7

FA5331P(M)/FA5332P(M)

6. ON/OFF control input circuit

Figure 7 shows the ON/OFF control input circuit. If pin 12 is set

to the high level (enable), this IC outputs pulses from the OUT

pin. If pin 12 is set to the low level (disable), the internal bias

power (reference voltage) goes off and the IC current

consumption becomes about 1/10 that of its ON state. The

output level of pin 11 (CS for soft start) also goes low.

7. Output circuit

As Fig. 8 shows, pin 9 is configured as the high power terminal

(VC), independent of the IC power terminal (VCC). This pin

allows an independent drive resistance when the power

MOSFET is ON and OFF. If the drive resistances in the ON and

OFF states are Rg (on) and Rg (off), the following formulas can

be used to determine the total gate resistance

Rg:

Rg (on) = Rg1 + Rg2 ............................................. (9)

Rg (off) = Rg2 ..................................................... (10)

In the standby state, the output level of pin 8 is held low.

If the potential at the drain terminal of the power MOSFET

fluctuates, the gate-drain capacitance may drive the IC output

voltage at pin 8 to below 0. Once the voltage at pin 8 reaches

–0.6V, an unwanted current flows in the IC and a large abnormal

current flows in the output circuit when the output transistor is

turned on. To prevent this, connect a Schottky diode across the

gate and source of the power MOSFET.

Fig. 7 ON/OFF control input circuit

Fig. 8 Output circuit

1k

100k

Vcc

12

ON/OFF

10

µA

Rg2

Rg1

GND

VCC

Cv

+

10

9

8

7

Pin7

Schottky

diode