SG6846A Просмотр технического описания (PDF) - Fairchild Semiconductor

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SG6846A

Highly Integrated Green-Mode PWM Controller

Fairchild Semiconductor 

Pin Configuration

Figure 5. Pin Configuration

Pin Definitions

Pin #

1

2

3

4

5

6

7

8

Name

GND

FB

VIN

RI

RT

SENSE

VDD

GATE

Function Description

Ground

Ground.

Feedback

The signal from the external compensation circuit is fed into this pin. The PWM

duty cycle is determined in response to the signal from this pin and the current-

sense signal from pin 6.

Line-voltage

Detection

Line-voltage detection is used for brownout protection with hysteresis. Constant

output power limit over universal AC input range is achieved using this pin. Add

a low-pass filter to filter out line ripple on the bulk capacitor.

Reference

Setting

A resistor from the RI pin to ground generates a reference current source that

determines the switching frequency. Increasing the resistance reduces the

switching frequency. A 26kΩ resistor results in a 65kHz switching frequency.

Temperature

Detection

For over-temperature protection, an external NTC thermistor is connected from

this pin to the GND pin. The impedance of the NTC decreases at high

temperatures. Once the voltage of the RT pin drops below a threshold, PWM

output is disabled.

Current Sense

The sensed voltage is used for peak-current-mode control and cycle-by-cycle

current limiting. If the switching current is higher than OCP threshold and lasts

for 1700ms, SG6846A turns off immediately. This two-level OCP feature is

especially suitable for SMPS with surge current output.

Power Supply

Driver Output

If an open-circuit failure occurs in the feedback loop, the internal protection

circuit disables PWM output as long as VDD exceeds a threshold.

The totem-pole output driver for the power MOSFET; internally clamped

below 18V.

© 2008 Fairchild Semiconductor Corporation

SG6846A • Rev. 1.1.4

4

www.fairchildsemi.com

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