Driving LEDs in Battery-Powered Applications: Efficiently Control Brightness Power

This application note describes how LEDs, including WLEDs, work. The note also explains how to drive them in battery-powered LED applications, including lithium-ion (Li+ or Li-ion), nickel-cadmium (NiCd), and nickel metal-hydride (NiMH) rechargeable handheld devices where power consumption is important. LED brightness matching and the value of series vs. parallel LEDs are discussed. Application information is also presented for several LED drivers that can efficiently drive and control LEDs.

About LEDs

Light-emitting diodes (LEDs) are the solid-state, highly reliable, efficient counterparts of the evacuated tungsten-filament light bulb. Epitaxial material based on gallium arsenide phosphide (GaAsP) produces red, green, or yellow outputs (Figure 1). Material based on indium gallium nitrate (InGaN) produces blue or white outputs (Figure 2). Different chemistries also produce different electrical characteristics.

Figure 1. Relative spectral response of red, green, and yellow diodes (IF = 2mA, TA = +25°C).

Figure 2. Relative spectral response of white diodes (IF = 20mA, TA = +25°C).

In Figures 1 and 2, the curve Vλ represents the standard response of a human eye. To obtain white light, a blue emitter is covered with material that emits yellow light when stimulated by blue light. The eye interprets the output as white and creates the spectral response of Figure 2.