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Transistor boosts regulator current

18 Mar 2015  | Yngve Linder

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Some circuits need a constant-current source that does not necessarily connect to a power-supply rail or to ground. The circuit in the figure shows a simple method for achieving that configuration.


Figure: The simple method achieves a constant-current source that does not necessarily connect to a power-supply rail or to ground.


The LM317Vage regulator develops 1.25V between the OUT and the ADJ pins. Placing a resistor between those pins produces a constant current. Thus, the circuit's output current is 1.25V/RADJ. The transistor lets the circuit source more current than the regulator alone can provide once the current through R1 creates enough voltage to turn the transistor on. Otherwise, the regulator supplies the load current.


About the author
Yngve Linder is from Örsundsbro, Sweden.


This article is a Design Idea selected for re-publication by the editors. It was first published on November 4, 2010 in EDN.com.




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