Драйвер для светодиода на PR4401

White LEDs have a forward voltage drop of around 3.6 V. If we wish to operate a white LED from a single 1.5 V dry cell or 1.2 V rechargeable cell we therefore need a voltage converter. Until now, solutions to this problem, whether discrete or based around an IC, have required several components. The PR4401 from PREMA is a special-purpose IC designed to drive a white LED with a single small coil as the only external component. In principle the IC would even work using a piece of wire for the inductor!


The device has just three connections and comes in an SOT-23 package which can be handsoldered. The voltage converter can be fitted into a very tiny space, for example in a pocket torch. Many SMD fixed inductors have the same pin spacing as the SOT-23 package and so the whole thing can be built without a printed circuit board, as the photograph illustrates.

The current through the LED is determined by the choice of inductor. With a 22 μH inductor the diode current is approximately 12 mA, and with a 10 μH inductor the current is approximately 23 mA. LED brightness is practically constant over a cell voltage range from 0.9 V to 1.5 V. Of course, the current drain increases as the cell voltage decreases.
Deviating from the design suggested in the data sheet we experimented with a 100 μH inductor to make a low-current version of the circuit. With a cell voltage of 1.5 V we measured a current consumption of 10 mA, and the LED remained lit down to a cell voltage of 0.7 V.