FIP4F8S datasheet(26/33 Pages) FUJI

26 / 33 page

AN-087E Rev.1.0

April-2011

FA5526/5527/5528/5536/5537/5538

Fuji Electric Co., Ltd.

http://www.fujielectric.co.jp/products/semiconductor/

When Vcc capacitance is too small, Vcc drops to the

OFF threshold voltage as shown in Fig.25 before the

auxiliary winding voltage increases sufficiently. In this case,

Vcc repeatedly goes up and down between the ON and

OFF threshold voltages, and the power supply can not

start up.

(1-3) At stopping

When the power supply is turned off by shutdown of

input voltage, output voltage remains low for certain period

of time before the IC stops operation.

During this period, FB pin voltage increases and the CS

pin clamp circuit is cancelled because output voltage

remains low. As a result, CS pin voltage increases as

shown in Fig.26.

CS pin voltage shall not reach the threshold voltage of

the latch mode. As shown in Fig.27, if CS pin voltage

reaches the threshold voltage, the latch mode is held for a

period of time until Vcc capacitor voltage drops to OFF

threshold voltage. As a result, the power supply cannot be

re-started even if input voltage is turned on again.

In such a case, the following measures shall be taken:

・Reduce the time taken until the IC stops operation after

the output voltage has dropped through reducing Vcc

capacitance.

・Suppress CS pin voltage rise through increasing CS pin

capacitance.

(2) Hold time of Vcc

In some cases, VCC pin capacitance shall be increased

to hold Vcc above the OFF threshold voltage at abrupt load

change after the power supply has started up.

However, when VCC pin capacitance becomes larger,

start-up time gets longer.

In such a case, the circuit shown in Fig.28 is effective.

Reducing C2 shortens start-up time, and hold time can

be kept long because power is supplied via C4 after

start-up.

Fig.28

Vcc circuit

Fig.25

Vcc waveform at start-up (2)

（when the power supply can not start up）

VCC pin

Voltage

8.5V

Secondary

Side DC

Output

Voltage

CS pin

Voltage

FB pin

Voltage

OFF

Threshold

Voltage

( 9V )

Fig.26

Waveform at stopping (1)

8.5V

Latch Mode

( The period when

IC can not re-start )

VCC pin

Voltage

Secondary

Side DC

Output

Voltage

CS pin

Voltage

FB pin

Voltage

OFF

Threshold

Voltage

( 9V )

Fig.27

Waveform at stopping (2)

FIP4F8S Datasheet(PDF) 26 Page - Fuji Electric

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