FA5590 FUJI [Fuji Electric], FA5590 Datasheet - Page 18

• Manufacturer Part Number: FA5590
• Description: FUJI Power Supply Control IC
• Manufacturer: FUJI [Fuji Electric]

Function
(i) Detect zero current through the inductor
(ii) Detect overcurrent and turn off OUT output
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.
Therefore, the value of RS (Ω) is determined as follows.
- 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.
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 47Ω.
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.
I
Rs
2
LP
(5) Terminal No. 5 (IS terminal)

AN-016E Rev.1.2
April-2011
(max)


I


LP
Fuji Electric Co., Ltd.
1
C

V
(max)
6
thIS

2



R
Vac

4
2
I
≒

LP
(min)
0
Po
.
(max)
1
58
～
[ 2 MHz
]
FA5590N,FA5591N
18
http://www.fujielectric.co.jp/products/semiconductor/
21.9kΩ
46.7kΩ
1.5V
1.4kΩ
Fig.20 IS pin protection circuit (1)
Fig.21 IS pin protection circuit (2)
IS
R4
5
Fig.19 IS pin circuit
100mV
20mV
R4
L1
Rs
ZD
C6
L1
ZD
Rs
R4
C1
C6
L1
C6
Rs
OCP.comp
ZCD.comp
Q1
Q1
7
5
Q1
7
5
GND
IS
GND
D1
IS
D1
D1
C2
C2
C2