FA5604 Datasheet, PDF (29/31 Pages) Fuji Electric – FUJI Power Supply Control IC

FA5604/5605/5606
(9)External latch (overvoltage protection on secondary side)
Fig.32 shows the overvoltage shutdown circuit based on the signal from the secondary side.
The optocoupler output transistor is connected between the CS and VCC pins. When the output voltage is put in
the overvoltage state, the optocoupler output transistor goes on to raise the CS pin voltage via resistor R2. When
the CS pin voltage exceeds the reference voltage (7.3V) of internal comparator, the IC enters the OFF latch mode
and shuts the output down. The IC consumes current 190A (typ.) (VCC=14V) in latch mode. This circuit must be
supplied via startup resistor R1.The overvoltage protection circuit can be reset by lowering the supply voltage VCC
to below 9.7V or forcing the CS pin voltage below 7.3V.
In normal operation, the CS pin voltage is clamped by the 3.8V zener diode with maximum sink current 100A
(max.). Therefore, to raise the CS pin voltage to 7.3V or more, 200A or a higher current needs to be supplied from
the optocoupler. Set the current input to the CS pin to 2mA or less.
Vin
C1
▼
R1 ^PGND
R2
PC
Vout
Cs
C2
VCC
6
▽
CS
8
▽
FA5604
4
GND
▽
Fig.32
(10)Not used the overload shutdown function
As explained by pre-block (6), the clamp CS pin voltage (3.8V) is canceled at overload (VFB>3.5V, VVF<3V), and
the CS pin voltage oscillates and counts between 3.8V and 5.7V. The overload shutdown function is prevented by
clamping the CS pin voltage on the outside.
Fig.33 shows the CS pin voltage is clamped by internal zener diode ZD. In this case, because of zener current Iz
is 10 A, please set the zener voltage to between 3.8V to 5.7V.
VCS=Vz=3.8V to 5.7V ····································· (11)
Moreover, when the overvoltage protection function is made effective, you set external pull-up to exceed the latch
threshold voltage 7.3V by using the zener diode ZD and the resistor R (Fig.28). in that case, the zener voltage VZ
should consider the voltage (R×Iz) of the resistor R.
VCS=Vz+(R×Iz)=3.8V to 5.7V ····················· (12)
When the IC operate of overvoltage protection, to raise the CS pin voltage to 7.3V or more, 200A to 2mA current
needs to be supplied. In this case, because the zener current Izovp (1mA etc) differs from Iz=10A, the zener
voltage Vzovp differs from Vz.
VCS=Vzovp+(R×Izovp)≧7.3V ····················· (13)
Because a method in which the VF pin is not reduced to less than 3 V interferes with reduction of the frequency, it
is recommended to use a method in which the CS pin is clamped by the Zener diode.
Cs
C2
VCC
6
▽
CS
8
▽ _FA5604/05/06
ZD Vz
▽
Fig.33
4
GND
▽
R2
PC
Cs
C2
VCC
6
▽
CS
8
R
▽ _FA5604/05/06
ZD Vz
Fig.34
4
▽
GND
▽
Fuji Electric Co., Ltd.
AN-033E Rev.1.2
April-2011
29
http://www.fujielectric.co.jp/products/semiconductor/