FA5640 Datasheet, PDF (24/35 Pages) Fuji Electric – Fuji Switching Power Supply Control IC

FA5640/41/42
(4) Overvoltage protection function on the secondary side
In case of overvoltage of output, the voltage of the auxiliary winding also rises. The ZCD pin has the function of
cycle-by-cycle detecting the voltage of the auxiliary winding of transformer. If the state in which the ZCD pin voltage is 6.0
V or higher continues for 60 s or longer, switching is stopped and the operation is latched-off. This state is maintained
until the input voltage is interrupted, and the VCC decreases to the OFF threshold voltage of the UVLO.
For example if switching is made at 40 kHz, 60 s  40 kHz = 2.4: detection of twice or more is required.
(5) External latch-off function
By pulling up the ZCD pin to 6.0 V or higher for 60 s or longer, the IC is latched-off. This state is maintained until the input
voltage is interrupted and VCC decreases to the OFF threshold voltage of the UVLO.
(6) Compensating each threshold level by high-line voltage detection (FA5640/FA5641)
By detecting the peak voltage of the VH pin, each threshold level is switched to compensate for high/low line voltage. The
threshold level to be switched by high-line voltage include the pulse shutdown FB voltage, which is related to the pulse
mode switching load, and the maximum input threshold voltage, which is the overcurrent limit level of the IS pin.
(7) Minimum switching frequency limitation and maximum ON width limitation
The maximum ON pulse width is limited to 24 s to reduce the audible noise of the transformer when it is started and
stopped. In addition, FA5641 is integrated in minimum switching frequency that is limited at 25 kHz to reduce audible noise
more. See 10. (9) “Other advice on designing” for details.
(8) Switching of overload protection levels due to external signal detection
By pulling up the voltage of IS pin to higher than the IS pin standby detection voltage during the OFF period of the
MOSFET using external signals, overload protection levels can be switched. More specifically, by switching the maximum
input threshold voltage, which is the overcurrent limit level of the IS pin, the power can be limited to approximately 1/7 of
the overload protection level of normal operation.
This function is useful for limiting the power in standby mode, for example.
(9) Restart operation
If the MOSFET cannot be turned on based on bottom detection of the ZCD pin at the time of startup, restart operation is
performed using a timer to forcibly turn on the MOSFET. If the condition in which the OUT is Low (MOSFET is OFF) and
the voltage of the ZCD pin is below to input threshold voltage (Vthzcd2) 150 mV or lower, the timer starts counting and the
MOSFET is turned ON when the timeout from the last ZCD trigger.
Fuji Electric Co., Ltd.
AN-064E Rev.1.1
April 2011
24
http://www.fujielectric.co.jp/products/semiconductor/