FIP4F8S datasheet(17/33 Pages) FUJI

17 / 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/

At start-up, soft start can be realized through gradually

increasing the threshold voltage based on CS pin

voltage.

At steady operation, the threshold voltage is varied

based on FB pin voltage to keep power supply output

voltage constant.

In addition, the maximum IS pin threshold voltage as

520mV limits MOSFET over-current when FB pin voltage

is very high like 4V by overload etc.

The oscillator generates a pulse to determine the

maximum duty cycle of an OUT pulse and the maximum

duty cycle is set at 80% (typ.) using this pulse..

For details, refer to “9-(14) Timing chart”.

(4) Blanking

When MOSFET turns on, a surge current is generated

due to discharge current from the capacitor in the main

circuit or gate drive current. If the surge current reaches

the IS pin threshold voltage, current comparator output

could be inverted and normal pulses would not be

generated from the OUT pin.

To avoid this, a blanking function is incorporated into

the current comparator. When a trigger signal is input

from the oscillator, the blanking circuit outputs a

certain-width pulse signal as a PWM latch (F.F.) set

signal.

Since the set signal is given a high priority in PWM

latch input signals, the output of PWM latch (F.F.) will not

be inverted while the set signal is input from the blanking

circuit, even if a rest signal is input from the current

comparator (IS comp.).

As a result, the IS pin input voltage is ignored for a

blanking

time

(200ns

for

FA5526/36,

400ns

for

FA5527/37 and 600ns for FA5528/38) immediately after

an output pulse has been generated from the OUT pin

and does not respond to a surge current at turn-on.

（See Fig.8.）

In general, the blanking circuit eliminates the need for

a noise filter at the IS pin.

Buf

20k

60k

IS comp.

0.52V

11µA

VCC

Fig.7

Current comparator

Fig.8

Blanking

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