FAN4800AU Fairchild Semiconductor, FAN4800AU Datasheet - Page 14

• Manufacturer Part Number: FAN4800AU
• Description: The highly integrated FAN4800AU/CU parts are specially designed for power supplies that consist of boost PFC and PWM
• Manufacturer: Fairchild Semiconductor

© 2011 Fairchild Semiconductor Corporation
FAN4800AU/CU • Rev. 1.0.0
To sense the RMS value of the line voltage, averaging
circuit with two poles is typically employed, as shown in
Figure 28. Notice that the input voltage of the PFC is
clamped at the peak of the line voltage once the PFC
stops switching because the junction capacitance of the
bridge diode is not discharged, as shown in Figure 31.
Therefore, the voltage divider for VRMS should be
designed considering the brownout protection trip-point
and minimum operation line voltage.
Figure 31. V
Figure 29. Modulation Gain Characteristics
V
Figure 30. Line Feed-Forward Operation
I
L
IN
V
EA
G
RMS

V
RMS
According to the PFC Operation
1
2
14
The rectified sinusoidal signal is obtained by the current
flowing into the IAC pin. The resistor R
large enough to prevent saturation of the gain
modulator, calculating as:
Current Control of Boost Stage
The FAN4800AU/CU employs two control loops for
power factor correction, as shown in Figure 32: a
current-control loop and a voltage-control loop. The
current-control loop shapes inductor current as shown
in Figure 33 based on the reference signal obtained at
the IAC pin calculated as:
The current-control feedback loop also has a pulse-by-
pulse current limit comparator that forces the PFC
switch to turn off until the next switching cycle if the
ISENSE pin voltage drops below -1.3V.
I
L
where V
protection, G
when V
datasheet), and 140µA is the maximum output
current of the gain modulator.

R
2
I
CS
V
MO
R
1
LINE
IAC
Figure 33. Inductor Current Shaping
R

R
CS
Figure 32. Gain Modulation Block
M
I
1
MIN
RMS
LINEMIN
MO


G
is 1.08V (which can be found in the
R
MAX
M
MAX
is the line voltage that trips brownout

is the maximum modulator gain

I
AC
140

G

A

R
M
IAC
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should be
(3)
(4)