5

FA531X series

4. CS pin circuit

When power is turned on, the constant current source (10

µA)

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.

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

µA 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

equation:

µF)...................................(2)

Fig. 5 CS pin circuit

Fig. 6 CS pin waveform

Fig. 7 Soft-start circuit

Fig. 8 Soft-start timing chart