November 1998
PRELIMINARY
ML4801
Variable Feedforward PFC/PWM Controller Combo
GENERAL DESCRIPTION
The ML4801 is a controller for power factor corrected,
switched mode power supplies. Key features of this
combined PFC and PWM controller are low start-up and
operating currents. 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 specifications. The ML4801 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 PFC frequency of the ML4801 is automatically set at
half that of the PWM frequency generated by the internal
oscillator. This technique allows the user to design with
smaller output components while maintaining the
optimum 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.
FEATURES
s
s
s
s
s
Internally synchronized PFC and PWM in one IC
Low start-up current (200µA typ.)
Low operating current (5.5mA typ.)
Low total harmonic distortion
Reduces ripple current in the storage capacitor
between the PFC and PWM sections
Average current continuous boost leading edge PFC
High efficiency trailing edge PWM optimized for
current mode operation
Current fed gain modulator for improved noise
immunity
Brown-out control, overvoltage protection, UVLO, and
soft start
s
s
s
s
BLOCK DIAGRAM
16
VEAO
VFB
15
2.5V
IAC
2
VRMS
4
ISENSE
3
RAMP 1
8
RTCT
7
RAMP 2
9
8V
VDC
6
VCC
SS
5
8V
25µA
1.25V
-
+
-
+
-
+
–
-
1
IEAO
POWER FACTOR CORRECTOR
OVP
1.6kΩ
+
+
13
VCC
VCC
7.5V
REFERENCE
S
-1V
+
-
VEA
IEA
2.75V
+
-
VREF
14
Q
Q
PFC OUT
Q
Q
12
GAIN
MODULATOR
1.6kΩ
R
S
R
PFC ILIMIT
OSCILLATOR
÷2
DUTY CYCLE
LIMIT
S
VFB
2.5V
-
+
Q
Q
PWM OUT
11
VIN OK
+
R
DC ILIMIT
1.5V
-
GND
10
PULSE WIDTH MODULATOR
VCC
UVLO
1
MICRO-LINEAR [ MICRO LINEAR CORPORATION ]
