FAN6921MR Fairchild Semiconductor, FAN6921MR Datasheet - Page 18

• Manufacturer Part Number: FAN6921MR
• Description: The highly integrated FAN6921MR combines Power Factor Correction (PFC) controller and Quasi-Resonant PWM controller
• Manufacturer: Fairchild Semiconductor

© 2009 Fairchild Semiconductor Corporation
FAN6921MR Rev. 1.0.2
PFC Peak Current Limiting (CSPFC pin)
During PFC stage switching operation, the PFC switch
current is detected by current-sense resistor on the
CSPFC pin and the detected voltage on this resistor is
delivered to input terminal of a comparator and
compared with a threshold voltage 0.82V (typical). Once
the CSPFC pin voltage is higher than the threshold
voltage, PFC gate is turned off immediately.
The PFC peak switching current is adjustable by the
current-sense resistor. Figure 33 shows the measured
waveform of PFC gate and CSPFC pin voltage.
Brown-In / Out Protection (VIN Pin)
With AC voltage detection, FAN6921MR can perform
brown-in/ out protection (AC voltage UVP). Figure 34
shows the key operation waveforms of brown-in / out
protection. Both use the VIN pin to detect AC input
voltage level and the VIN pin is connected to AC input
by a resistor divider (refer to Figure 1); therefore, the
V
When the AC voltage drops, and V
than 1V for 100ms, the UVP protection is activated and
the COMP pin voltage is clamped to around 1.6V.
Because PFC gate duty is determined by comparing
sawtooth waveform and COMP pin voltage, lower
COMP voltage results in narrow PFC on-time, so that
the energy converged is limited and the PFC output
voltage decreases. When INV pin is lower than 1.2V,
FAN6921MR stops all PFC and PWM switching
operation immediately until V
voltage then raises to turn-on voltage again (UVLO).
When the brownout protection is activated, all switching
operation is turned off, V
up and down continuously. Until V
than 1.3V (typical) and V
again, the PWM and PFC gate is sent out.
The measured waveforms of brown-in / out protection
are shown in Figure 35.
VIN
CSPFC
OPFC
Figure 33. Cycle-by-Cycle Current Limiting
voltage is proportional to the AC input voltage.
PFC MOS Current Limit
DD
DD
voltage enters hiccup mode
DD
reaches turn-on voltage
voltage drops to turn-off
VIN
VIN
voltage is higher
voltage is lower
0.82V
18
Figure 34. Operation Waveforms of Brown-In/ Out
AC Input
Figure 35. Measured Waveform of Brown-In/ Out
Brownout
V
DD
Protection (Adapter Application)
OPWM
OPFC
V
DD
Hiccup Mode
Protection
Brown-In
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