FIP4F8S datasheet(19/33 Pages) FUJI

19 / 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/

(7) Soft start circuit

The CS pin is connected to a built-in constant current

source. The current for soft start is 11uA.

The capacitor externally connected to the CS pin is

charged by the constant current source, gradually

increasing CS pin voltage.

MOSFET current gradually increases at start-up

because CS pin voltage is input to the current

comparator (IS comp.), realizing soft start.

As a guide for soft start time, the time tss taken until

CS pin voltage increases from 0V to 3V is given by the

following equation.

tss [ s ] = 0.27 * Cs [ uF ] ( typical value )

Here, Cs is a capacitance connected to CS pin [ uF ].

In steady operation, CS pin voltage is clamped at

approximately 4V by a zener diode in the IC.

The CS pin is provided with a built-in circuit to stop

pulses when CS pin voltage is 0.68V or less, same as

FB pin.

(See “9-(14) Timing chart.”)

(8) Overload protection of FA5526/27/28

FA5526, FA5527 and FA5528 have built-in time-latch

type overload protection. Fig.12 shows its block diagram

and Fig.13 its Timing chart.

In steady operation, FB pin voltage is 3V or less and

CS pin voltage is clamped at 4V by a zener diode in the

IC.

When power supply voltage drops on account of

overload or short-circuit on the load side, FB pin voltage

increases. If FB pin voltage exceeds the 3.6V threshold

voltage for overload protection, output voltage of a

comparator for overload detection (Overload) is inverted

and 4V clamp of the CS pin is canceled, increasing CS

pin voltage again due to a built-in constant current

source. The current supplied from the CS pin becomes

4µA.

If the power supply voltage continues to decrease and

CS pin voltage reaches the threshold voltage (8.5V) of

the comparator (Latch), the output of the comparator

(Latch) is inverted, turning off a 5V circuit in the IC and

forcing OUT pin voltage to be low.

This status is the latch mode of the IC. In the latch

mode, the start-up circuit resumes operation to supply

current to Vcc and to hold the latch mode.

Buf

20k

60k

IS comp.

0.52V

11µA

VCC

Fig.11

Soft start circuit

ENB

VCC

5V Reg.

UVLO

5V Reg.

ENB

UVLO

F.F.

11uA/4uA

8.5V/7.7V

4.8V

2.8V

OUT

VCC

Overload

Latch

Fig.12

Overload protection circuit

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