SG6846B Fairchild Semiconductor, SG6846B Datasheet - Page 4

• Manufacturer Part Number: SG6846B
• Description: The highly integrated PWM controller, SG6846B, provides several features to enhance the performance of fly-back converters
• Manufacturer: Fairchild Semiconductor

© 2008 Fairchild Semiconductor Corporation
SG6846B • Rev. 1.3.4
Pin Configuration
Pin Definitions
Pin #
1
2
3
4
5
6
7
8
SENSE
Name
GATE
GND
VDD
VIN
RT
FB
RI
Current Sense
Power Supply
Driver Output
Temperature
Line-voltage
Function
Reference
Feedback
Detection
Detection
Ground
Setting
Description
Ground.
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 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.
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.
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.
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 200ms, SG6846B turns off immediately. This two-level OCP feature is
especially suitable for SMPS with surge current output.
If an open-circuit failure occurs in the feedback loop, the internal protection
circuit disables PWM output as long as V
The totem-pole output driver for the power MOSFET; internally clamped
below 18V.
Figure 5. Pin Configuration
4
DD
exceeds a threshold.
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