FA5640 Datasheet, PDF (28/35 Pages) Fuji Electric – Fuji Switching Power Supply Control IC

FA5640/41/42
(3) Pin No. 3 (IS pin)
Function
(ⅰ) Detects and limits the current value of the MOSFET.
(ⅱ) Switches the maximum threshold voltage of current
limit by external signals.
How to use
(ⅰ) Current detection and current limiting
• Connection method
Connect a current detecting resistor Rs between the
MOSFET source pin and the GND. The current signals of
the MOSFET generated in the resistor are input (Fig. 20).
• Operation
The current signals of the MOSFET input to the IS pin is
then input to the current comparator, and if it reaches the
threshold voltage determined by the FB pin, the MOSFET is
turned off. This FB pin voltage fluctuates due to the
feedback circuit from the output voltage to control the
MOSFET current.
In addition, since the maximum input threshold voltage is
also input to the current comparator, the MOSFET current
is limited by the current equivalent to this voltage even in an
emergency state such as transient state at the time of
startup or overload status.
If overload state continues, the latch-off stop is performed
by the overload protection function. Generally, the output
current value that is stopped in the latch-off mode varies
depending on the high-line voltage, and there may be a
case in which the higher the line voltage, the larger the
output current that is stopped in the latch mode. To
compensate the dependency of overload detection level on
the line voltage, the maximum input threshold voltage is
switched between 0.5 V (low line voltage) and 0.45 V (high
line voltage).
operation. But it is necessary to confirm output power in
actual power supply unit because the output power may
vary with specification of transformer and circuit constant.
For example, the power of the power supply is limited in
standby mode. Specifically, the maximum input threshold
voltage is switched between 0.15 V (low line voltage) and
0.1 V (high line voltage).
If Low signals are input from the external signal, transistor
Tr1 and the optocoupler are set to OFF, and transistor Tr2
is brought into continued state, and the IS pin is subjected
to the effect of the auxiliary winding voltage. The auxiliary
winding voltage remain positive while the MOSFET is set to
OFF, and negative while it is set to ON, and thanks to the
function of diode D1, the IS pin voltage is increased to
positive side only during the period in which the MOSFET
remains OFF.
Current
Comparator
3
IS
Rs
Fig.20 IS pin circuit
vcc
6
D1
R3
D2
Vout
( ⅱ ) Switching of current limiting maximum threshold
voltage by external signals
• Connection method
As shown in Fig. 21, a diode, current limiting resistor,
transistor switch, optocoupler, etc. are added between the
auxiliary winding and the IS pin.
• Operation
While the MOSFET remains ON, MOSFET current signals
are kept input to the IS pin for comparison with the
threshold voltage that is determined by the FB pin. With this
IC, IS pin voltage level is detected during this OFF period.
By increasing the IS pin voltage to 0.55 V, which is the IS
pin standby detection voltage, or higher within 2 s after the
MOSFET is turned off, the maximum input threshold
voltage of the IS pin is switched. In this case, the detection
level is decreased to approximately 1/7 of that of normal
Tr2
IS
3
Tr1
Rs
External
switch
signal
Standby
Standby detection
voltage 0.55V(typ.)
IS pin
Delay
2.0us
Normal
Fig.21 Power limiting circuit and waveform at
standby detection
Fuji Electric Co., Ltd.
AN-064E Rev.1.1
April 2011
28
http://www.fujielectric.co.jp/products/semiconductor/