AN4105 Просмотр технического описания (PDF) - Fairchild Semiconductor
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AN4105
Fairchild Semiconductor
AN4105 Datasheet PDF : 22 Pages
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AN4105
APPLICATION NOTE
Vo, 80V 12V PC817
R3
C1
1k
3 3n
R1
3 3k
R4
3.3k
1 0n
R2
C2
1k
KA431
LM431
Vfb
#4
Cfb
3 3n
2uA 1mA
2k
2R
Vfb*
1k
R
Vck Fairchild Power SwitchS(FPPSS)
OSC.
S
Q
R
Rsense
Ioffset
Sense
(a) CCoonntrtorol lCicrciruciut iutsitnhgatKAus4e3d1(LKMA44331)1C(LoMnt4ro3l1I)C
Vo, 80V 12V PC817
R1
33k R3
1k
2.2k
C1
0.1u
R2
Vz
Vfb
#4
Cfb
33n
2uA 1mA
2k
2R
Vfb*
1k
R
Vck Fairchild Power SwitcSh(PFPSS)
OSC.
S
Q
R
Rsense
Ioffset
Sense
(b) CCoonnttrrool lCicricruciut iutstinhgata uZseender tDhieodzeeCnoenrtrdoiloICde
Figure 5. Fairchild Power Switch(FPS) feedback control circuit.
1.5 Soft Start Operation
Normally, the SMPS output voltage increases from start up
with a fixed time constant. This is due to the capacitive
component of the load. At start up, therefore, the feedback
signal applied to the PWM comparator's inverting input
reaches its maximum value (1V), This is because the
feedback loop is effectively open. Also at this time, the drain
current is at its peak value (Ipeak) and maximum allowable
power is being delivered to the secondary load. With that
said, note that when the SMPS pushes maximum power to
the secondary side for this initial fixed time, the entire circuit
is seriously stressed. Use of a soft start function avoids such
stresses. Figure 6 shows how to implement a soft start for a
Fairchild Power Switch(FPS). At turn on, the soft start
capacitor CS on pin 5 of the Fairchild Power Switch(FPS)
starts to charge through the 1mA current source. When the
voltage across CS reaches 3V, diode DS turns off. No more
current flows to it from the 1mA current source. Cs then
continues to charge to 5V through the 50kΩ resistor.
4
©2002 Fairchild Semiconductor Corporation
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