SG6846 データシートの表示（PDF） - Fairchild Semiconductor
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SG6846 Datasheet PDF : 16 Pages
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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
Feedback
Line-Voltage
Detection
Reference
Setting
Temperature
Detection
Current Sense
Power Supply
Driver Output
Ground.
The signal from the external compensation circuit is fed into this pin. The FB
pin and the current-sense signal from Pin 6 determine the PWM duty cycle.
Line-voltage detection. The line-voltage detection is used for brownout
protection with hysteresis. Constant-output-power limit over universal-AC-input
range is also achieved using the VIN signal. It is suggested to add low-pass
filter to filter out line ripple on bulk capacitor.
A resistor from the RI pin to ground generates a reference current source
inside SG6846. This determines the switching frequency. Increasing the
resistance reduces the switching frequency. Using a 26kΩ resistor RI results in
a 65KHz switching frequency.
For OTP. An external negative temperature coefficient (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 2/3 of the
peak-current threshold for longer than 96ms, OTP is activated. This two-level
OCP feature is especially suitable for SMPS with surge current output.
Power Supply. The internal protection circuit disables PWM output in
response to VDD over voltage.
The totem-pole output driver for the power MOSFET. A soft driving waveform is
implemented for improved electromagnetic interference (EMI).
© 2008 Fairchild Semiconductor Corporation
SG6846 • Rev. 1.3.1
4
www.fairchildsemi.com
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