November 1998
PRELIMINARY
ML4805
Variable Feedforward PFC/PWM Controller Combo
GENERAL DESCRIPTION
The ML4805 is a controller for power factor corrected,
switched mode power supplies. Similar to the ML4801,
the ML4805 may be used for voltage mode operation. 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
ML4805 includes circuits for the implementation of a
leading edge, average current “boost” type power factor
correction and a trailing edge pulse width modulator.
The PFC frequency of the ML4805 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
voltage 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
18
VEAO
VFB
17
2.5V
IAC
2
VRMS
4
ISENSE
3
RAMP 1
8
RTCT
7
RAMP 2
9
8V
VDC
6
VCC
SS
5
AGND
11
PULSE WIDTH MODULATOR
VCC
UVLO
8V
25µA
1.25V
-
+
-
+
-
+
–
-
1
IEAO
POWER FACTOR CORRECTOR
OVP
+
+
15
VCC
VCC
7.5V
REFERENCE
S
-1V
+
-
VEA
1.6kΩ
+
IEA
2.75V
VREF
16
-
Q
Q
PFC OUT
Q
Q
14
GAIN
MODULATOR
1.6kΩ
R
S
R
PFC ILIMIT
OSCILLATOR
÷2
PGND
DUTY CYCLE
LIMIT
12
VIN OK
VFB
2.5V
-
+
+
S
R
DC ILIMIT
Q
Q
PWM OUT
13
1.5V
-
ILIM
10
1
MICRO-LINEAR [ MICRO LINEAR CORPORATION ]
