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NCP1654
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14
V
ramp
 is the internal ramp voltage, the positive input of the
PFC modulation comparator,
V
m
 is the multiplier voltage appearing on V
m
 pin,
I
ch
 is the internal charging current,
C
ramp
 is the internal ramp capacitor, and
V
REF
 is the internal reference voltage, the negative input of
the PFC modulation comparator.
I
ch
, C
ramp
, and V
REF
  also act as the ramp signal of
switching frequency. Hence the charging current I
ch
  is
specially designed as in (Equation 5). The multiplier
voltage V
m
  is therefore expressed in terms of t
1
  in
(Equation 6).
(eq. 5)
I
ch
+
C
ramp
V
REF
T
(eq. 6)
V
m
+ V
REF
*
t
1
C
ram
C
ramp
V
REF
T
+ V
REF
T * t
1
T
From (Equation 3) and (Equation 6), the input impedance
Z
in
 is reformulated in (Equation 7).
(eq. 7)
Z
in
+
V
m
V
REF
V
out
I
L*50
Because V
REF
 and V
out
 are roughly constant versus time,
the multiplier voltage V
m
 is designed to be proportional to
the I
L50
 in order to have a constant Z
in
 for PFC purpose.
It is illustrated in Figure 34.
Figure 34. Multiplier Voltage Timing Diagram
Time
Time
Time
V
M
I
L
I
in
V
in
It can be seen in the timing diagram in Figure 33 that V
m
originally consists of a switching frequency ripple coming
from the inductor current I
L
. The duty ratio can be
inaccurately generated due to this ripple. This modulation
is the socalled peak current mode. Hence, an external
capacitor C
M
 connected to the multiplier voltage V
m
 pin is
essential to bypass the highfrequency component of V
m
.
The modulation becomes the socalled average current
mode with a better accuracy for PFC.
Figure 35. External Connection on the Multiplier
Voltage Pin
PFC Duty
Modulation
V
m
+
R
M
I
cs
V
bo
4(V
control
* V
CONTROL(min)
)
R
M
C
M
V
m
I
m
2
The multiplier voltage V
m
  is generated according to
(Equation 8).
(eq. 8)
V
m
+
R
M
I
cs
V
bo
4(V
control
* V
CONTROL(min)
)
Where,
R
M
 is the external multiplier resistor connected to V
m
 pin,
which is constant.
V
bo
 is the input voltage signal appearing on the BO pin,
which is proportional to the rms input voltage,
I
cs
 is the sense current proportional to the inductor current
I
L
 as described in (Equation 11).
V
control
 is the control voltage signal, the output voltage of
Operational Transconductance Amplifier (OTA), as
described in (Equation 12).
R
M
 directly limits the maximum input power capability
and hence its value affects the NCP1654 to operate in either
follower boost mode or constant output voltage mode.
Figure 36. External Connection on the Brown Out Pin
Vbo
BO
4
+
R
boL
V
in
R
boU
C
BO
+
-
V
boH
 / V
boL
V
boH
 = 1.3 V, V
boL
 = 0.7 V
Refer to Figure 36,
(eq. 9)
V
bo
+ K
BO
(V
in
) + K
BO
@
2  2
p
V
ac
(eq. 10)
K
BO
+
R
boL
R
boU
) R
boL
where
V
bo
 is the voltage on BO pin.
K
BO
 is the decay ratio of V
in
 to V
bo
.
<V
in
> is the average voltage signal of V
in
, the voltage
appearing on C
filter
.
V
ac
 is the RMS input voltage.
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