Back-Lighting with LEDs

Liquid crystal displays require back-lighting. This is because liquid crystal displays do not generate any light, and their visibility depends on light passing through the display. Opaque sections in the display become visible when they block light from behind the display. To make the display readable, manufacturers resort to providing them with light from behind the display.

The back-lighting in liquid crystal display panels may come from sources like incandescent, fluorescence, electroluminescence, woven fiber optics, or LEDs. Appearance, cost, and features consideration decide the ultimate choice for the selection of source for the back-lighting. The most popular is solid-state lighting using LEDs, as these devices offer better luminance and power efficiency, as compared to all other sources. Another advantage of LED back-lighting for LCD panels is the long life of LEDs.

Earlier, LED back-lighting typically used direct lighting, with large numbers of LEDs mounted behind the display. This arrangement provided excellent image quality along with the ability of local dimming. However, the high cost of this method did not allow it to gain market share. Rather, back-lighting technology changed over to edge-mounted LED back-lighting. An added advantage of edge-mounted LED back-lighting was that the edge-lit LCD panels could be made in extremely thin designs.

By using edge-lit LED back-lighting, manufacturers could reduce the number of LEDs necessary, by concentrating them along the edges. Initially, manufacturers used LEDs on all four edges. Very soon, they placed the LEDs along two shorter edges only, and eventually, they were placing the LEDs on only a single short edge of the LCD panel.

LEDs are a good choice for back-lighting. They are compact, operate in a wide temperature range, offer a good color selection, have a low operating voltage, and have a long operating life of at least 50 thousand hours. Over a specified range of drive current levels, LEDs offer a predominantly fixed voltage drop. However, LED back-lighting also offers some challenges. For instance, the light provided by the LEDs is uneven, which improves with a suitable light pipe or diffuser. Another challenge is the current through the LEDs depends on the ambient temperature, and requires close monitoring to allow safe operation over a wide range of temperatures.

The driver for such constant-current devices requires building up the drive voltage until it is supplying the desired current level. It reaches a stabilization point when the drive voltage equals the sum of the forward drops of all the LEDs in series. The maximum voltage of the driver limits the number of LEDs in series that it can drive at a time. However, even the simplest of drivers requires holding some voltage in reserve, for dropping across a current limiting resistor. This means a driver will never be able to apply the entire power supply voltage across a chain of LEDs.

The number of LEDs required depends on the size of the LCD panel, and its brightness. High-brightness and ultra-high-brightness LCD panels require a larger number of LEDs. Driving large numbers of LEDs requires sophisticated constant current drivers and high-efficiency power supplies.