FAN6921ML Datasheet, PDF (17/23 Pages) Fairchild Semiconductor – Integrated Critical Mode PFC/Quasi-Resonant Current Mode PWM Controller

PFC Peak Current Limiting (CSPFC pin)
During PFC stage switching operation, the PFC switch
current is detected by a current-sense resistor on the
CSPFC pin and the detected voltage on this resistor is
delivered to an input terminal of a comparator and
compared with a threshold voltage 0.6V (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.
PFC MOS Current Limit
CSPFC
OPFC
Figure 33. Cycle-by-Cycle Current Limiting
Brownin/out Protection (VIN Pin)
With AC voltage detection, FAN6921ML can perform
brownin/out protection (AC voltage UVP). Figure 34
shows the key operation waveforms. The VIN pin is
used to detect AC input voltage level and is connected
to AC input by a resistor divider (refer to Figure 1);
therefore, the VVIN voltage is proportional to the AC
input voltage. When the AC voltage drops; and VVIN
voltage is lower 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 the 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 voltage
is lower than 1.2V, FAN6921ML stops all PFC and
PWM switching operation immediately until VDD
voltage drops to turn-off voltage then rises to turn-on
voltage again (UVLO).
When the brownout protection is activated, all switching
operation is turned off, the VDD voltage enters hiccup
mode up and down continuously. Until VVIN voltage is
higher than 1.2V (typical) and VDD reaches turn-on
voltage again, the PWM and PFC gate is sent out.
The measured waveforms of brownin/out protection are
shown in Figure 35.
Figure 34. Operation Waveforms of Brownin/out
Protection
V_DD
V_DD Hiccup Mode
Brownout
AC Input
OPWM
OPFC
Brownin
AC Input
Figure 35. Measured Waveform of Brownin/out
Protection (Adapter Application)
© 2010 Fairchild Semiconductor Corporation
FAN6921ML • Rev. 1.0.1
17
www.fairchildsemi.com