FAN7528 查看數據表（PDF） - Fairchild Semiconductor

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产品描述 (功能)

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FAN7528

Dual-Output, Critical Conduction Mode PFC Controller

Fairchild Semiconductor 

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 1V and the slope

is proportional to the current flowing out of the MOT pin.

The internal ramp signal has 1V offset; therefore, 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 pro-

grammed. 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.

MOT

3

1V

1V

Sawtooth

Generator

Off Signal

Error Amp

Output

FAN7528 Rev. 1.00

Figure 37. Sawtooth Generator Block

4. Over-Current Protection Block

The MOSFET current is sensed using an external sens-

ing resistor for the over-current protection. If the CS pin

voltage is higher than 0.8V, the over-current protection

comparator generates a protection signal. An internal RC

filter is included to filter switching noise.

40k

CS 4

OCP

Signal

8pF

0.8V

Over-Current

Protection

Comparator

FAN7528 Rev. 1.00

Figure 38. Over-Current Protection Block

5. Switch Drive Block

The FAN7528 contains a single totem-pole output stage

designed for a direct drive of power MOSFET. The drive

output is capable of up to 400mA peak current with a typ-

ical rise and fall time of 50ns with 1nF load. The output

voltage is clamped to be 13V to protect MOSFET gate

even if the VCC voltage is higher than 13V.

6. Under-Voltage Lockout Block

If the VCC voltage reaches 12V, the IC’s internal blocks

are enabled and start operation. If the VCC voltage drops

below 8.5V, most of the internal blocks are disabled to

reduce the operating current. VCC voltage should be

higher than 8.5V under normal conditions.

© 2005 Fairchild Semiconductor Corporation

FAN7528 Rev. 1.0.6

14

www.fairchildsemi.com

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