The LM317 regulator (Figure 1) is well known for its versatility in a variety of different functions and applications.
The standard and classic of the LM317 is that it is a three-terminal positive voltage regulator. Another simple and important configuration is its current regulator with two terminals.
However, when the adjustment terminal is connected to the output terminal, the LM317 is converted into a two-terminal component that functions like a 1.5V high-current Zener diode (Figure 2). This approach provides a practical and versatile circuit configuration that can sink or reduce high currents at positive or negative supply voltages.
Connecting the trim terminal to the output terminal causes the internal regulator to provide compensation, thereby causing the internal NPN transistor to saturate. The saturated NPN transistor will have a VCE(sat) voltage drop of about 0.20V~0.25V, plus the internal regulator's 1.25V voltage drop, and the total voltage drop is 1.45V~1.50V.
The LM317 in this configuration is similar in function to a regulated diode with a dynamic internal resistance of approximately 1 Ω or less. This dynamic internal resistance provides a tangible reference that keeps the pressure drop of the modified LM317 within 0.1V each time a 100mA change in current through the device occurs.
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