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

P A T E N T P E N D I N G LIN D O C #: 1570

LX1570/1571

PHASE MODULATED AC SYNCHRONOUS SECONDARY-SIDE CONTROLLER

T H E I N F I N I T E P O W E R O F I N N O V A T I O N

P R E L I M I N A R Y D A T A S H E E T

D E S C R I P T I O N

K E Y F E AT U R E S

The LX1570/71 series of controller ICs are

approach a certain maximum duty cycle, lim-

p REPLACES COSTLY MAG-AMP CORES WITH

A LOW ON-RESISTANCE MOSFET

designed to provide all control functions in

ited by the minimum delay and the mag-

p LOOK-AHEAD SWITCHINGTM ENSURES

a secondary-side regulator for isolated auxil-

netic BH loop characteristic of the Mag Amp

SWITCH TURN ON BEFORE THE AC INPUT

iary or secondary power supplies.

Auxiliary

core.

This forces an increase in the size of

TO ACHIEVE 100% ENERGY TRANSFER

or secondary-side controllers are used in a

the main transformer as well as the output

p LOWER OVERALL SYSTEM COST

variety

of applications including

multiple

inductor, resulting in higher overall system

p LOWER PEAK CURRENT STRESS ON THE

output off-line power supplies, commonly

cost.

The LX1570/71 eliminates all the

PRIMARY SWITCH

found in desktop computers, as well as tele-

disadvantages of the Mag Amp approach

p ALLOWS HIGHER OPERATING FREQUENCY

communications applications. Although they

as well as improving system perfor-

AND SMALLER OUTPUT INDUCTOR

can be used in all secondary output applica-

mance and reducing overall system cost.

p EASY SHORT-CIRCUIT PROTECTION

tions requiring precision regulation, they are

The LX1570/71 is a current mode control-

mainly optimized for outputs delivering more

ler IC that controls the duty cycle of a switch

p CURRENT MODE APPROACH ACHIEVES

than 3A current where standard three-termi-

in series with the secondary AC output of

EXCELLENT DYNAMIC RESPONSE

nal regulators lack the desired efficiency. For

the power transformer in buck-derived ap-

these applications, the Mag Amp regulators

plications, such as forward or bridge topolo-

A P P L I C AT I O N S

have traditionally been used. However, Mag

gies.

It offers features such as 100% duty

Amps have several disadvantages.

First, be-

cycle operation for maximum energy trans-

■ SECONDARY-SIDE REGULATOR IN OFF-LINE

cause they have to withstand the maximum

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

POWER SUPPLIES

input voltage during a short-circuit condition,

ing with long off-time between the cycles

■ COMPUTER POWER SUPPLIES, 3.3V OUTPUT

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

for reduced power dissipation, high-fre-

FOR NEW LOW-VOLTAGE PROCESSORS

increasing the cost and size of the power

quency operation for smaller magnetics, soft-

AND MEMORIES

supply.

Second, Mag Amps are inherently

start, and current mode control for excel-

■ TELECOMMUNICATION AND MILITARY

leading edge modulators, so they can only

lent dynamic response.

DC/DC CONVERTERS

P R O D U C T H I G H L I G H T

A VA I L A B L E OP T I O N S P E R P A R T #

Aux Output

Part #

C.L.

C.S.

Application

12V/8A

Threshold

Option

LX1570

Resistive

Output

-0.2V

Currents

Sensing

< 4A

LX1571

Current

Output

1V

Transformer

Currents

Sensing

> 4A

OUT

C.S.

DRV

VCC

VFB

LX1571

COMP

CT

S.S.

GND

PA C K A G E O R D E R I N F O R M AT I O N

Plastic DIP

Plastic SOIC

Ceramic DIP

TA (°C)

M

DM

Y

8-pin

8-pin

8-pin

0 to 70

LX157xCM

LX157xCDM

—

-40 to 85

LX157xIM

LX157xIDM

—

-55 to 125

—

—

LX157xMY

Copyright © 1997

Rev. 0.9.3 1/97

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

11861 WESTERN AVENUE, GARDEN GROVE, CA. 92841

1

P R O D U C T D A T A B O O K 1 9 9 6 / 1 9 9 7

LX1570/1571

PHASE MODULATED AC SYNCHRONOUS SECONDARY-SIDE CONTROLLER

P R E L I M I N A R Y D A T A S H E E T

	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, θJA	95°C/W
DM PACKAGE:
THERMAL RESISTANCE-JUNCTION TO AMBIENT, θJA	165°C/W
Y PACKAGE:
THERMAL RESISTANCE-JUNCTION TO AMBIENT, θJA	130°C/W
Junction Temperature Calculation: TJ = TA + (PD x θJA).
The θJA numbers are guidelines for the thermal performance of the device/pc-board system.
All of the above assume no ambient airflow.

