ML4801 Datasheet, PDF (1/14 Pages) Fairchild Semiconductor – Variable Feedforward PFC/PWM Controller Combo

March 2001
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
I Internally synchronized PFC and PWM in one IC
I Low start-up current (200µA typ.)
I Low operating current (5.5mA typ.)
I Low total harmonic distortion
I Reduces ripple current in the storage capacitor
between the PFC and PWM sections
I Average current continuous boost leading edge PFC
I High efficiency trailing edge PWM optimized for
current mode operation
I Current fed gain modulator for improved noise
immunity
I Brown-out control, overvoltage protection, UVLO, and
soft start
BLOCK DIAGRAM
16
VEAO
1
IEAO
POWER FACTOR CORRECTOR
VFB
15
2.5V
IAC
2
VRMS
4
ISENSE
3
VEA
-
+
1.6kΩ ^IEA
+
–
GAIN
MODULATOR
1.6kΩ
RAMP 1
8
RTCT
7
+
-
OSCILLATOR
÷2
2.75V
OVP
+
-
-1V +
-
PFC ILIMIT
VCC
13
VCC
7.5V
REFERENCE
VREF
14
SQ
RQ
SQ
PFC OUT
12
RQ
RAMP 2
9
8V
VDC
6
VCC
SS
5
25µA
8V
1.25V
-
+
-
+
DUTY CYCLE
LIMIT
VFB
2.5V
VIN OK
-
+
1.5V
+
- _{DC ILIMIT}
PULSE WIDTH MODULATOR
VCC
SQ
RQ
UVLO
PWM OUT
11
GND
10
REV. 1.1 3/9/2001