FAN4802 データシートの表示（PDF） - Fairchild Semiconductor

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FAN4802

FAN480X PFC+PWM Combination Controller Application

Fairchild Semiconductor 

AN-8027

RT/

CT

PFC dead time

OPFC

OPWM

OPWM (FAN4800C, FAN4802/2L)

VBOUT

RRAMP

CRAMP

REF

RAMP

1.5V

PWM

-

+

FBPWM

Figure 9. FAN480X Timing Diagram

Figure 10. PWM Ramp Generation Circuit

PWM Stage

The PWM stage is capable of current-mode or voltage-

mode operation. In current-mode applications, the PWM

ramp (RAMP) is usually derived directly from a current

sensing resistor or current transformer in the primary of the

output stage and is thereby representative of the current

flowing in the converter’s output stage. ILIMIT, which

provides cycle-by-cycle current limiting, is typically

connected to RAMP in such applications.

For voltage-mode operation, RAMP can be connected to a

separate RC timing network to generate a voltage ramp

against which FBPWM voltage is compared. Under these

conditions, the use of 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 a programmable

shunt regulator, such as 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 that 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

until the start of the next PWM clock cycle.

VIN OK Comparator

The VIN OK comparator monitors the output of the PFC

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 to soft-start.

PWM Soft-Start (SS)

PWM startup is controlled by the soft-start capacitor. A

10µA current source supplies the charging current for the

soft-start capacitor. Startup of the PWM is prohibited until

the soft-start capacitor voltage reaches 1.5V.

© 2009 Fairchild Semiconductor Corporation

Rev. 1.0.0 • 8/26/09

4

www.fairchildsemi.com

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