1.5V battery powers white-LED driver

Although white LEDs are common in a variety of lighting applications, their 3 to 4 V forward-voltage drop makes low-voltage applications challenging. Charge pumps and other ICs are available for driving white LEDs, but they generally don’t work with the low supply voltage of 1.5 V in single-cell-battery applications.

The low-voltage circuit of Figure 1 provides a current-regulated output suitable for driving white LEDs. The boost converter, IC₁, can supply load currents to 62 mA with input voltages as low as 1.2 V, making it suitable for use with a 1.5 V, single-cell battery. Because a white LED draws negligible load current until the output voltage rises above 3 V, the boost converter can start with input voltages as low as 0.8 V.

By deriving feedback from a high-side current-sense amplifier, IC₂, the circuit allows current regulation without sacrificing efficiency. IC₂‘s 1.8-MHz bandwidth also eliminates instability in the feedback loop. IC₂ amplifies the voltage across R₁ with a gain of 20. This high gain boosts efficiency by enabling use of a small-valued current-sensing resistor. You can calculate the value of R₁ from the desired output current:

For 1.5 V input and 62-mA output, the circuit efficiency of Figure 1 is approximately 80%. Zener diode D₁ provides overvoltage protection at the output. When the output voltage rises above the sum of the zener voltage (V_Z) and IC₁‘s 1.235 V feedback voltage (V_FB), the feedback voltage (Pin 3) rises and causes IC₁ to stop switching. Thus, for an open-circuit output, the output voltage is regulated at V_Z + V_FB.

by Steve Caldwell via EDN

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