FA5695 Datasheet, PDF(18/21 Page) Fuji Electric – Fuji Switching Power Supply Control IC

English

English German Russian Spanish Italian Polish Chinese Japanese Korean French Portuguese	Language :

FA5695 Datasheet, PDF (18/21 Pages) Fuji Electric – Fuji Switching Power Supply Control IC

◁

(5) Terminal No. 5 (IS terminal)

Function

(i) Detect zero current through the inductor

(ii) Detect overcurrent and turn off OUT output

FA5695

Application

(i) Detect the current value through the inductor

The maximum threshold voltage Vthis of the IS terminal is -0.58V (max).

The current detection resistance Rs is set so that necessary current can

be supplied for this VthIS.

With maximum output Po (W) and minimum input voltage Vac (min), the

maximum value of peak current (ILP (max)) through the inductor can be

approximately expressed by the following formula.

2 ï´ 2 ï´ Po

I LP(max) ï½

ï¨ ï´ Vac(min)

Therefore, the value of RS (â¦) is determined as follows.

Rs ï¼ ï VthIS ï½ 0.58

ILP(max) ILP(max)

- Wiring

Connect the current detection resistor Rs between the source terminal

(GND) of MOSFET and the minus lead of the input capacitor (C1). The

voltage across Rs is fed to the IC as the current/voltage conversion

signal.

- Operation

(i) The internal reference voltage and the internally divided voltage of the

IS terminal are inputted to the ZCD comparator, and when the IS

terminal voltage becomes larger than -10mV, the comparator output

inverts and turns on the OUT output.

(ii) When the IS terminal voltage becomes smaller than -0.6V, the

comparator output signal inverts and turns off the OUT output.

- Additional explanation

When MOSFET turns on, the gate driving current of MOSFET and surge

current due to discharging the parasitic capacitors run to the current

detection resistance Rs. Large surge current may cause malfunction

following disturbed input current waveform. Depending on the amperage

of the surge current or timing, whisker-like pulse may be mixed in the

turn-on portion of the OUT pulse of the IC. Normally, therefore, a CR

filter is connected as shown in Fig.20. The cutoff frequency of this CR

filter must be set sufficiently higher than the switching frequency so that

it will not affect the normal operation.

It is recommended to set this cutoff frequency to about 1~2MHz.

â 1ï½2[MHz]

2 ï´ ï° ï´ C6 ï´ R4

Since the threshold level is made through resistance dividing voltage as

shown in Fig.19, the input resistor R4 is recommended to be not higher

than 47â¦.

The voltage rating of the IS terminal is -5V. In case of an ordinary

boosting circuit, rush current to charge the output smoothing capacitor

C2 runs at the moment the ac input voltage is connected. This current

may become far larger in comparison with the input current during

normal operation.

As a result, far larger voltage may also be applied to the IS terminal than

the ordinary case.

In order to avoid damage, protective circuit must be taken in design so

that voltage higher than -5V, absolute maximum rating, will not be

applied to the IS terminal even when such ac input voltage is connected.

If voltage higher than the rating is predicted to be applied to the IS

terminal, use rush preventive circuit suppress rushing current or place

Zener diode shown in Fig. 20 and Fig. 21.

1.4kÎ©

21.9kÎ© 20mV

ZCD.comp

OCP.comp

46.7kÎ© 100mV

1.5V

Fig.19 IS pin circuit

GND

Fig.20 IS pin protection circuit (1)

R4 ZD

GND

Fig.21 IS pin protection circuit (2)

Fuji Electric Co., Ltd.

AN-073E Rev.1.1

April-2011

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

▷