FA531X series
4.2 Overload shutdown
Figure 9 shows the overload shutdown circuit, and Figure 10 is
a timing chart which illustrates overload shutdown operation.
If the output voltage drops due to an overload or short-circuit,
the output voltage of the FB pin rises. If FB pin voltage
exceeds the reference voltage (2.8V) of comparator C3, the
output of comparator C3 switches low to turn transistor Q off.
In normal operation, transistor Q is on and the CS pin is
clamped at 3.6V by zener diode Zn. With Q off, the clamp is
released and the 10µA constant-current source begins to
charge capacitor CS again and the CS pin voltage rises. When
the CS pin voltage exceeds the reference voltage (7.0V) of
comparator C2, the output of comparator C2 switches high to
turn the bias circuit off. The IC then enters the latched mode
and shuts the output down. Shutdown current consumption is
400µA(VCC=9V). This current must be supplied through the
startup resistor. The IC then discharges the MOSFET gates.
Shutdown operation initiated by an overload can be reset by
lowering supply voltage VCC below VCC OFF or forcing the CS
pin voltage below 7.0V.
Fig. 9 Overload shutdown circuit
The period tOL from the time that the output is short-circuited to
the time that the bias circuit turns off is given by the following
equation:
tOL(mS)=340Cs(µF)........................................... (3)
4.3 Overvoltage shutdown
Figure 11 shows the overvoltage shutdown circuit, and Figure
12 is a timing chart which illustrates overvoltage shutdown
operation.
The optocoupler PC1 is connected between the CS and VCC
pins. If the output voltage rises too high, the PC1 turns on to
raise the voltage at the CS pin via resistor R6. When the CS
pin voltage exceeds the reference voltage (7.0V) of
comparator C2, comparator C2 switches high to turn the bias
circuit off. The IC then enters the latched mode and shuts the
output down. The shutdown current consumption of the IC is
400µA(VCC=9V). This current must be applied via startup
resistor R5.
Fig. 10 Overload shutdown timing chart
The IC then discharges the MOSFET gates.
The shutdown operation initiated by an overvoltage condition
can be reset by lowering supply voltage VCC below VCC OFF or
forcing the CS pin voltage below 7.0V.
During normal operation, the CS pin is clamped by a 3.6V
zener diode with a sink current of 65µA max. Therefore, a
current of 65µA or more must be supplied by the optocoupler
in order to raise the CS pin voltage above 7.0V.
Fig. 11 Overvoltage shutdown circuit
Fig. 12 Overvoltage shutdown timing chart
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