FA5604 Datasheet, PDF (21/31 Pages) Fuji Electric – FUJI Power Supply Control IC

FA5604/5605/5606
(6)Overload protection circuit
The IC contains an overload protection circuit (operation that switching turned ON/OFF in fixed time ratio). Fig.16
shows a timing chart at overload.
During normal operation, the CS pin voltage is clamped at 3.8V by internal zener diode. The VF pin is input to the
voltage proportional to the output voltage.
If the output voltage drops due to an overload and so on, the FB pin voltage rises to raise the output voltage. If the
FB pin voltage exceeds 3.5V, the IC toggles to overload protection (OLP) mode. The VF pin voltage decreases at the
same time as output voltage. If the VF pin voltage reduced to 4.2V or less, switching frequency control is began at
overload. Moreover, if the VF pin voltage becomes 3.0V or less, the clamp voltage is canceled and the capacitor Ccs,
which is connected the CS pin, is charged by constant current source (10A).
If the CS pin voltage rises to 5.8V or more, it toggles to discharge mode, and the capacitor is discharged by
constant current source (10A). If the CS pin voltage reduced to 3.9V or less again, it toggles to charge mode,
and the capacitor is charged.
The CS terminal performs an oscillation operation between 3.8 V and 5.7 V by repeating the above operation.
The intermittent operations are performed by counting the oscillation waveforms inside the IC and by making a
setting so that the count period of 1 to 64 is the switching operation period and the count period of 65 to 512
(FA5604/06) or 65 to 1024 (FA5605) is the switching shutdown period (switching-on period: switching-off period:
64:448 = 1:7 or 64:960 = 1:15). The operation frequency during the intermittent operation is under the condition in
which the frequency is reduced.
OLP Mode
Switching ON
Switching OFF
Switching ON
Switching OFF
A
B
C
C
OUT
3.5V
FB
VF
4.2V
3.0V
5.7V
CS _3.8V
A : Normal operation B : OLP operation (frequency contorol)
C : OLP operation (frequency contorol + hiccup)
Fig.16
(7)Undervoltage lockout circuit (U.V.L.O)
The IC incorporates a circuit that prevents the IC from malfunctioning when the supply voltage drops. When the
supply voltage is raised from 0V, the IC starts operation with VCC=17.5V (typ.). If the supply voltage drops, the
output is shut down when VCC =9.7V (typ.). When the undervoltage lookout circuit operates, the output of the the
OUT and CS pins go low to reset the IC.
(8)Output circuit
The IC contains a push-pull output stage and can directly drive the MOSFET. The absolute maximum rating of
OUT pin peak current is +1.0A/-0.5A. But when using in actual circuit, the output peak current depends on the
characteristics of the MOSFET, the resistance between the OUT pin and the MOSFET, supply voltage,
temperature conditions.
The output current causes loss of the output stage. The total loss caused by the operating current and the output
current should be within the ratings in actual circuit. (See item 11 in “Design advice”)
If the circuit turned off by undervoltage lockout circuit, the output of OUT pin become low level and the MOSFET is
shut down.
Fuji Electric Co., Ltd.
AN-033E Rev.1.2
April-2011
21
http://www.fujielectric.co.jp/products/semiconductor/