FA5604 Datasheet, PDF (20/31 Pages) Fuji Electric – FUJI Power Supply Control IC

FA5604/5605/5606
(4)Overcurrent limiting circuit
This circuit detects the peak value of every drain current pulse (pulse by pulse method) of the main switching
MOSFET to limit the overcurrent, and the detection threshold voltage is -0.17V with respect to the ground level. Fig.
13 shows overcurrent limiting circuit, and Fig.14 is the overcurrent timing chart.
The drain current of the MOSFET is converted to voltage by resistor Rs and fed to the IS pin of the IC. If the
detection voltage becomes -0.17V or less, the O.C.P comparator output is high, and the RS-FF output QB to Low.
The OUT of this IC becomes L level at the moment, the MOSFET is turned off to shut off the current. The flip-flop
output is reset on the next cycle to turn on the output again. This operation is repeated to limit the overcurrent.
If the overcurrent limiting circuit malfunctions due to noise, place an RC filter between the IS pin and MOSFET as
shown in Fig. 13. (See item 6 in “Design advice”)
OSC output
-0.17V
▽
3
IS(-)
O.C.P.
C
R
QB
S
F.F
OUT
4
GND
R
▽
OSC output
Rs
▼
PGND
OUT pin voltage ^H
L
IS pin voltage
OCP comparator
(-0.17V)
Fig.13
Fig.14
Over
Over
Current Current
detection detection
(5)Overload protection circuit (switching frequency control by VF pin)
The voltage proportional to the output voltage is input the VF pin. If the output voltage drops due to an overload
and so on, the FB pin voltage rises and the VF pin voltage decreases. When the FB pin voltage exceeds 3.5V, IC is
operating the OLP mode. The VF pin voltage becomes 4.2V or less, the oscillatory frequency decreases according
to the voltage value. In adds to overcurrent limiting (preceding clause 4), overload protect by decreasing the
oscillatory frequency (See item 5 in “Design advice”)
To a voltage proportional to the output voltage, apply the OUT terminal voltage smoothed in the case of the
forward circuit, or apply the VCC voltage in the case of the flyback circuit (see Fig. 15).
OUT
FA5604 ⁵
7
VF
R4
C6
Forward circuit
VCC
6
FA5605/06
7
VF
R4
C6
R6
Fig.15
Flyback circuit
Fuji Electric Co., Ltd.
AN-033E Rev.1.2
April-2011
20
http://www.fujielectric.co.jp/products/semiconductor/