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The LM317 is an adjustable 3-terminal positive voltage regulator capable of supplying different DC voltage outputs other than the fixed voltage power supply of +5 or +12 volts, or as a variable output voltage from a few volts up to some maximum value all with currents of about 1.5 amperes.

The LM317 has three pins: INput, OUTput, and ADJustment. Internally the device has a bandgap voltage reference which produces a stable reference voltage of Vref= 1.25 V followed by a feedback-stabilized amplifier with a relatively high output current capacity. How the adjustment pin is connected determines the output voltage as follows.

If the adjustment pin is connected to ground the output pin delivers a regulated voltage of 1.25 V at currents up to the maximum. Higher regulated voltages are obtained by connecting the adjustment pin to a resistive voltage divider between the output and ground. Then

Vout = Vref (1 + RL/RH)

Vref is the difference in voltage between the OUT pin and the ADJ pin. Vref is typically 1.25 V during normal operation.

Because some quiescent current flows from the adjustment pin of the device, an error term is added:

Vout = Vref (1 + RL/RH) + IQRL
To make the output more stable, the device is designed to keep the quiescent current at or below 100µA, making it possible to ignore the error term in nearly all practical cases.

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