FA531X series
4.4 Output ON/OFF control
The IC can be turned on and off by an external signal applied
to the CS pin.
Figure 13 shows the external output on/off control circuit, and
Figure 14 is the timing chart.
The IC is turned off if the CS pin voltage falls below 0.42V. The
output of comparator C1 switches high to turn the bias circuit
off. This shuts the output down. The IC then discharges the
MOSFET gates.
The IC turns on if the CS pin is opened for automatic soft start.
The power supply then restarts operation.
5. Overcurrent limiting circuit
The overcurrent limiting circuit detects the peak value of every
drain current pulse of the main switching MOSFET to limit the
overcurrent.
The detection threshold is + 0.24V for FA5310B/11B/16/17
with respect to ground as shown in Figure 15.
The drain current of the MOSFET is converted to voltage by
resistor R7 and fed to the IS pin of the IC. If the voltage
exceeds the reference voltage (0.24V) of comparator C4, the
output of comparator C4 goes high to set flip-flop output Q
high. The output is immediately turned off to shut off the
current. Flip-flop output Q is reset on the next cycle by the
output of the oscillator to turn the output on again. This
operation is repeated to limit the overcurrent.
Fig. 13 External output on/off control circuit
If the overcurrent limiting circuit malfunctions due to noise,
place an RC filter between the IS pin and the MOSFET.
Figure 16 is a timing chart which illustrates current-limiting
operations.
Fig. 14 Timing chart for external output on/off control
Fig. 15 Overcurrent limiting circuit for FA5310/11/16/17
Fig. 16 Overcurrent timing chart for FA5310/11/16/17
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