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What is the absolute max input voltage for your LM317, NCV317 voltage regulator IC? Is it 77 volts because of the max output voltage of 37 volts and the differential 40 volt float?

The LM317 does not have a ground pin, rather an adjust pin. This allows the part to float. The voltage differential between input to output is specified and can be as great as 40 Volts. Resistor R2, typically shown in the application circuits will drop a significant amount of voltage across it. Extra care or additional protection circuitry may need to be added to protect the part when operating at high voltage differentials between input and output. There is a current fold back circuit implemented in the device. This is demonstrated in figure 6 of the datasheet. With a 30 V differential, you will only be able to draw about 600 mA of current through the part. Additionally, significant heatsinking will be needed. 30 V * 600 mA = 18 Watts. Please refer to our adjustable output voltage regulator LM317 datasheet .

Answered by:
ON Semiconductor
2014-06-05 10:46:01.264