Single FET controls LED array


Posted on Feb 1, 2013

The circuit`s 90% conversion efficiency offers a distinct power-saving advantage over resistor-limited and linearly regulated designs. It might appear that a series-LED connection is unsuitable for applications in which some (but not all) LEDs must be off. A cell phone, for example, sometimes needs that capability for occasions when the display is off but the keypad remains lit. Or, a PDA might need to play a sound file while maintaining illumination in the buttons but not the display. In the circuit of Figure 1, switching off individual LEDs or groups of LEDs is not a problem, even with series drive. Applying a logic-high level to the gate of a simple MOSFET switch, Q2, turns off a subset of LEDs by shunting their current. The remaining LEDs (for the keypad, for example) remain on, and their intensity remains constant because IC1 regulates their current, by sensing the voltage across R2 (300 mV at full brightness).






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