4V (1570)

INPUT

LX1570/1571

Min. Typ. Max.

2.475 2.500 2.525 V
±1 %

±1.5 %

90 100 110 kHz

85 100 115 kHz

±1 %

3mA

3.5 mA
4µA

0.6 V

6V

0.005 µΩ

0.1 1 µ A

60 70 dB

200 400 µA
200 400 µA

5.1 V

0.8 V

1 V/µSec

35 50 65 ms/µF

TBD mA

-0.8 V

-0.3 6 V
25 µA

1µA

-13.5 -15 -16.5 V/V

2.7 3 3.3 V/V

-50 mV
250 mV

100 200 ns

-0.18 -0.2 -0.22 V

0.9 1 1.1 V

-0.3 V

1.5 V

2V

Units

Copyright © 1997
Rev. 0.9.3 1/97

3

PRODUCT DATABOOK 1996/1997

LX1570/1571

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ELECTRICAL CHARACTERISTICS (Con't.)

Parameter

PWM Section

E.A. Output to PWM Drive Offset V
Fixed Duty Cycle D

Output Drive Section

Rise / Fall Time tR / tFCL = 1000pF
Output HI V

Output LO V
Output Pull Down V

UVLO Section

Start-Up Threshold V
Turn Off Threshold V

Hysterises V

Supply Current Section

Dynamic Operating Current I

Start-Up Current I

AC S

RELIMINARY DAT A SHEET

Symbol

OFS

DHISOURCE

DLISINK

DPDVCC

ST

OFF

H

Qd

ST

YNCHRONOUS SECONDARY-SIDE CONTROLLER

Test Conditions Units

= 200mA, VCS = 0V, VFB = 2.3V

= 200mA, VCS = 1.2V, VFB = 2.3V

= 0V, I

Out Freq = 100kHz, CL = 0

PULL UP

= 2mA

LX1570/1571

Min. Typ. Max.

1.7 2.0 2.4 V
52 54 56 %

50 ns

13.5 V

0.8 V

1V

15 16 17 V

91011V

5.5 6 6.5 V

18 30 mA

150 250 µA

Pin # Description

S.S. 1

V

FB

COMP 3

C.S. 4

GND 5

OUT 6
DRV

V

CC

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

2

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.

7

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.

FUNCTIONAL PIN DESCRIPTIONFUNCTIONAL PIN DESCRIPTION

FUNCTIONAL PIN DESCRIPTION

FUNCTIONAL PIN DESCRIPTIONFUNCTIONAL PIN DESCRIPTION

C

4

8

T

The free running oscillator frequency is programmed by connecting a capacitor from this pin to GND.

Copyright © 1997
Rev. 0.9.3 1/97