Steve Caldwell
EDN
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.
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| Figure 1. | Powered from a single-cell battery, this circuit provides a regulated output current suitable for driving a white LED. |
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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 (VZ) and IC1's 1.235 V feedback voltage (VFB), the feedback voltage (Pin 3) rises and causes IC1 to stop switching. Thus, for an open-circuit output, the output voltage is regulated at VZ + VFB.
