FA5640 Datasheet, PDF (19/35 Pages) Fuji Electric – Fuji Switching Power Supply Control IC

FA5640/41/42
9. Description of the function (The values in the following description are typical values unless otherwise specified.)
(1) Steady-state operation, bottom-skip operation, and burst operation
• Steady-state operation, bottom-skip operation
The ON/OFF cycle, which is from turn-on of the driver signal to the end of fly-back voltage, is detected, and bottom-skip
operations are performed at the time detected. Since the relation between the ON/OFF width and the number of times of
bottom skip operations is exhibited in the hysteresis as shown in Fig. 4, waveform fluctuations can be prevented and
transformer audible noise can be decreased. Fig. 5 shows the change image of the switching frequency to the output
electric power. Fig. 6 shows the change image of the ON/OFF width.
Load decrease
Vds
1st
2nd
3rd
4th
On/Off
pulse
width
ON-OFF width
T>9us
9us>T>8us
8us>T>7us
T<7us
Bottom
Signal
OUT
pulse
Vds
On/Off
pulse
width
T<10us
Bottom
Signal
Load increase
4th
3rd
2nd
1th
10us<T<11.67us
11.67us<T<14us
T>14us
OUT
pulse
Fig.3 Steady-state operation and bottom-skip operation timing chart
Fuji Electric Co., Ltd.
AN-064E Rev.1.1
April 2011
19
http://www.fujielectric.co.jp/products/semiconductor/