LED stepdown power supply

  
Below is a proof of concept circuit which needs improvement. The part values shown will run a high efficiency red LED with a Vf of 1.9V from only 2 nicads, right down to 2 volts. For a white LED you will need more batteries and you must increase the 1.0K resistor to a higher value. Current sensing at a mere 18mV over 1ohm makes this stepdown switcher efficient, but efficiency can be increased by using a larger inductor, improving transistor switching speed, or by increasing the value of the 1.0K resistor. Positive feedback from the driver transistor back into the current mirror through the 47K hysterisys resistor ensures that this circuit behaves as a switching power supply, however it could be used in linear mode if this resistor and the inductor were removed.
LED stepdown power supply - schematic




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