May 1997
Micro Linear
ML4821*
Power Factor Controller
GENERAL DESCRIPTION
FEATURES
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.
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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.
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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
* Some Packages Are Obsolete
BLOCK DIAGRAM
11
1
OVP
+
VREF
ILIM
–
+
SLEEP
–
0.7V
–
UNDER
VOLTAGE
LOCKOUT
+
2
3
IA OUT
IA–
+
IA+
S
VLIMIT
R
+
6
7
OUT
VRMS
GAIN
MODULATOR
PGND
RT
OSC
SYNC
CT
EA OUT
15
14
13
12
10
17
VREF
EA–
–
VREF
9
18
17V
OUT
ISINE
16
Q
4
8
GND
VCC
–
–
5
VREF
+
SOFT START
Micro Linear
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