FA551 Datasheet, PDF (19/22 Pages) List of Unclassifed Manufacturers – CMOS IC For Switching Power Supply Control

12. 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.
• 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 limitimg function,
insert the RC filter into IS pin as shown in Figure 13 and 14.
Connect this capacitor as near the IC as possible to suppress
noise effectively.
• Connect a noise prevention capacitor (0.1µF or more) to the
REF pin that outputs the reference voltage for each
component.
• If noise is applied to the FB pin, the output pulses may be
disturbed. In this case, see item 7 in “Design advice.”
• Relatively large noise may occur at the VCC pin because
large current flows from VCC pin to drive the MOSFET. Then
this 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, suppress the noise width below about
0.5µs or less and noise voltage below about ±0.6V or less.
13. Preventing malfunction caused by negative voltage
applied to 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 to 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. 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 to
fall below –0.3V.
14. 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 Figure 34. 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.
FA551X
FA551X
4
GND
OUT
5
SBD
Fig. 33 Protection of OUT pin against the negative voltage
Rg1 Rg2
FA551X ⁵
OUT
Fig. 34 Gate circuit
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