FA5613N datasheet(21/30 Pages) FUJI

21 / 30 page

.0.08AN-049J Rev.1.0

AN-054E Rev.1.2

Apr.-2011

FA5612, FA5613

Fuji Electric Co., Ltd.

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

Besides, Soft start function is realized by slowing a voltage rise of COMP pin

with adjustment of capacitance connected between COMP and GND.

When starting, VCMP voltage rises and a switching operation starts at 0.5 V.

At this time, MOSFET ON width increases gradually in proportion to the COMP

voltage rise. The soft start time is adjustable with adjustment with changing

capacitors C4 and C5.

Increasing C4, C5 capacitance → prolongs soft start time

Decreasing C4, C5 capacitance → shortens soft start time

(3) Terminal No.3 (VDET pin)

Function

(i) Input voltage detection (multiplier input)

Application

(i) Input voltage setting for VDET pin

Multiplier generates current reference signal. Its VDET pin (pin 3) receives

sinusoidal full-waveform rectified waveforms. Considering the dynamic range of

multiplier operation, VDET pin peak value would be used within 0.65 to 2.4 V.

Therefore, set R6 and R7 in Fig. 17 so that the peak voltage of sinusoidal

waveforms at VDET pin will be between 0.65 and 2.4 V with the entire range of

AC input voltage.

Recommended dividing ratio: R6:R7 = 160:1

Caution:

Because multiplier has dispersions of characteristics, even if VDET pin voltage

is 2.4 V or lower, multiplier might be saturated and the input peak current would

slightly be truncated like trapezoidal waves.

(4) Terminal No.4 (IS pin)

Functions

(i) Inductor current detection

(ii) Turn off OUT output upon detecting an overcurrent

Applications

(i) Via IS pin, inductor current signal is applied to current error amplifier

(CUR_AMP) that constitutes a current loop for making the input current follow

sinusoidal waveforms. CUR_AMP that is connected with multiplier output

receives current reference signal. IS pin is current input terminal, and receives a

potential of 0 to -0.4 V with respect to GND level.

(ii) When IS pin voltage is lower than IS threshold voltage, comparator output

signal is inverted to turn off OUT output.

▪ Supplement

When MOSFET turns on, MOSFET gate drive current and surge current are

generated by discharge of stray capacitance, and then those currents flow to

current detecting resistance Rs. An excessively large surge current may cause

an erratic operation to disturb the input current waveform. Depending on the

surge current intensity, timing, etc., pulse shoots may mix with turn-on parts of

IC's OUT pulses. So, RC filter shown in Fig. 18 is connected in generally.

Since the level is determined by resistive division as shown in Fig. 18, 100 Ω or

lower is recommended for input resistance R4.

Rated voltage at IS pin for input of inductor current signal is -5 V.

In case of a general boost circuit, a rush current flows for charging the output

smoothing capacitance C2 the instant an AC input voltage has been connected.

This current may be considerably greater than at a steady operation.

As a result, IS pin may receive a voltage that is far higher than usual. Even if

such AC voltage is connected, IS pin must not be exposed to a potential higher

than -5 V that is an absolute maximum rated voltage.

There may be the case that a voltage higher than the rated is applied to IS pin.

In such case, it needs to use a preventive circuit for suppression of a rush

current or to add the Zener diode to IS pin as shown in Fig. 19.

Fig.19 IS pin circuit (2)

GND

FA5610

12kΩ

28kΩ

Detector

Fig.18 IS pin circuit

Fig.17 VDET terminal circuit

VDET

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