fa5311bs Fuji Electric holdings CO.,Ltd, fa5311bs Datasheet - Page 5

• Manufacturer Part Number: fa5311bs
• Description: Bipolar Ic For Switching Power Supply Control
• Manufacturer: Fuji Electric holdings CO.,Ltd

FA531X series
5
4. CS pin circuit
As shown in Figure 5 capacitor C
When power is turned on, the constant current source (10 A)
begins to charge capacitor C
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
C
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:
t
S
S
(m
. When power is turned on, a 10 A constant-current source
S
)=160C
S
( F)...................................(2)
S
. Accordingly, the CS pin
S
is connected to the CS pin.
Fig. 8 Soft-start timing chart
Fig. 6 CS pin waveform
Fig. 7 Soft-start circuit
Fig. 5 CS pin circuit