?2010 Fairchild Semiconductor Corporation
www.fairchildsemi.com
FAN6920MR " Rev. 1.0.8
18
PFC Peak Current Limiting (CSPFC Pin)
During PFC stage switching operation, the PFC switch
current is detected by the current-sense resistor on the
CSPFC pin and the detected voltage on this resistor is
delivered to the input terminal of a comparator and
compared with a threshold voltage 0.82 V (typical).
Once the CSPFC pin voltage is higher than the
threshold voltage, the 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.
Figure 33. Cycle-by-Cycle Current Limiting
Brownout Protection (VIN Pin)
With AC voltage detection, FAN6920MR can perform
brownout / in protection (AC voltage UVP). Figure 34
shows the key operation waveforms of brownout / in
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
VIN
voltage is proportional to the AC input voltage.
When the AC voltage drops and V
VIN
voltage is lower
than 1 V for 100 ms, the UVP protection is activated and
the COMP pin voltage is clamped to around 1.6 V.
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
1.2 V, FAN6920MR stops all PFC and PWM switching
operation immediately until V
DD
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 and V
DD
voltage enters Hiccup
Mode up and down continuously. Once V
VIN
voltage is
higher than 1.3 V (typical) and V
DD
reaches turn-on
voltage again, the PWM and PFC gate is sent.
The measured waveforms of brownout / in protection
are shown in Figure 35.
C Input
OPFC
VIN-UVP
V
VIN
V
VIN-RE-UVP
V
0V
Brownout
Protection
OPWM
V
INV
2.5V
Hiccup
M
V
INV-BO
V
COMP
1.2V
Brownout
Protection
Debounce
Time 100ms
1.6V
V
COMP-BO
Figure 34. Operation Waveforms of Brownout /
In Protection
Figure 35. Measured Waveform of Brownout / In
Protection (Adapter Application)
PF
OPWM
AC In ut
V
V Hi
M
Brownout
Brown-In
PF
CSPFC
PF M
rr n Limi
0.82V
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