FA3641 데이터 시트보기 (PDF) - Fuji Electric

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FA3641

PWM control IC with light load power saving function For Switching Power Supply Control

Fuji Electric 

FA3641P(N), FA3647P(N)

5. Overcurrent limiting circuit

The overcurrent limiting circuit detects the peak value of every

drain current pulse (pulse by pulse method) of the main

switching MOSFET to limit the overcurrent. The detection

threshold voltage is +0.235V for FA3641 or –0.168V for FA3647

with respect to the ground as shown in Figure 14.

The drain current of the MOSFET is converted to voltage by

resistor Rs and fed to the IS pin of the IC. If the voltage

exceeds the reference voltage +0.235V (FA3641) or –0.168V

(FA3647) of comparator C4, comparator C4 works to set flip-

flop output Q to high. The output is immediately turned off to

shut off the current. Flip-flop output Q 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

Figure 14. (See item 14 in “Design advice.”)

Figure 15 is a timing chart that illustrates overcurrent-limiting

operations.

6. Vcc overvoltage protection circuit

The IC contains a Vcc overvoltage protection circuit to protect

the IC from damage by overvoltage. Figure 16 shows the

overvoltage protection circuit. Figure 17 is a timing chart that

illustrates overvoltage protection operations.

Overvoltage is detected if the supply voltage Vcc rises to 32V

(Icc = 13mA) or more and current flows in the built-in zener

diode. The output of comparator C5 then goes high and the

constant current source (0.9mA) raises the CS pin voltage.

When the CS pin voltage exceeds 8.5V, the output of

comparator C2 goes high to turn off the 5V REF circuit. The IC

then enters the latched mode and the IC output is put in the off

(low voltage) state. When latched mode, the IC current

consumption is 45µA (typ) (Vcc = 10V). This current must be

supplied through the startup resistor.

The overvoltage shutdown operation can be reset by lowering

the supply voltage to below 9.0V or forcing the CS pin voltage

below 7.9V.

(When you want to enable Vcc overvoltage shutdown at a

desired voltage, see item 7 in “Design advice.”

Fig. 14 Overcurrent limiting circuit

Fig. 15 Overcurrent timing chart

Fig. 16 Overvoltage shutdown circuit

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Fig. 17 Overvoltage shutdown timing chart

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