Micro Linear

Micro Linear

May 1997

ML4821

*

Power Factor Controller

1

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

* Some Packages Are Obsolete

which reduces output power below 90V RMS input.

FEATURES

s

Average current sensing for lowest possible

harmonic distortion

s

Average line compensation with brown-out control

s

Precision buffered 5V reference

s

1A peak current totem-pole output drive

s

Overvoltage comparator eliminates output “runaway”

due to load removal

s

Wide common mode range in current sense

comparators for better noise immunity

s

Large oscillator amplitude for better noise immunity

s

Output driver internally limited to 17V

s

“Sleep mode” shutdown 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