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LM317 Current Boosting Secrets!

LM317 is one of the most popular adjustable regulator chips. The output voltage of the regulator can be adjusted from 1.25V to 35V. However, the chip can deliver currents up to 1.5A which is not enough for some power applications. In this article, I will discuss two methods of LM317 current boosting, using power PNP and NPN pass transistors.

[A] Circuit Analysis
According to the LM317 datasheet: “The LM317 [1, 2] device is an adjustable three-terminal positive-voltage regulator capable of supplying more than 1.5 A over an output-voltage range of 1.25 V to 37 V. It requires only two external resistors to set the output voltage. The device features a typical line regulation of 0.01% and a typical load regulation of 0.1%. It includes current limiting, thermal overload protection, and safe operating area protection. Overload protection remains functional even if the ADJUST terminal is disconnected.”

This information proves to us that this cheap 3-terminal device is suitable for many applications but it comes with a drawback for power applications and that is the limitation of the regulator’s output current handling (1.5A in the best conditions). This problem can be solved using a pass power transistor.

[A-1] Current Boosting Using a PNP Power Transistor (MJ2955)
Figure-1 shows the schematic diagram of the circuit. This is an adjustable high current regulator circuit that the output voltage can be adjusted using a 5K potentiometer.”

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