AN-6083

APPLICATION NOTE

© 2008 Fairchild Semiconductor Corporation

www.fairchildsemi.com

Rev. 1.0.2 • 3/11/09

3

function internally. The frequency variation ranges from

around 61KHz to 69KHz for a center frequency 65KHz. The

frequency hopping function helps reduce EMI emission of a

power supply with minimum line filters.

RI

GND

FAN6791

Figure 5. Setting the PWM Frequency

For power saving, flyback PWM stage has a green mode

frequency disables, and it enters burst mode.

Figure 6. Oscillation Frequency in Green Mode

Xxx Figure 7 shows a resistive divider with low-pass

below 0.8V, OPWM turns off.

Figure 7. Line-Voltage Detection on VRMS Pin

Remote On/Off

Figure 8 shows the remote on / off function. When the

supervisor FPO pin pulls down and enables the system by

pin to enable forward PWM stage.

Figure 8. Remote On/Off

Interleave Switching

The FAN6791 uses interleaved switching to synchronize the

stand-by PWM / forward PWM stages. This reduces

switching noise and spreads the EMI emissions. Figure 9

stand-by gate drives and the turn-on of the forward PWM.

Figure 9. Interleaved Switching

FBFYB and FBPWM

The FAN6791 is designed for peak-current-mode control. A

current-to-voltage conversion is done externally with a

inductor is controlled by an FBFYB level as:

1.3

3.2

−

=

×

FBFYB

PEAK

S

V

I

R

(5)

output pulses.

Figure 10 is a typical feedback circuit consisting mainly of a

are adjusted for control-loop compensation. A small-value

to the GND can further increase the stability.