The advent of high-power LEDs that can replace incandescent bulbs has produced a need for circuits that efficiently match the LEDs to available power sources (batteries, car electrical systems, etc.). The circuits should also be able to regulate and control the LED light output, despite variations in output voltage from the power source.
The circuit of Figure 1 starts and operates a 1-A LED from a voltage that ranges from 2.7 V minimum (the end of life for three alkaline primary cells) to 16.5 V maximum (the maximum voltage for 12-V automobile systems). Operation over that range is stable and smooth, even with more than 1 Ω of internal resistance at the power source.
An inductive buck-boost converter controller (IC1) with an external MOSFET power switch is set up in a current-regulation loop using a 0.1-Ω current-sense resistor in series with the power LED. (If the output circuit opens, the 6.2-V zener diode turns on, kicking in a voltage regulation loop.) Figure 2 shows the current regulation for 0.5- and 1-A output currents. The corresponding efficiencies are plotted versus the input-voltage operating range in Figure 3.
Replacing the 10-kΩ feedback resistor (connected to the current-sense resistor) with a linear potentiometer of the same value allows for efficient dimming of the LED. Starting from the maximum 1-A regulation point set by the circuit values shown, the power-LED current (and hence its light output) can then be linearly swept through a range of more than 20:1.