AN-6921 查看數據表（PDF） - Fairchild Semiconductor
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AN-6921
Fairchild Semiconductor
AN-6921 Datasheet PDF : 16 Pages
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AN-6921
[STEP-B4] Design the Feedback Circuit
Figure 17 is a typical feedback circuit mainly consisting of a
shunt regulator and a photo-coupler. R01 and R02 form a
voltage divider for output voltage regulation. RF and CF are
adjusted for control-loop compensation. A small-value RC
filter (e.g. RFB = 100Ω, CFB = 1nF) placed from the FB pin
to GND can increase stability substantially. The maximum
source current of the FB pin is about 1.2mA. The
phototransistor must be capable of sinking this current to
pull the FB level down at no load. The value of the biasing
resistor, RBIAS, is determined as:
VO − VOPD − VKA ⋅ CTR > 1.2 ×10−3
RBIAS
(42)
where VOPD is the drop voltage of photodiode, about 1.2V;
VKA is the minimum cathode to anode voltage of shunt
regulator (2.5V); and CTR is the current transfer rate of the
opto-coupler.
APPLICATION NOTE
[STEP-B5] Design the Over-Temperature
Protection Circuit
The adjustable Over-Temperature Protection (OTP) circuit
is shown in Figure 18. As can be seen, a constant sourcing
current source (IRT) is connected to the RT pin. Once VRT is
lower than 0.8V for longer than 10ms debounce time,
FAN6921 is latched off. RRT can be determined by:
0.8V = (RRT + RNTC@OT ) ×100μ A
(43)
Figure 18. Adjustable Over-Temperature Protection
and External Latched-off Function
(Design Example) Assuming the resistance of
NTC at over-temperature protection point is 4.3kΩ;
RRT
= 0.8V - 4.3kΩ = 3.7kΩ
100μ A
Figure 17. Feedback Circuit
(Design Example) Assuming CTR is 100%;
VO −VOPD −VKA ⋅ CTR > 1.2 ×10−3
RBIAS
RBIAS
<
VO −VOPD −VKA
1.2 ×10−3
=
19 −1.2 − 2.5
1.2 ×10−3
= 12.75kΩ
220Ω resistor is selected for RBIAS.
The voltage divider resistors for VO sensing are
selected as 68kΩ and 10kΩ.
© 2010 Fairchild Semiconductor Corporation
Rev. 1.0.1 • 8/24/10
12
www.fairchildsemi.com
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