FA5640 Datasheet, PDF (27/35 Pages) Fuji Electric – Fuji Switching Power Supply Control IC

FA5640/41/42
(2) Pin No. 2 (FB pin)
Function
(ⅰ)Input feedback signals from the error amplifier on the
secondary side.
(ⅱ)Detects overload status.
(ⅲ)Stops switching for burst operation.
How to use
(ⅰ) Feedback signal input
• Connection method
Connect the optocoupler corrector to this pin will allow
reguration. At the same time, to prevent generation of noise,
connect a capacitor in parallel to the optocoupler (Fig. 19).
• Operation
Pin No. 2 is biased from the IC internal power supply via the
resistance. The FB pin voltage is level-shifted and input into
the current comparator to provide the threshold voltage of
the MOSFET current signals to be detected with the IS pin.
(ⅱ) Overload detection
• Connection method
The same as the feedback signal input in (ⅰ).
• Operation
In case of overload, the output voltage decreases to lower
than the setting, therefore the FB pin overshoots to the high
side. This state is detected to judge overload status. The
threshold voltage for overload judgment is 3.5 V.
By the automatic recovery function, overload status brings
about hiccup operation, and once the overload state is
reset, operation is automatically resumed. See 9. (3)
“Operation under overload” for details of operation.
(ⅲ) Stopping switching for burst operation
• Connection method
The same as feedback signal input in (ⅰ)
• Operation
FB pin voltage decreases under light load. If this voltage
decreases to threshold voltage of stopping on-pulse or
lower, switching is stopped, and switching is resumed if the
voltage increases to the threshold voltage of stopping
on-pulse or higher. By repeating this operation, burst
operation is achieved. To undershoot the FB pin voltage
significantly at the time of burst operation, the internal FB
pin resistance is switched (Fig. 19).
To compensate the dependence of load point for entering
burst operation on the high-line voltage, the pulse shutdown
FB threshold voltage is switched to 0.45 V for high line
voltage, and 0.35 V for low line voltage.
6.0V
4.5us
0V
6.0V
0V
Fig.18 ZCD pin waveform at overvoltage
Advice for designing
The FB pin provides threshold voltage of the current
comparator. If noise is added to the pin, output pulse
fluctuation may result. To prevent generation of noise, a
capacitor having the capacitance of approximately 1000 pF
to 0.01 F is connected for use as shown in Fig. 19.
4.8V
216kΩ/
108kΩ
24kΩ
2
FB
1000pF～
0.01uF
Fig.19 FB pin circuit
Fuji Electric Co., Ltd.
AN-064E Rev.1.1
April 2011
27
http://www.fujielectric.co.jp/products/semiconductor/