FAN6862L Fairchild Semiconductor, FAN6862L Datasheet - Page 11

• Manufacturer Part Number: FAN6862L
• Description: A highly integrated PWM controller, FAN6862R/L provides several features to enhance the performance of flyback converters
• Manufacturer: Fairchild Semiconductor

© 2009 Fairchild Semiconductor Corporation
FAN6862R/L • Rev. 1.0.0
Protections
Self-protective functions include V
Protection (OVP), Open-Loop / Overload Protection
(OLP), Over-Current Protection (OCP), Short-Circuit
Protection, and Over-Temperature Protection (OTP).
FAN6862R uses auto-restart mode protections and
FAN6862L uses latch-mode protections.
Auto-Restart Mode Protection: Once a fault condition
is detected, switching is terminated and the MOSFET
remains off. This causes V
power is delivered from auxiliary winding. When V
falls to V
operating current reduces to startup current, which
causes V
operation when V
manner, the auto-restart can alternately enable and
disable the switching of the MOSFET until the fault
condition is eliminated (see Figure 25).
Latch-Mode Protection: Once this protection is
triggered, switching is terminated and the MOSFET
remains off. The latch is reset only when V
discharged below 4V by unplugging AC power line.
Over-Current Protection (OCP)
FAN6862R/L has over-current protection thresholds. It
is for pulse-by-pulse current limit, which turns off the
MOSFET for the remainder of the switching cycle when
the sensing voltage of MOSFET drain current reaches
the threshold. The other threshold is for the over-current
protection, which shuts down the MOSFET gate when
the sensing voltage of MOSFET drain current is above
the threshold longer than the shutdown delay (56ms).
8.5V
3mA
8uA
16V
V
V
DS
DD
Operating Current
DD-OFF
Figure 25. Auto-Restart Operation
Power
DD
On
to rise. FAN6862R resumes normal
Operation
(8.5V), the protection is reset and the
Normal
DD
Protection Triggers
reaches V
Situation
DD
Fault
to fall because no more
DD-ON
DD
Fault Removed
Operation
(16V). In this
Normal
Over-Voltage
DD
DD
is
11
Open-Loop / Overload Protection (OLP)
When the upper branch of the voltage divider for the
shunt regulator (KA431 shown) is broken, as shown in
Figure 26, no current flows through the opto-coupler
transistor, which pulls up the feedback voltage to 5.2V.
When the feedback voltage is above 4.6V longer than
56ms, OLP is triggered. This protection is also triggered
when the SMPS output drops below the nominal value
longer than 56ms due to the overload condition.
V
V
by over voltage on the VDD pin. The OVP is triggered
when V
30µs) prevents false triggering by switching noise.
Over-Temperature Protection (OTP)
The OTP circuit is composed of current source and
voltage comparators. Typically, an NTC thermistor is
connected between the RT and GND pins. Once the
voltage of this pin drops below a threshold of 1.02V,
PWM output is disabled after t
this pin drops below 0.7V, it triggers the latch-off
protection immediately after t
DD
DD
Over-Voltage Protection (OVP)
over-voltage protection prevents IC damage caused
V
FB
DD
reaches 25V. A debounce time (typically
Figure 26. OLP Operation
OLP Shutdown Delay Time
DOTP2
DOTP
debounce time.
debounce time. If
V
OLP Triggers
FB-OLP
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5.2V
(4.6V)