© 2008 Fairchild Semiconductor Corporation

www.fairchildsemi.com

FAN6791 / FAN6793 • Rev. 1.0.2

5

Pin Definitions

Pin #

Name

Description

1

HV

For startup

, this pin is pulled HIGH to the line input or bulk capacitor via resistors.

2

NC

No connection.

3

GND

Ground

.

4

RI

Oscillator Setting

. One resistor connected between RI and ground pins determines the switching

frequency (resistance between 12 ~ 47kΩ is recommended). The switching frequency is equal to

[1560 / RI]kHz, where RI is in k

Ω. For example, if RI is equal to 24kΩ, then the switching frequency

is 65kHz.

5

FBFYB

Voltage Feedback for Flyback PWM Stage

. It is internally pulled HIGH through a 6.5k

Ω resistor.

An external opto-coupler from secondary feedback circuit is usually connected to this pin.

6

IFYB

PWM Current Sense for Flyback PWM Stage

. The sensed voltage is used for peak-current-

mode control and cycle-by-cycle current limiting.

7

FBPWM

Voltage Feedback for Forward PWM Stage

. It is internally pulled HIGH through a 6.5k

Ω resistor.

An external opto-coupler from secondary feedback circuit is usually connected to this pin.

8

IPWM

PWM Current Sense for Forward PWM Stage

. Via a current sense resistor, this pin provides the

control input for peak-current-mode control and cycle-by-cycle current limiting.

9

VREF

Reference voltage

. This pin can provide a reference voltage 5V.

10

SS

PWM Soft-Start

. During startup, the SS pin charges an external capacitor with a 20µA constant

current source. The voltage on FBPWM is clamped by SS during startup. In the event of a

protection condition occurring and/or forward PWM being disabled, the SS pin quickly discharges.

11

ON/OFF

PWM Remote ON/OFF

. Active HIGH. The forward PWM is disabled whenever the voltage at this

pin is lower than 0.8V or the pin is open.

12

PGND

Ground

. The power ground.

13

OPWM

Forward PWM Gate Drive

. The totem-pole output drive for the forward PWM MOSFET. This pin is

internally clamped under 16V to protect the MOSFET.

14

VDD

Power Supply

OVP trigger point.

15

OFYB

Flyback PWM Gate Drive

. The totem-pole output drive for the forward PWM MOSFET. This pin is

internally clamped under 16V to protect the MOSFET.

16

VRMS

Line-Voltage Detection

. The pin is used for line compensation, for forward, and brownout

protection.