FAN7554

11

Gate Driver

The gate drive circuit has the totem-pole output configuration. The output has 1A peak current and 200mA average current

drive ability.

ON/OFF Control

The FAN7554 is able to use the feedback pin for ON/OFF control by placing NPN transistor between the cathode of the

KA431 and ground as shown in figure 19. When the transistor turns on, the current flows through the photo diode and

saturates the photo transistor. As a result, the feedback voltage is dropped to zero. When the feedback voltage is below 0.3V,

the soft start voltage starts to discharge by connecting the internal resistor 1k

Ω in parallel with the external capacitor Cs. When

the soft start voltage becomes less than 1.5V, all the blocks in the FAN7554 are turned off , with the exceptions of the UVLO

block and ON/OFF control block. The operation current is about 200uA. So the stand-by power is reduced and SMPS

efficiency is improved. When the feedback voltage exceeds 0.3V, the FAN7554 normally operates by turning on Vref block.

Figure 17-B . Delayed Shutdown & Feedback Waveform

t

Vfb

Slope (dv/dt) = 5uA / Cfb

3V

6V

Shutdown start point

t2

t1

t

Vfb

Slope (dv/dt) = 5uA / Cfb

3V

6V

Shutdown start point

t2

t1

Figure 18. Gate Drive Circuit

DRAIN

FAN7554

OUT

7

6

Clock

Shutdown

Q

DRAIN

FAN7554

OUT

7

6

Clock

Shutdown

Q

FAN7554

OUT

7

6

Clock

Shutdown

Q