LED driver delivers constant luminosity

Posted on Mar 17, 2013

The circuit in Figure 1 is similar in principle to that of a previous Design Idea (Reference 1) but offers improved, more reproducible performance. The output current is almost constant over an input-voltage range of 1.2 to 1.5V and is insensitive to variations of transistor gain. Transistors Q1 and Q2 form an astable flip-flop. R1 and C define the on-time of Q2. During that time, Q1 is off, and the voltage at the base of Q1 and the current in inductor L ramp up. When the voltage at the base of Q1 reaches approximately 0.6V, Q1 turns on, and Q2 turns off. This switching causes `flyback` action in inductor L. The voltage across the inductor reverses, and the energy stored in the inductor transfers to the LED in the form of a down-ramping pulse of current. During flyback time, voltage across the LED is approximately constant.

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