AN-6920
(Design Example)
0.7 > 30µ A ,
RDET 2
RDET 2 < 23.3kΩ
Setting the OVP trip point at 22.5V,
K DET
=
RDET 1
RDET 2
=
N AUX
NS
⋅ VOVP
2.5
−1=
3 ⋅ 22.5 −1 = 5.75
4 2.5
Then RDET1 = K DET⋅R ⋅ RDET 2 < 134kΩ
I PK.L
DS
I PK.K
DS
= VO.PFC.H
VO.PFC.L
⋅ VO.PFC.L + VRO
VO.PFC.H + VRO
= 400 ⋅ 300 + 240 = 1.125
300 400 + 240
Using 113% of 1.125,
VLIMIT . L
= 1.27 =
− 994 ⋅VO.PFC.L
NA
NP
+ RDET1
VLIMIT . H
− 994 ⋅VO.PFC.H
NA
NP
+ RDET1
=
− 994 ⋅ 300 ⋅
− 994 ⋅ 400 ⋅
3
48
3
48
+
+
RDET1
RDET 1
=
−18637.5 + RDET1
− 24850 + RDET1
Then, RDET1 = 47.5KΩ and REDT2=8.25KΩ.
RDET1 and RDET2 are selected from the off-the-shelf
components as 150kΩ and 18kΩ, respectively.
Then, the pulse by pulse current limit threshold voltage
is obtained as:
VLIMIT
=
VO.PFC.L
−877 ⋅ (
NA
NP
RDET1
− 0.7
+
0.7
) + 0.882
RDET 2
= 0.474V
To set current limit level at low line as 115% of IDSPK
0.63 = 0.27Ω
1.53A ×1.15
APPLICATION NOTE
[STEP-B4] Design the Feedback Circuit
Figure 19 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
(40)
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.
Figure 19. 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
330Ω resistor is selected for RBIAS.
The voltage divider resistors for VO sensing are selected as
66.5kΩ and 10kΩ.
© 2010 Fairchild Semiconductor Corporation
Rev. 1.0.0 • March 10, 2011
12
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