AN-6083 Datasheet, PDF (3/9 Pages) Fairchild Semiconductor – Highly Integrated, Dual-PWM Combination Controller

AN-6083
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
APPLICATION NOTE
Remote On/Off
Figure 8 shows the remote on / off function. When the
supervisor FPO pin pulls down and enables the system by
connecting an opto-coupler, V_REF applies to the ON/OFF
pin to enable forward PWM stage.
R_I
FAN6791
GND
Figure 5. Setting the PWM Frequency
For power saving, flyback PWM stage has a green mode
function. Frequency linearly decreases when V_FBFYB is
within V_G and V_N. Once V_FBFYB is lower than V_G, switching
frequency disables, and it enters burst mode.
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
shows off-time t_OFF is inserted in between the turn-off of the
stand-by gate drives and the turn-on of the forward PWM.
Figure 6. Oscillation Frequency in Green Mode
Line Voltage Detection (V_RMS)
Xxx Figure 7 shows a resistive divider with low-pass
filtering for line-voltage detection on VRMS pin. The V_RMS
voltage is used for brownout protection: when V_RMS drops
below 0.8V, OPWM turns off.
Figure 7. Line-Voltage Detection on VRMS Pin
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
current-sense resistor R_S. Under normal operations, the peak
inductor is controlled by an FBFYB level as:
I _PEAK
= ^{VFBFYB −1.3}
3.2 × R_S
(5)
when V_FBFYB is less than 1.3V, the FAN6791 terminates the
output pulses.
Figure 10 is a typical feedback circuit consisting mainly of a
shunt regulator and an opto-coupler. R₁ and R₂ form a
voltage divider for the output voltage regulation. R₃ and C₁
are adjusted for control-loop compensation. A small-value
RC filter (e.g. R_FB = 47Ω, C_FB = 1nF) placed on the FB pin
to the GND can further increase the stability.
© 2008 Fairchild Semiconductor Corporation
Rev. 1.0.2 • 3/11/09
3
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