PACKAGE PIN OUTS

S.S.		1	8		CT
VFB		2	7		VCC
COMP		3	6		OUT DRV
C.S.		4	5		GND

M & Y PACKAGE

(Top View)

S.S.				1	8			CT
VFB				2	7			VCC
COMP				3	6			OUT DRV
C.S.				4	5			GND

DM PACKAGE

(Top View)

		L X 1 5 7 1 B L O C K D I A G R A M
	2.5V			Minimum
			0.25V	Current Comp	PWM Latch
					R	Q	OUT DRV
		Error Amp				6
S.S.	1
		2R	C.S. Comp		S
VFB	2	1V	R
COMP 3				Current Mode
						2.5V
C.S. 4			1.5V	Hiccup Comp
	0.5V	Voltage Hiccup				2.5V
		Comp.
						REF
			Voltage Mode Hiccup			5V
						Internal
						Bias
			QUICK
		CHG	CHG
		CONTROL	CONTROL	LATCH RESET
				CONTROL
CT	8	Timing / Duty Cycle				7	VCC
			Control	LATCH		6V
		DISCH	VALLEY	SET CONTROL		16V
		CONTROL	THRESHOLD
			CONTROL			5	GND
				Q	R
				Q	S
				Hiccup
				Latch
2							Copyright © 1997
							Rev. 0.9.3	1/97

P R O D U C T D A T A B O O K 1 9 9 6 / 1 9 9 7

LX1570/1571

PHASE MODULATED AC SYNCHRONOUS SECONDARY-SIDE CONTROLLER

P R E L I M I N A R Y D A T A S H E E T

E L E C T R I C A L C H A R A C T E R I S T I C S

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

Parameter		Symbol	Test Conditions	LX1570/1571			Units
				Min.	Typ.	Max.
Reference Section
Initial Accuracy		VRI	TA = 25ºC, measured at F.B pin	2.475	2.500	2.525	V
Line Regulation		VRL	11V < VCC < 25			±1	%
Temp Stability		VRT	Note 2			±1.5	%
Timing Section
Initial Accuracy		fO	CT = , TJ = 25°C, measured at pin 6	90	100	110	kHz
			Over Temp, measured at pin 6	85	100	115	kHz
Line Voltage Stability		fOL				±1	%
Charging Current		ICHG			3		mA
Discharging Current		IDISCH			3.5		mA
Leakage Current		ILK	C.S.INPUT = 1.5V		4		µA
Ramp PK to PK		VRPP	C.S.INPUT = 0V		0.6		V
			C.S.INPUT = 1.5V (1571), C.S.INPUT = -0.4V (1570)		6		V
Error Amp / Soft Start Comp Section
Transconductance		gm			0.005		µΩ
Input Bias Current		IB			0.1	1	µA
Open Loop Gain		AVOL		60	70		dB
Output Sink Current		IEA(SINK)	VFB = 2.6V	200	400		µA
Output Source Current		IEA(SOURCE)	VFB = 2.4V	200	400		µA
Output HI Voltage		VCOMP-HI			5.1		V
Output LO Voltage		VCOMP-LO				0.8	V
Slew Rate		S			1		V/µSec
Soft-Start Section
Soft Start Timing Factor		KSS		35	50	65	ms/µF
Soft Start Discharge Current		ISS-DIS			TBD		mA
Current Sense Section
Input Range	LX1570	VCSI				-0.8	V
	LX1571			-0.3		6	V
Input Current	LX1570	ICSB				25	µA
	LX1571					1	µA
C.S. Amplifier Gain	LX1570	ACS		-13.5	-15	-16.5	V/V
	LX1571			2.7	3	3.3	V/V
Minimum Current Threshold Voltage	LX1570	VCSMIN			-50		mV
	LX1571				250		mV
C.S. Delay to Driver Output			10% Overdrive		100	200	ns
C.L. Pulse-By-Pulse Threshold Voltage	LX1570	VCLP		-0.18	-0.2	-0.22	V
	LX1571			0.9	1	1.1	V
C.L. Hiccup Threshold Voltage	LX1570	VCLH			-0.3		V
	LX1571				1.5		V
Voltage Hiccup Threshold		VHCCP			2		V

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

Copyright © 1997	3
Rev. 0.9.3 1/97

P R O D U C T D A T A B O O K 1 9 9 6 / 1 9 9 7

LX1570/1571

PHASE MODULATED AC SYNCHRONOUS SECONDARY-SIDE CONTROLLER

P R E L I M I N A R Y D A T A S H E E T

			E L E C T R I C A L C H A R A C T E R I S T I C S			(Con't.)
		Parameter		Symbol	Test Conditions		LX1570/1571			Units
							Min.	Typ.	Max.
	PWM Section
	E.A. Output to PWM Drive Offset			VOFS			1.7	2.0	2.4	V
	Fixed Duty Cycle			D			52	54	56	%
	Output Drive Section
	Rise / Fall Time			tR / tF	CL = 1000pF			50		ns
	Output HI			VDH	ISOURCE = 200mA, VCS = 0V, VFB = 2.3V			13.5		V
	Output LO			VDL	ISINK = 200mA, VCS = 1.2V, VFB = 2.3V			0.8		V
	Output Pull Down			VDPD	VCC = 0V, IPULL UP = 2mA			1		V
	UVLO Section
	Start-Up Threshold			VST			15	16	17	V
	Turn Off Threshold			VOFF			9	10	11	V
	Hysterises			VH			5.5	6	6.5	V
	Supply Current Section
	Dynamic Operating Current			IQd	Out Freq = 100kHz, CL = 0			18	30	mA
	Start-Up Current			IST				150	250	µA
			F U N C T I O N A L P I N D E S C R I P T I O N
	Pin	# Description

S.S.1 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.

VFB	2	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.
COMP	3	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.
C.S.	4	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.
GND	5	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.
OUT	6	This pin drives a gate drive transformer which drives the power mosfet. A Schottky diode such as 1N5817 must be
DRV		connected from this pin to GND in order to prevent the substrate diode conduction.
VCC	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.
CT	8	The free running oscillator frequency is programmed by connecting a capacitor from this pin to GND.

4		Copyright © 1997
		Rev. 0.9.3 1/97