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AN3142 bảng dữ liệu(PDF) 27 Page - STMicroelectronics

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Giải thích chi tiết về linh kiện  Solution for designing a 400 W fixed-off-time controlled
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AN3142
Designing a fixed-off-time PFC
Doc ID 17005 Rev 3
27/44
Figure 17.
Brownout function in L6563S and L6563H
The divider replaces the discharge resistor RFF shown in Figure 16. It should be selected in
order to have a similar time constant of
(16) but also to obtain the PFC startup at minimum
input mains voltage VACmin, (in this design 90 VAC), as specified in (1).
Therefore, we can set:
Referring to
Figure 17 and considering the peak of the minimum input mains voltage
, the corresponding voltage on the VFF pin is:
Equation 52
∆V is the voltage drop between the VFF and MULT pins.
Now, considering the RUN pin enable threshold (0.88 V is the typical value given on the
datasheet), the RUN pin divider ratio can be calculated as follow:
Equation 53
Setting up RFF_L = 1M
Ω, RFF_H can be calculated from Equation 53:
Equation 54
The result of the previous formula (
Equation 54) is based on typical values and doesn't take
into account the VRUN_EN threshold and the resistor tolerances. In order to have the startup
(17)
RUN
VFF
MULT
MULTIPLIER
1/V
2
Vin
-
0 .8/0.88V
PWM_STOP
to IC
R
FF_ H
R
FF_ L
F
1
CFF
µ
=
Vac
90
2
⋅
V
R
R
R
V
2
V
multH
multL
multL
START
V
@
FF
START
∆
−
+
⋅
⋅
=
V
973
.
0
mV
20
M
6
.
6
k
51
k
51
Vac
90
2
V
START
V
@
FF
=
−
Ω
+
Ω
Ω
⋅
⋅
=
904
.
0
R
R
R
V
V
H
_
FF
L
_
FF
L
_
FF
V
@
FF
EN
_
RUN
START
=
+
=
L
_
FF
EN
_
RUN
V
@
FF
H
_
FF
R
1
V
V
R
START
⋅
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
−
=
Ω
=
Ω
⋅
⎟
⎠
⎞
⎜
⎝
⎛
−
=
k
105
M
1
1
V
88
.
0
V
973
.
0
R
H
_
FF



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