May 1997

ML4841

*

Variable Feedforward PFC/PWM Controller Combo

1

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

Internally synchronized PFC and PWM in one IC

s

Low total harmonic distortion

s

Reduced ripple current in the storage capacitor

between the PFC and PWM sections

s

Average current, continuous mode, boost type,

leading edge PFC

s

High efficiency trailing edge PWM can be configured

for current mode or voltage mode operation

s

Average line voltage compensation with brown-out

control

s

PFC overvoltage comparator eliminates output

“runaway” due to load removal

s

Current-fed multiplier for improved noise immunity

s

Overvoltage protection, UVLO, and soft start

BLOCK DIAGRAM

*Some Packages Are End Of Life Or Obsoete

15

VEAO

IEAO

VFB

IAC

VRMS

ISENSE

RAMP 1

OSCILLATOR

OVP

PFC ILIMIT

UVLO

VREF

PULSE WIDTH MODULATOR

POWER FACTOR CORRECTOR

2.5V

+

-

-

+

16

2

4

3

7.5V

REFERENCE

14

VCC

13

VCCZ

VEA

8

-

+

IEA

1

+

-

+

-

PFC OUT

12

S

R

Q

Q

S

R

Q

Q

2.7V

-1V

RAMP 2

9

PWM OUT

11

S

R

Q

Q

VDC

6

SS

5

RTCT

7

VCC

DUTY CYCLE

LIMIT

+

-

1V

-

+

2.5V

VFB

-

+

8V

8V

VIN OK

GAIN

MODULATOR

VCCZ

÷2

3.5k

Ω

3.5k

Ω

1.25V

50µA

-

+

13.5V

DC ILIMIT

8V