FA7711V Datasheet, PDF (16/25 Pages) Fuji Electric – FUJI Power Supply Control IC

FA7711V
（5）Soft start circuit
This IC has a soft start function to protect DC-to-DC
converter circuits from damage when starting
operation. CS1 pin (Pin11), CS2 pin (pin14) and CS3
pin (Pin16) are used for soft start function of ch1, ch3
and ch2 respectively. (Fig. 5)
When the supply voltage is applied to the VCC pin
and UVLO is cancelled, capacitor CCS1, CCS2 and
CCS3 is charged by VREF through the resistor RCS1 ,
RCS2 or RCS3. Therefore, CS1 and CS3 voltage
gradually increases and CS2 voltage gradually
decreases. Since CS1, CS2 and CS3 pin are
connected to the PWM comparator internally, the
pulses gradually widen and then the soft start
function operates. (Fig. 6)
The maximum duty cycle can be set by using the CS
pins. (See Design Advice about the detail)
RCS1
CCS1
4 VREF
11 CS1
CCS2
RCS2
4 VREF
14 CS2
RCS3
CCS3
4 VREF
16 CS3
Fig.5
Oscillator output
Error Amplifier 1,3 output
CS1，CS3 pin voltage
（6）Timer latched short-circuit protection
This IC has the timer latch short-circuit protection
circuit. This circuit cuts off the output of all channels
when the output voltage of DC-to-DC converter drops
due to short circuit or overload. To set delay time for
timer latch operation, a capacitor CCP should be
connected to the CP pin (Fig. 7).
ICP
1 CP
VCP
CCP
Fig.7
When one of the output voltage of the DC-to-DC
converter drops due to short circuit or overload, the
FB1 and FB3 pin voltage increases up to around the
VREF voltage for ch1 and ch3, or the FB2 pin
voltage drops down to around 0 V for ch2. When FB1
and FB3 pin voltage exceeds 3.2V(max.) or FB2 pin
voltage falls below 0.4V(min.), constant-current
source (2.5μA typ.) starts charging the capacitor CCP
connected to the CP pin. If the voltage of the CP pin
exceeds 3.2 V (max.), the circuit regards the case as
abnormal. Then the IC is set to off latch mode and
the output of all channels is shut off, (Fig. 8) and the
current consumption become 4mA(typ.) The period
(tp) between the occurrence of short-circuit in the
converter output and setting to off latch mode can be
calculated by the following equation:
tp[s] = CCP × ^VTHCPON
ICP
VTHCPON: CP pin latched mode threshold voltage [V]
ICP: CP charge source current[μA]
CCP: capacitance of CP pin capacitor[μF]
PW M output 1，3
OUT1
OUT3 Pch driven
(S E L 3 ：V R E F )
OUT3 Nch driven
(SEL3：G ND)
CS2 pin voltage _{Oscillator output}
Error Amplifier 2 output
PW M output 2
OUT2 Pch driven
(S E L 2 ：V R E F )
OUT2 Nch driven
(SEL2：G ND)
Fig.6
3.0V
3.2V(max)
momentary
short circuit
2.0V _Start-up
1.0V
short circuit
tp
VREF voltage
short circuit
protection
Time t
Fig.8
You can reset off latched mode of the short-circuit
protection by either of the following ways about 1) CP
pin, or 2) VCC pin:
1) CP voltage = 0V
2) VCC voltage UVLO voltage (3.3V, typ.) or below
If the timer-latched mode is not necessary, connect
the CP pin to GND.
Fuji Electric Systems Co., Ltd.
AN-060E Rev.1.0
16
Jun-2010
http://www.fujielectric.co.jp/fdt/scd/