FA5546 Datasheet, PDF (22/27 Pages) Fuji Electric – Switching Power Supply Control IC

9. Advice for designing
FA5546/47
(1) Startup
To properly start or stop the power supply, a capacitor
having appropriate capacitance must be selected. (See
Fig.21)
Fig.22 shows the VCC voltage at the time of startup when
an appropriate capacitor is connected.
When the power is turned on, the capacitor of the VCC is
charged with the current supplied from the startup circuit,
and the voltage increases.
When the VCC reaches the ON threshold voltage, the IC
starts operating. The IC is operated based on the voltage
supplied from the auxiliary winding. Note that during the
period immediately after startup until the voltage of the
auxiliary winding starts up, the VCC decreases. Select a
capacitor for the VCC that does not allow the VCC to
decrease down to the OFF threshold voltage.
Specifically, a VCC terminal capacitor whose OFF threshold
voltage is 11V or higher is recommended.
If the capacitance of the VCC terminal is too small, VCC
decreases to lower than the OFF threshold voltage before
the voltage of the auxiliary winding starts up as shown by
Fig.23. In this case, the VCC repeats up/down operation
between ON and OFF threshold voltages, and
consequently the power supply cannot be turned on.
To prevent the VCC terminal voltage from decreasing to
lower than the UVLO OFF threshold voltage due to sudden
load change and other reasons, it may be desirable that the
capacitance of the capacitor to be connected to the VCC
terminal be made larger.
However, if the capacitance of the capacitor of the VCC
terminal is increased, the startup time is made longer.
In such cases, the circuit shown in Fig.24 can balance the
capacitance and the startup time.
By setting C2 to less than C3, the startup time can be kept
short. Since current is supplied via C3 after startup, the
VCC terminal voltage hold time can be kept long even
under sudden change conditions.
(2) VCC hold time
When the load of the power circuit is short-circuited, power
supply from the auxiliary winding is interrupted, causing
VCC to decrease. In this case, by resetting the VCC
terminal when it decreases to lower than the OFF threshold
voltage, VCCoff (9.8V typ), before the overload protection
delay time (TdOLP) elapses, startup and stop are repeated.
By this operation, the latch type is not stopped by latch
shutdown, but run and stop are repeated irrespective of the
auto reset wait time TdOLP2 (1400ms typ). (See Fig.25)
To avoid this phenomenon, connect a capacitor whose
capacitance does not allow VCC voltage to decrease down
to the OFF threshold voltage on occurrence of short circuit
of the load of the power circuit.
Fig. 21 VCC terminal circuit
Select a capacitor whose
capacitance does not allow VCC
voltage to decrease down to
VCCoff.
Auxiliary winding voltage
Time t
Fig. 22 VCC terminal voltage at startup
Time t
Fig. 23 VCC terminal voltage at startup (when
capacitance is too small)
Fig. 24 VCC circuit (shortening of startup time)
Fig. 25 VCC hold time (at output short circuit)
Fuji Electric Co., Ltd.
AN-027E Rev.0.4
April -2011
22
http://www.fujielectric.co.jp/products/semiconductor/