www.fairchildsemi.com
REV. 1.0.4 5/31/01
Features
• Precision buffered 5V reference (±0.5%)
• Current-input gain modulator reduces external
components and improves noise immunity
• Programmable ramp compensation circuit
• 1A peak current totem-pole output drive
• Overvoltage comparator helps prevent output voltage
“runaway”
• Wide common mode range in current sense comparators
for better noise immunity
• Large oscillator amplitude for better noise immunity
Description
The ML4812 is designed to optimally facilitate a peak
current control boost type power factor correction system.
Special care has been taken in the design of the ML4812 to
increase system noise immunity. The circuit includes a
precision reference, gain modulator, error amplifier, over-
voltage protection, ramp compensation, as well as a high
current output. In addition, start-up is simplified by an under-
voltage lockout circuit with 6V hysteresis.
In a typical application, the ML4812 functions as a current
mode regulator. The current which is necessary to terminate
the cycle is a product of the sinusoidal line voltage times the
output of the error amplifier which is regulating the output
DC voltage. Ramp compensation is programmable with an
external resistor, to provide stable operation when the duty
cycle exceeds 50%.
Block Diagram
(Pin Configuration Shown is for DIP Version)
5V
OVP
I
SENSE
GM OUT
EA OUT
I
SINE
RAMP COMP
C
T
R
T
ERROR
AMP
I
EA
OSC
1kΩ
UNDER
VOLTAGE
LOCKOUT
SHDN
OUT
PWR GND
V
REF
V
CC
GND
CLOCK
V
CC
EA–
32V
5
1
2
10
12
11
14
13
15
9
3
4
6
7
16
8
+
–
5V
+
–
5V
+
–
–
5V
S
R
Q
Q
GAIN MODULATOR
ML4812
Power Factor Controller
