June 1998

ML4824

*

Power Factor Correction and PWM Controller Combo

1

GENERAL DESCRIPTION

The ML4824 is a controller for power factor corrected,

switched mode power supplies. Power Factor Correction

(PFC) allows the use of smaller, lower cost bulk capacitors,

reduces power line loading and stress on the switching

FETs, and results in a power supply that fully complies

with IEC1000-2-3 specification. The ML4824 includes

circuits for the implementation of a leading edge, average

current, “boost” type power factor correction and a trailing

edge, pulse width modulator (PWM).

The device is available in two versions; the ML4824-1

Doubling the switching frequency of the PWM allows the

user to design with smaller output components while

maintaining the best operating frequency for the PFC. An

over-voltage comparator shuts down the PFC section in the

event of a sudden decrease in load. The PFC section also

includes peak current limiting and input voltage brown-

out protection. The PWM section can be operated in

current or voltage mode at up to 250kHz and includes a

duty cycle limit to prevent transformer saturation.

FEATURES

s

Internally synchronized PFC and PWM in one IC

s

Low total harmonic distortion

s

Reduces ripple current in the storage capacitor between

the PFC and PWM sections

s

Average current, continuous boost leading edge PFC

s

Fast transconductance error amp for voltage loop

s

High efficiency trailing edge PWM can be configured

for current mode or voltage mode operation

s

Average line voltage compensation with brownout

control

s

PFC overvoltage comparator eliminates output

“runaway” due to load removal

s

Current fed gain modulator for improved noise immunity

s

Overvoltage protection, UVLO, and soft start

15

VEAO

IEAO

VFB

IAC

VRMS

ISENSE

RAMP 1

OSCILLATOR

OVP

PFC ILIMIT

UVLO

VREF

PULSE WIDTH MODULATOR

POWER FACTOR CORRECTOR

2.5V

+

–

–

+

16

2

4

3

7.5V

REFERENCE

14

VCC

13

VCCZ

VEA

7

+

–

IEA

1

+

–

+

–

PFC OUT

12

S

R

Q

Q

S

R

Q

Q

2.7V

–1V

RAMP 2

8

PWM OUT

11

S

R

Q

Q

VDC

6

SS

5

DC ILIMIT

9

VCC

DUTY CYCLE

LIMIT

–

+

1V

–

+

2.5V

VFB

–

+

8V

8V

VIN OK

GAIN

MODULATOR

VCCZ

x 2

(-2 VERSION ONLY)

3.5k

Ω

3.5k

Ω

1.25V

50µA

–

+

13.5V

DC ILIMIT

BLOCK DIAGRAM

*Some Packages Are Obsolete