FA5604 Datasheet, PDF (28/31 Pages) Fuji Electric – FUJI Power Supply Control IC

FA5604/5605/5606
(6)Preventing malfunction caused by noise
Noise applied to each pin may cause malfunction of the IC. If noise causes malfunction, see the notes
summarized below and confirm in actual circuit to prevent malfunction.
○1 The IS pin for overcurrent limiting function detects the MOSFET current converted to the voltage. The parasitic
capacitor and inductor of the MOSFET, transformer, wiring, etc. cause a noise in switching operation. If this
switching noise causes a malfunction of overcurrent limiting function, insert the RC filter into IS pin as shown in
Fig.13. Connect this capacitor as near the IC as possible to suppress noise effectively.
○2 If noise is applied to the FB pin, the output pulses may be disturbed. In this case, see .item (4) in Design advice.
○3 Relatively large noise may occur at the VCC pin because large current flows from VCC pin to drive the MOSFET.
Then noise may cause malfunction of the IC. In addition, the IC may stop operation when VCC voltage drops
below the off threshold voltage by noise. Mind that capacitance and characteristics of the capacitor connected
between VCC and GND pin not to allow the large noise at the VCC pin. To prevent malfunction, connect several
10F electrolytic capacitor and about 0.1F ceramic capacitor as near the IC as possible to suppress noise
effective (Connect the ceramic capacitor nearer the IC than the electrolytic capacitor by priority).
○4 RT terminal may cause a malfunction. It is recommended to install a capacitor of about 100 pF between the RT
terminal and the GND. The line regulation may also be improved by this measure.
(7)Preventing malfunction caused by negative voltage applied ton a pin
When large negative voltage is applied to each IC pin, a parasitic element in the IC may operate and cause
malfunction. Be careful not allow the voltage applied to each pin to drop below -0.3V.
Especially for the OUT pin, voltage oscillation caused after the MOSFET turns off may be applied to the OUT pin
via the parasitic capacitance of the MOSFET, causing the negative voltage to be applied to the OUT pin. If the
voltage falls below -0.3V, add a Schottky diode between the OUT pin and the ground as shown in Fig.30.
The forward voltage of the Schottky diode can suppress the voltage applied to the OUT pin.
Use the low forward voltage of the Schottky diode.
Similarly, be careful not to cause the voltages at other pins fall below -0.3V.
OUT
FA5604/05/06 ⁵
GND ⁴
SBD
▽
Fig.30
(8)Gate circuit configuration
To adjust switching speeds or prevent oscillation at gate terminals, resistors are normally inserted between the
power MOSFET gate terminal to be driven and the OUT pin of the IC.
You may prefer to decide on the drive current independently, to turn the MOSFET on and off.
If so, connect the MOSFET gate terminal to the OUT pin of the IC as shown in Fig.31.
In this circuit, Rg1 and Rg2 restrict the current when the MOSFET is turned on, and only Rg1 restricts the current
when it is turned off.
Rg1 Rg2
FA5604/05/06 ⁵
OUT
Fig.31
Fuji Electric Co., Ltd.
AN-033E Rev.1.2
April-2011
28
http://www.fujielectric.co.jp/products/semiconductor/