April 1997

Flyback Power Factor Controller

1

GENERAL DESCRIPTION

The ML4813 is a PWM controller designed for use in a

discontinuous "flyback" or "buck-boost" type power factor

correction (PFC) system for low power, low cost

applications.

The circuit includes a precision reference, oscillator, error

amplifier, over-voltage comparator, over-current

comparator, and an extra op-amp as well as a high

current output. In addition, start-up is simplified by an

under-voltage lockout circuit.

In a typical application, the ML4813 functions as a

voltage mode regulator. By maintaining a constant duty

cycle, the current follows the input voltage, making the

impedance of the entire circuit appear purely resistive.

With the flyback circuit, power factors of 0.99 are easily

achievable with a small output inductor and a minimum

of external components.

FEATURES

s

Precision buffered 5V reference

s

Extra op-amp for output voltage instrumentation

amplifier

s

Overcurrent comparator for switch protection

s

Soft start and under-voltage lockout for easy low surge

off-line starting

s

1A peak current Totem-pole output drive

s

Overvoltage comparator eliminates output "runaway"

due to load removal

s

Large oscillator amplitude for better noise immunity

* This Part Is End Of Life As Of August 1, 2000

BLOCK DIAGRAM

-

+

+

-

S

R

Q

Q

12

OUT

4

VFB

ERROR

AMPLIFIER

5V

2

SOFT START

+

-

1

ILIMIT

CURRENT LIMIT

COMPARATOR

1V

5

OVP

OVP

COMPARATOR

5.6V

-

+

3

COMP

PWM

COMPARATOR

16

CT

9

RT

10

SYNC

OSCILLATOR

15

GND

UNDERVOLTAGE

LOCKOUT

13

VCC

14

VREF

32V

+

-

11

PWR GND

6

OA OUT

7V

7

OA-

8

OA+

60µA