Microsemi Corporation LX1571MY, LX1571IM, LX1571IDM, LX1571CM, LX1571CDM Datasheet

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PACKAGE ORDER INFORMATION

(°C)

Plastic DIP 8-pin

0 to 70 LX157xCM LX157xCDM —

-40 to 85 LX157xIM LX157xIDM —

-55 to 125 — — LX157xMY

Plastic SOIC 8-pin

Ceramic DIP 8-pin

DESCRIPTION KEY FEATURES

p REPLACES COSTLY MAG-AMP CORES WITH

A LOW ON-RESISTANCE MOSFET

p LOOK-AHEAD SWITCHING

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

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.

PRODUCT HIGHLIGHT

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

LX1571

OUT DRV

C.S.

COMP

S.S.

GND

Aux Outpu

12V/8A

HASE MODULATED

AC S

YNCHRONOUS SECONDARY-SIDE CONTROLLER

P RELIMINARY DATA SHEET

THE INFINITE POWER OF INNOVATION

FOR FURTHER INFORMATION CALL (714) 898-8121

11861 WESTERN AVENUE, GARDEN GROVE, CA. 92841

LX1570/1571

LIN DOC #:

1570

ATENT PENDING

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

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

A VAILABLE OPTIONS PER PART #

Part #

C.L. C.S.

Application

Threshold

Option

Resistive

Output

LX1570 -0.2V

Sensing

Currents

< 4A

Current Output

LX1571 1V

Transformer

Currents

Sensing > 4A

HASE MODULATED

AC S

YNCHRONOUS SECONDARY-SIDE CONTROLLER

LX1570/1571

PRODUCT DATABOOK 1996/1997

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

PACKAGE PIN OUTS

OUT DRV GND

S.S.

COMP

C.S.

1 8

M & Y PACKAGE

(Top View)

DM PACKAGE

(Top View)

OUT DRV GND

S.S.

COMP

C.S.

1 8

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.

LX1571 BLOCK DIAGRAM

THERMAL DATA

M PACKAGE:

THERMAL RESISTANCE-JUNCTION TO AMBIENT,

θθ

95°C/W

DM PACKAGE:

THERMAL RESISTANCE-JUNCTION TO AMBIENT,

θθ

165°C/W

Y PACKAGE:

THERMAL RESISTANCE-JUNCTION TO AMBIENT,

θθ

130°C/W

Junction Temperature Calculation: T

= TA + (P

x θ

The θ

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

All of the above assume no ambient airflow.

Error Amp

Voltage Hiccup

Comp.

2.5V

Minimum Current Comp

C.S. Comp

Current Mode Hiccup Comp

RSQ

PWM Latch

GND

OUT DRV

RSQQ

Hiccup

Latch

Internal

Bias

2.5V REF

2.5V

16V

1.5V

0.25V

0.5V

S.S.

OMP

C.S.

Voltage Mode Hiccup

Timing / Duty Cycle

Control

CHG

CONTROL

QUICK

CHG

CONTROL

LATCH RESET CONTROL

LATCH SET CONTROL

VALLEY

THRESHOLD

CONTROL

DISCH

CONTROL

HASE MODULATED

AC S

YNCHRONOUS SECONDARY-SIDE CONTROLLER

LX1570/1571

PRODUCT DATABOOK 1996/1997

P RELIMINARY DATA SHEET

Input Range LX1570 V

CSI

LX1571

Input Current LX1570 I

CSB

LX1571

C.S. Amplifier Gain LX1570 A

LX1571

Minimum Current Threshold Voltage LX1570 V

CSMIN

LX1571 C.S. Delay to Driver Output 10% Overdrive

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

CLP

LX1571

C.L. Hiccup Threshold Voltage LX1570 V

CLH

LX1571 Voltage Hiccup Threshold V

HCCP

Initial Accuracy fOCT = , TJ = 25°C, measured at pin 6

Over Temp, measured at pin 6

Line Voltage Stability ∆f

Charging Current I

CHG

Discharging Current I

DISCH

Leakage Current I

C.S.

INPUT

= 1.5V

Ramp PK to PK V

RPP

C.S.

INPUT

= 0V

C.S.

INPUT

= 1.5V (1571), C.S.

INPUT

= -0.4V (1570)

ELECTRICAL CHARACTERISTICS

Reference Section

Parameter

Symbol

Test Conditions

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

Line Regulation ∆V

11V < VCC < 25

Temp Stability ∆V

Note 2

Units

LX1570/1571

Min. Typ. Max.

2.475 2.500 2.525 V ±1 %

±1.5 %

Timing Section

90 100 110 kHz

85 100 115 kHz

±1 %

3mA

3.5 mA 4µA

0.6 V

Error Amp / Soft Start Comp Section

Transconductance g

Input Bias Current I

Open Loop Gain A

VOL

Output Sink Current I

EA(SINK)VFB

= 2.6V

Output Source Current I

EA(SOURCE)VFB

= 2.4V

Output HI Voltage V

COMP-HI

Output LO Voltage V

COMP-LO

Slew Rate S

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

(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

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

Soft-Start Section

Soft Start Timing Factor K

Soft Start Discharge Current I

SS-DIS

35 50 65 ms/µF

TBD mA

Current Sense Section

-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

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

HASE MODULATED

AC S

YNCHRONOUS SECONDARY-SIDE CONTROLLER

LX1570/1571

PRODUCT DATABOOK 1996/1997

RELIMINARY DAT A SHEET

S.S. 1

COMP 3

C.S. 4

GND 5

OUT 6 DRV

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.

Pin # Description

FUNCTIONAL PIN DESCRIPTIONFUNCTIONAL PIN DESCRIPTION

FUNCTIONAL PIN DESCRIPTION

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

DHISOURCE

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

Output LO V

DLISINK

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

Output Pull Down V

DPDVCC

= 0V, I

PULL UP

= 2mA

ELECTRICAL CHARACTERISTICS (Con't.)

Parameter

Symbol

Test Conditions Units

LX1570/1571

Min. Typ. Max.

1.7 2.0 2.4 V 52 54 56 %

E.A. Output to PWM Drive Offset V

OFS

Fixed Duty Cycle D

UVLO Section

15 16 17 V

91011V

5.5 6 6.5 V

Start-Up Threshold V

Turn Off Threshold V

OFF

Hysterises V

Supply Current Section

Dynamic Operating Current I

Out Freq = 100kHz, CL = 0

Start-Up Current I

18 30 mA

150 250 µA

50 ns

13.5 V

0.8 V

PWM Section

Output Drive Section

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