Many LED drivers, using both charge pumps and inductors, are available to boost the 1.2 to 2.4V available from single- and dual-cell NiMH (nickel-metal-hydride) batteries to the 3.6V that white LEDs require. However, most of these circuits, such as the Maxim MAX1595, require a minimum input voltage of approximately 2.5V to operate properly. The MAX1595 works with an input voltage of 2.4V but does not ensure an adequate output until the input voltage reaches approximately 3V. Furthermore, as the battery voltage decreases to the threshold level, the output becomes erratic. The circuit in Figure 1 uses a flip-flop to generate flux in an inductor, which then charges a capacitor in the common boost configuration. US Patent 4,068,149 describes the flip-flop’s operation in an application for operating an incandescent safety lamp’s flasher (Reference 1).

In Figure 1, R₁ provides a path for starting current through the base-emitter junctions of Q₁ and Q₂. Q₂ thus turns on and, in so doing, turns on Q₁, rapidly forcing both transistors into saturation. However, C₁ charges through R₂ to the battery voltage minus the base-emitter drop of Q₁ and the saturated collector-emitter voltage of Q₂, eventually causing Q₁ to turn off and thereby also turning off Q₂. C₁ then discharges through R₁ and R₂ and the forward-biased base-collector junction of Q₂. The R₂C₁ time constant determines the turn-on time, and (R₁+R₂)(C₂) determines the turn-off time. C₂ acts as the capacitive input filter for the current flowing from L₁ when Q₂ is off and provides a substantially constant voltage to power D₂, a standard white LED. The output voltage is proportional to the battery voltage.

With the component values in Figure 1 and with L₁, a Coilcraft MSS7341-104MLB, the operating frequency is approximately 60 kHz. With a battery voltage of 2.36V from two NiMH cells, approximately 20 mA of current flows through the LED. In tests simultaneously driving two LEDs, each with its own current-limiting resistor, R₃, the energy-conversion efficiency of the circuit at this battery voltage is approximately 80%. Operation continues with battery voltages of slightly more than 1V, and the delivered current diminishes but still provides usable illumination.