May 1997
ML4841*
Variable Feedforward PFC/PWM Controller Combo
GENERAL DESCRIPTION
The ML4841 is a controller for power factor corrected,
switched mode power supplies. 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 ML4841 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 ML4841 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
Internally synchronized PFC and PWM in one IC
Low total harmonic distortion
Reduced ripple current in the storage capacitor
between the PFC and PWM sections
Average current, continuous mode, boost type,
leading edge PFC
High efficiency trailing edge PWM can be configured
for current mode or voltage mode operation
Average line voltage compensation with brown-out
control
PFC overvoltage comparator eliminates output
“runaway” due to load removal
Current-fed multiplier for improved noise immunity
Overvoltage protection, UVLO, and soft start
s
s
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
50µA
1.25V
-
+
-
+
-
+
+
-
*Some Packages Are End Of Life Or Obsoete
1
IEAO
POWER FACTOR CORRECTOR
OVP
+
+
13
VCCZ
13.5V
VCC
7.5V
REFERENCE
S
-1V
+
-
VEA
3.5kΩ
-
IEA
2.7V
VREF
14
-
Q
Q
PFC OUT
Q
Q
12
GAIN
MODULATOR
3.5kΩ
8V
R
S
R
PFC ILIMIT
OSCILLATOR
÷2
DUTY CYCLE
LIMIT
S
VFB
2.5V
-
+
Q
Q
PWM OUT
11
VIN OK
+
R
DC ILIMIT
1V
-
PULSE WIDTH MODULATOR
VCCZ
UVLO
1
MICRO-LINEAR [ MICRO LINEAR CORPORATION ]
