FAN6921ML Fairchild Semiconductor, FAN6921ML Datasheet - Page 21

• Manufacturer Part Number: FAN6921ML
• Description: The highly integrated FAN6921ML combines a Power Factor Correction (PFC) controller and a Quasi-Resonant PWM controller
• Manufacturer: Fairchild Semiconductor

© 2010 Fairchild Semiconductor Corporation
FAN6921ML • Rev. 1.0.2
Open-Loop, Short-Circuit, and Overload Protection
(FB Pin)
Referring to Figure 45, outside of FAN6921ML; the FB
pin is connected to the collector of transistor of an opto-
coupler. Inside of FAN6921ML, the FB pin is connected
to an internal voltage bias through a resistor of ~5k.
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.
Figure 45. FB Pin Open-Loop, Short Circuit,
and Overload Protections
Under-Voltage Lockout (UVLO, VDD Pin)
Referring to Figure 36 and Figure 37, the turn-on and
turn-off V
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
FAN6921ML starts all switching operation if no
protection is triggered before V
off voltage V
21
DD
capacitor) is charged by the HV startup current
DD
DD
voltage reaches the turn-on voltage. Before the
DD
capacitor from auxiliary winding, this hold-up
DD-PWM-OFF
threshold voltages are fixed at 18V and
DD
.
voltage reaches turn-on voltage,
DD
DD
voltage drops to turn-
voltage energy for
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