www.fairchildsemi.com

REV. 1.0.2 6/19/01

Features

• Average current sensing for lowest possible harmonic

distortion

• Average line compensation with brown-out control

• Precision buffered 5V reference

• 1A peak current totem-pole output drive

• Overvoltage comparator eliminates output “runaway” due

to load removal

• Wide common mode range in current sense comparators

for better noise immunity

• Large oscillator amplitude for better noise immunity

• Output driver internally limited to 17V

• “Sleep mode” shutdown input

General Description

The ML4821 provides complete control for a “boost” type

power factor correction system using the average current

sensing method. Special care has been taken in the design of

the ML4821 to increase system noise immunity. The circuit

includes a precision reference, gain modulator, average

current error amplifier, output error amplifier, over-voltage

protection comparator, shutdown logic, as well as a high

current output. In addition, start-up is simplified by an under-

voltage lockout circuit.

In a typical application, the ML4821 controls the AC input

current by adjusting the pulse width of the output MOSFET.

This modulates the line current so that its shape conforms to

the shape of the input voltage. The reference for the current

regulator is a product of the sinusoidal line voltage times the

output of the error amplifier which is regulating the output

DC voltage. Average line voltage compensation is provided

in the gain modulator to ensure constant loop gain over a

wide input voltage range. This compensation includes a

special “brown-out” control which reduces output power

below 90V RMS input.

Block Diagram

11

1

2

3

4

5

8

6

7

9

16

18

15

14

13

12

10

17

+

–

+

–

–

+

–

+

–

+

–

+

SOFT START

EA–

EA OUT

OUT

GAIN

MODULATOR

IA–

IA+

IA OUT

OVP

0.7V

R

S

Q

UNDER

VOLTAGE

LOCKOUT

OSC

17V

GND

OUT

PGND

SYNC

SLEEP

ML4821

Power Factor Controller