FA5527 FUJI [Fuji Electric], FA5527 Datasheet - Page 25

• Manufacturer Part Number: FA5527
• Description: Fuji Switching Power Control IC
• Manufacturer: FUJI [Fuji Electric]

10．Design advice
(1) Start-up and stop
values shall be set for capacitors connected to the CS pin
and VCC pin.
(1-1) At start-up (1)
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.
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)
capacitance is connected.
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.
connected to VCC pin so that the bottom of Vcc becomes
larger than 11V as the result of typical experiment.
AN-087E Rev.1.0
April-2011
To properly start up and stop the power supply, optimum
It takes certain time until the output voltage reaches to
On the other hand, when CS pin capacitance is too small,
Fig.24 shows Vcc voltage at start-up when proper
When input power is turned on, the VCC capacitor is
We recommend that you choose the capacitance
Fuji Electric Co., Ltd.
FA5526/5527/5528/5536/5537/5538
Secondary
Side DC
Output
Voltage
FB pin
Voltage
CS pin
Voltage
25
Fig.22 CS pin voltage waveform at start-up (1)
Fig.23 CS pin voltage waveform at start-up (2)
Secondary
Side DC
Output
Voltage
FB pin
Voltage
CS pin
Voltage
a) After CS pin exceeds 4V,
（when the power supply can not start up）
FB pin voltage drops.
Fig.24 Vcc waveform at start-up (1)
Start-up
of IC
（at normal start-up）
Start-up
of IC
http://www.fujielectric.co.jp/products/semiconductor/
DC output set voltage
( normal start )
Latch Mode
8.5V
4V
3V
b) Before CS pin exceeds 4V,
Start-up
FB pin voltage drops.
of IC
8.5V