PAGE 129 OF 150 · POWER SUPPLIES & BATTERIES
Boost Converters for LEDs
Boost converters raise voltage from a small battery.
LED SYMBOLTriangle points from anode to cathode; the arrows show light leaving the part.
Step-up
A boost converter raises a low voltage using an inductor and fast switching. It can light a white LED from a single 1.5 V cell, which cannot do so directly.
Example
A 3.3 V white LED at 20 mA needs about 0.066 W. At 85 percent efficiency from 1.5 V, the cell supplies roughly 52 mA.
- Input current is always higher than output current.
- Use a constant-current boost driver to power LED strings.
- Expect some switching noise.
Never power the output with no load, as the voltage can climb above the rating of the parts.
REMEMBER
A boost converter trades higher input current for higher voltage.Try it yourself
Sketch the circuit on paper first, calculate the values, then build it on a solderless breadboard using a low-voltage supply. Measure with a multimeter before you make anything permanent.
Explore LED clocks, stopwatches and electronics at cosycom.com.