FIP4F8S datasheet(25/33 Pages) FUJI

25 / 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/

10．Design advice

(1) Start-up and stop

To properly start up and stop the power supply, optimum

values shall be set for capacitors connected to the CS pin

and VCC pin.

(1-1) At start-up (1)

It takes certain time until the output voltage reaches to

the set voltage after the IC has been activated. During this

period, FB pin voltage reaches its maximum voltage and

the 4V clamp circuit does not operate. As a result, with

proper CS pin capacitance and proper start-up, CS pin

voltage waveform during start-up will be as shown in

Fig.22.

On the other hand, when CS pin capacitance is too small,

CS pin voltage may reach the threshold voltage of the latch

mode as shown in Fig.23 before the output voltage

increases to the set value. The IC changes into a latch

mode and the power supply cannot start properly. In cases

like this, increase CS pin capacitance.

(1-2) At start-up (2)

Fig.24 shows Vcc voltage at start-up when proper

capacitance is connected.

When input power is turned on, the VCC capacitor is

charged by the current supplied from the start-up circuit

and its voltage increases. Then, when Vcc reaches the ON

threshold voltage, the IC starts operation. In steady

operation, the IC operates at the voltage supplied from an

auxiliary winding. Right after IC start-up, however, Vcc

drops until the auxiliary voltage increases sufficiently.

Determine the capacitance connected to VCC pin so that

Vcc does not drop to the OFF threshold voltage in any

condition.

We recommend that you choose the capacitance

connected to VCC pin so that the bottom of Vcc becomes

larger than 11V as the result of typical experiment.

Secondary

Side DC

Output

Voltage

CS pin

Voltage

FB pin

Voltage

8.5V

Start-up

of IC

a) After CS pin exceeds 4V,

FB pin voltage drops.

Start-up

of IC

b) Before CS pin exceeds 4V,

FB pin voltage drops.

Fig.22

CS pin voltage waveform at start-up (1)

( normal start )

8.5V

Latch Mode

Secondary

Side DC

Output

Voltage

CS pin

Voltage

FB pin

Voltage

Start-up

of IC

DC output set voltage

Fig.23

CS pin voltage waveform at start-up (2)

（when the power supply can not start up）

Fig.24

Vcc waveform at start-up (1)

（at normal start-up）

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