June 1998
ML4824*
Power Factor Correction and PWM Controller Combo
GENERAL DESCRIPTION
The ML4824 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 specification. The ML4824 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 device is available in two versions; the ML4824-1
(f
PWM
= f
PFC
) and the ML4824-2 (f
PWM
= 2 x f
PFC
).
Doubling the switching frequency of the PWM allows the
user to design with smaller output components while
maintaining the best 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. The PWM section can be operated in
current or voltage mode at up to 250kHz and includes a
duty cycle limit to prevent transformer saturation.
FEATURES
s
s
s
Internally synchronized PFC and PWM in one IC
Low total harmonic distortion
Reduces ripple current in the storage capacitor between
the PFC and PWM sections
Average current, continuous boost leading edge PFC
Fast transconductance error amp for voltage loop
High efficiency trailing edge PWM can be configured
for current mode or voltage mode operation
Average line voltage compensation with brownout
control
PFC overvoltage comparator eliminates output
“runaway” due to load removal
Current fed gain modulator for improved noise immunity
Overvoltage protection, UVLO, and soft start
s
s
s
s
s
s
s
BLOCK DIAGRAM
16
1
*Some Packages Are Obsolete
13
VEAO
VFB
VEA
–
+
–
IEAO
POWER FACTOR CORRECTOR
OVP
3.5kΩ
+
VCCZ
13.5V
VCC
15
2.5V
IEA
+
–
+
2.7V
–
7.5V
REFERENCE
S
Q
Q
VREF
14
IAC
2
VRMS
4
GAIN
MODULATOR
3.5kΩ
–1V
+
–
R
S
R
PFC OUT
ISENSE
3
PFC ILIMIT
Q
Q
12
RAMP 1
7
RAMP 2
8
8V
OSCILLATOR
(-2 VERSION ONLY)
x2
DUTY CYCLE
LIMIT
VDC
6
VCC
1.25V
–
+
–
S
VFB
–
+
Q
Q
PWM OUT
11
VIN OK
SS
5
50µA
+
1V
–
+
2.5V
8V
R
DC ILIMIT
DC ILIMIT
9
PULSE WIDTH MODULATOR
VCCZ
UVLO
1