FA531X series
4. CS pin circuit
As shown in Figure 5 capacitor CS is connected to the CS pin.
When power is turned on, the constant current source (10µA)
begins to charge capacitor CS. Accordingly, the CS pin
voltage rises as shown in Figure 6. The CS pin is connected to
an input of the PWM comparator. The device is in soft-start
mode while the CS pin voltage is between 0.9V and 1.9V
(FA5310/14/16) and between 0.9V and 2.3V(FA5311/15/17).
During normal operation, the CS pin is clamped at 3.6V by
internal zener diode Zn. If the output voltage drops due to an
overload, etc., the clamp voltage shifts from 3.6V to 8.0V. As a
result, the CS pin voltage rises to 8.0V. The CS pin is also
connected to latch comparator C2. If the pin voltage rises
above 7.0V, the output of comparator C2 goes high to turn off
the bias circuit, thereby shutting the output down. Comparator
C2 can be used not only for shutdown in response to an
overload, but also for shutdown in response to an overvoltage.
Comparator C1 is also connected to the CS pin, and the bias
circuit is turned off and the output is shut down if the CS pin
voltage drops below 0.42V. In this way, comparator C1 can
also be used for output on/off control.
Fig. 5 CS pin circuit
As explained above, the CS pin can be used for soft-start
operation, overload and overvoltage output shutdown and
output on/off control.
Further details on the four functions of the CS pin are given
below.
4.1 Soft start function
Figure 7 shows the soft start circuit. Figure 8 is the soft-start
operation timing chart. The CS pin is connected to capacitor
CS. When power is turned on, a 10µA constant-current source
begins to charge the capacitor. As shown in the timing chart,
the CS pin voltage rises slowly in response to the charging
current. The CS pin is connected internally to the PWM
comparator. The comparator output pulse slowly widens as
shown in the timing chart.
The soft start period can be approximately evaluated by the
period ts from the time the IC is activated to the time the output
pulse width widens to 30%. Period ts is given by the following
Fig. 6 CS pin waveform
equation:
tS(mS)=160CS(µF)...................................(2)
Fig. 7 Soft-start circuit
Fig. 8 Soft-start timing chart
5
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