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Figure 9
If the bottom level of the ripple voltage touches 0.7V, PFC is in standby mode and gate is off. The ripple
voltage defines PFC brown out off threshold of AC input voltage (RMS), V
following equation. Assuming
(RMS) to switch off PFC, for example 65VAC.
R
⋅
⋅
6
2 (
V
AC
_
off
+
R
R
5
6
assuming t
is equal to half cycle time of line frequency, ie.
discharge
⋅
2
R
=
C
2
f
R
ln
4
L
6
=
⋅
⋅
C
2
50
Hz
120
k
4
2.12
IC supply
The IC supply voltage operating range is 11~26V.
There are two stages during IC turned on. First Vcc capacitor is charged from 0V to 7V, the IC internal
regulator block starts to reset voltage at all external pins. The reset process will take about 10us. And then
when Vcc voltage is charged to Vcc_on threshold, IC starts the soft start with gate switching. In the case of
Vcc decoupling capacitance is too low such as 0.1uF, Vcc voltage may be charged up too fast and the time
interval from Vcc=7V to Vcc_on is less than the reset time. Then the IC will not go through a proper soft start
as the voltages at IC pins are not yet properly reset. To avoid such a problem, the delay circuitry is needed.
Application Note
Timing diagram of VINS Pin when IC enters brown-out shutdown
=
V
INS
_
AVE
R
5
t
disch
arg
e
−
−
⋅
=
R
C
. 0
) 7
e
6
4
R
−
6
V
0
7 .
V
AC
_
off
+
R
5
6
0
7 .
V
Ω
120
k
⋅
2
Ω
+
7
8 .
M
120
Ω
ln
0
7 .
R
⋅
6
V
, where, V
AC
_
off
+
R
6
0
7 .
V
t
disch
−
1
−
1
−
65
V
0
7 .
V
Ω
k
=
140
V
15
. C
can be obtained from the
AC_off
4
is the maximum AC input voltage
AC_off
1
=
, then
arg
e
2
f
L
nF
(27)
(28)
2008-08-01
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