Many LED drivers, using both charge pumps and inductors, are available to boost the 1.2 to 2.4 V available from single- and dual-cell NiMH (nickel-metal-hydride) batteries to the 3.6 V that white LEDs require. However, most of these circuits, such as the MAX1595, require a minimum input voltage of approximately 2.5 V to operate properly. The MAX1595 works with an input voltage of 2.4 V but does not ensure an adequate output until the input voltage reaches approximately 3 V. 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).
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| Figure 1. | In this circuit, transistors Q1 and Q2 form a flip-flop that toggles at 60 kHz, providing a drive current for the output LED down to the 1 V battery voltage.. |
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 2.36 V 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 1 V, and the delivered current diminishes but still provides usable illumination.
