FAN6921MR Fairchild Semiconductor, FAN6921MR Datasheet - Page 22

• 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
Open-Loop, Short-Circuit, and Overload Protection
(FB Pin)
Referring to Figure 45, outside of FAN6921MR, the FB
pin is connected to the collector of transistor of an opto-
coupler. Inside of FAN6921MR, the FB pin is connected
to an internal voltage bias through a resistor around
5k.
Figure 45. FB Pin Open-Loop, Short Circuit, and
Overload Protection
As the output loading is increased, the output voltage is
decreased and the sink current of transistor of opto-
coupler on primary side is reduced. So the FB pin
voltage is increased by internal voltage bias. In the case
of an open loop, output short circuit, or overload
conditions, this sink current is further reduced and the
FB pin voltage is pulled to high level by internal bias
voltage. When the FB pin voltage is higher than 4.2V for
50ms, the FB pin protection is activated.
Under-Voltage Lockout (UVLO, VDD Pin)
Referring to Figure 36 and Figure 37, the turn-on and
turn-off V
at 18V and 10V, respectively. During startup, the hold-
up capacitor (V
until V
output voltage rises to rated voltage and delivers energy
to the V
capacitor has to sustain the V
operation. When V
FAN6921MR starts all switching operation if no
protection is triggered before V
off voltage V
22
DD
DD
voltage reaches the turn-on voltage. Before the
DD
capacitor from auxiliary winding, this hold-up
threshold voltages of FAN6921MR are fixed
DD-PWM-OFF
DD
cap.) is charged by HV startup current
DD
.
voltage reaches turn-on voltage,
DD
DD
voltage drops to turn-
voltage energy for
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