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

µA typ.)

between the PFC and PWM sections

voltage mode operation

immunity

soft start

BLOCK DIAGRAM

17

VEAO

IEAO

VFB

IAC

VRMS

ISENSE

RAMP 1

OSCILLATOR

OVP

PFC ILIMIT

UVLO

VREF

PULSE WIDTH MODULATOR

POWER FACTOR CORRECTOR

2.5V

+

-

-

+

18

2

4

3

7.5V

REFERENCE

16

VCC

15

VCC

VEA

8

+

–

IEA

1

+

-

+

-

PFC OUT

14

S

R

Q

Q

S

R

Q

Q

2.75V

-1V

RAMP 2

9

PWM OUT

PGND

ILIM

13

S

R

Q

Q

VDC

6

SS

AGND

5

RTCT

7

VCC

DUTY CYCLE

LIMIT

+

-

1.5V

-

+

2.5V

VFB

-

+

8V

8V

VIN OK

GAIN

MODULATOR

VCC

÷2

1.6k

Ω

1.6k

Ω

1.25V

25µA

-

+

DC ILIMIT

10

12

11

REV. 1.1 3/9/2001