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FAN7530M

Critical Conduction Mode PFC Controller

Fairchild Semiconductor 

Applications Information

1. Error Amplifier Block

The error amplifier block consists of a transconductance

amplifier, output OVP comparator, and disable compara-

tor. For the output voltage control, a transconductance

amplifier is used instead of the conventional voltage

amplifier. The transconductance amplifier (voltage con-

trolled current source) aids the implementation of OVP

and disable function. The output current of the amplifier

changes according to the voltage difference of the invert-

ing and non-inverting input of the amplifier. The output

voltage of the amplifier is compared with the internal

ramp signal to generate the switch turn-off signal. The

OVP comparator shuts down the output drive block when

the voltage of the INV pin is higher than 2.675V and

there is 0.175V hysteresis. The disable comparator dis-

ables the operation of the FAN7530 when the voltage of

the inverting input is lower than 0.45V and there is

100mV hysteresis. An external small signal MOSFET

can be used to disable the IC, as shown in Figure 33.

The IC operating current decreases below 65µA to

reduce power consumption if the IC is disabled.

OVP

2.675V 2.5V

Disable

0.45V 0.35V

Error Amp

Gm

Vout

Vref1 (2.5V)

INV

1

3

COMP

Disable

Signal

rent detector turns on the MOSFET. The ZCD pin is pro-

tected internally by two clamps, 6.7V-high clamp and

0.65V-low clamp. The 150µs timer generates a MOSFET

turn-on signal if the drive output has been low for more

than 150µs from the falling edge of the drive output.

Vin

ZCD

5

RZCD

6.7V

1.4V

1.5V

150μs

Timer

Turn-on

Signal

Zero Current

Detector

S

Q

R

FAN7529 Rev. 00

Figure 34. Zero Current Detector Block

3. Sawtooth Generator Block

The output of the error amplifier and the output of the

sawtooth generator are compared to determine the

MOSFET turn-off instance. The slope of the sawtooth is

determined by an external resistor connected to the

MOT pin. The voltage of the MOT pin is 2.9V and the

slope is proportional to the current flowing out of the

MOT pin. The internal ramp signal has a 1V offset; there-

fore, the drive output is shut down if the voltage of the

COMP pin is lower than 1V. The MOSFET on-time is

maximum when the COMP pin voltage is 5V. According

to the slope of the internal ramp, the maximum on-time

can be programmed. The necessary maximum on-time

depends on the boost inductor, lowest AC line voltage,

and maximum output power. The resistor value should

be designed properly.

FAN7530 Rev. 00

Figure 33. Error Amplifier Block

2. Zero Current Detection Block

The zero current detector (ZCD) generates the turn-on

signal of the MOSFET when the boost inductor current

reaches zero using an auxiliary winding coupled with the

inductor. If the voltage of the ZCD pin goes higher than

1.5V, the ZCD comparator waits until the voltage goes

below 1.4V. If the voltage goes below 1.4V, the zero cur-

MOT

3

2.9V

Sawtooth

Generator

1V Offset

Off Signal

Error Amp

Output

FAN7529 Rev. 00

Figure 35. Sawtooth Generator Block

© 2006 Fairchild Semiconductor Corporation

FAN7530 Rev. 1.0.2

12

www.fairchildsemi.com

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