Economical White LED

The general assumption is that white LEDs operate at a voltage of 3.6 V and a current of 20 mA, which is about right. Lithium Ion cells coincidentally have a voltage of exactly 3.6 V, which seems to be convenient. However, we can’t just connect an LED to a voltage source (the cell), because the current could become too large and the LED could be damaged. That is why they are usually driven by a current source, but the energy dissipated in the current source is of course lost. Besides, a current source can only function properly when it drops a few volts, which we don’t have in this case.