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Constant Current LED Driver 5   December 13, 2010

LEDs are current driven devices.  The light they produce is proportional to the current flowing though them.  One way to maintain a fairly steady light output is to keep the current through the LED fixed, even as the supply voltage may change.  A classic way to do this is shown below.  This works fairly well but it has two weaknesses.  It requires a 0.6v voltage drop across the current shunt resistor and due to the base-emitter voltage temperature sensitivity, the current increases with temperature.

  A simple way to counteract both of these flaws is to use a voltage reference as shown in the second circuit below.  Two NPN transistors form a cheap 1.2v reference.  The reference voltage change with temperature is balanced by the Q3 base emitter voltage change.  Also, by biasing the base of Q3 to 0.5v, the voltage across the current monitoring shunt resistor R5 only has to increase to 0.1v to control the LED current.  The overall effect is a circuit that maintains a fixed LED current with only a 0.1v drop and is much more stable with temperature.

The reference voltage change with temperature is balanced by the Q3 base emitter voltage change.  Also, by biasing the base of Q3 to 0.5v, the voltage across the current monitoring shunt resistor R5 only has to increase to 0.1v to control the LED current.  The overall effect is a circuit that maintains a fixed LED current with only a 0.1v drop and is much more stable with temperature.

   

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