FAN4800CU Fairchild Semiconductor, FAN4800CU Datasheet - Page 15

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FAN4800CU

Manufacturer Part Number
FAN4800CU
Description
The highly integrated FAN4800A/C and FAN4801/02/02L are specially designed for power supplies that consist of boost PFC and PWM
Manufacturer
Fairchild Semiconductor
Datasheet
© 2011 Fairchild Semiconductor Corporation
FAN4800AU/CU • Rev. 1.0.0
Voltage Control of Boost Stage
The voltage-control loop regulates PFC output voltage
using an internal error amplifier such that the FB
voltage is the same as the internal reference of 2.5V.
Brownout Protection
The built-in internal brownout protection comparator
monitors the voltage of the VRMS pin. Once VRMS pin
voltage is lower than 1.05V, the PFC stage is shut down
to
FAN4800AU/CU starts up the boost stage once VRMS
voltage increases above 1.9V.
TriFault Detect™
To improve power supply reliability, reduce system
component count, and simplify compliance to UL 1950
safety
Fairchild’s TriFault Detect technology.
In a feedback path failure, the output voltage of the PFC
can exceed safe operating limits. TriFaultDetect
protects the power supply from a failure related to the
output feedback by monitoring the FBPFC voltage.
TriFaultDetect is an entirely internal circuit. It requires
no external components to serve its protective function.
PWM Stage
The PWM stage is capable of Current Mode or Voltage
Mode operation. In Current-Mode, the PWM ramp
(RAMP) is usually derived directly from a current-
sensing resistor or current transformer in the primary
side of the output stage, and is thereby representative
of the current flowing in the converter’s output stage.
I
typically connected to RAMP in such applications.
LIMIT
, which provides cycle-by-cycle current limiting, is
R
R
protect
FB1
FB2
V
standards,
BOUT
Figure 34. TriFault Detect™
the
FBPFC
system
the
V
2.75V
DD
0.5V
300nA
FAN4800AU/CU
from
TriFaultDetect
+
+
-
-
Disable OPFC
over
includes
current.
15
For Voltage-Mode operation, RAMP can be connected to
a separate RC timing network to generate a voltage ramp
against which the FBPWM voltage is compared. Under
these conditions, the voltage feed-forward from the PFC
bus can be used for better line transient response.
No voltage error amplifier is included in the PWM stage,
as this function is generally performed by KA431, in the
secondary side. To facilitate the design of opto-coupler
feedback circuitry, an offset voltage is built into the
inverting input of PWM comparator. This allows
FBPWM to command a zero percent duty cycle when
its pin voltage is below 1.5V.
PWM Current Limit
The ILIMIT pin is a direct input to the cycle-by-cycle
current limiter for the PWM section. If the input voltage
at this pin exceeds 1V, the output of the PWM is
disabled for until the start of the next PWM clock cycle.
V
The V
stage and inhibits the PWM stage if this voltage is less
than 2.4V (96% of its nominal value). Once this voltage
goes above 2.4V, the PWM stage begins soft-start. The
PWM stage is shut down when FBPFC voltage drops
below 1.3V.
PWM Soft-Start (SS)
PWM startup is controlled by the soft-start capacitor. A
current source of 10µA supplies the charging current for
the soft-start capacitor. PWM startup is prohibited until
the soft-start capacitor voltage reaches 1.5V.
IN
OK Comparator
Figure 35. PWM Ramp Generation Circuit
IN
V
OK comparator monitors the output of the PFC
BOUT
C
R
RAMP
RAMP
RAMP
REF
1.5V
FBPWM
www.fairchildsemi.com
PWM
+
-

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