LCD Backlight
A backlight is a form of illumination used in LCD. As LCD do not produce light by itself, unlike CRT. LCD display needs illumination, ambient light or backlight, to produce image. Currently, there are two types of backlight in the market, CCFL and LED.
CCFL：Traditionally, TFT LCD used Cold Cathode Fluorescent Lamp as backlight. It was the major player in market, before LED backlight was introduced. CCFL is a mature technology, and provides stable color light source. However, CCFL backlight is relatively bulky and is not energy efficient. It is phasing out of market now.
LED: LED backlight on the other hand is small, low power consumption, has high brightness and long service time. That is why LED is taking over LCD backlight market share from CCFL.
LCD Display's LED Backlight
LED, in full Light Emitting Diode, is a semiconductor diode that emits light when electric voltage is applied to it. We can separate LED backlights into two groups, based on their installation locations.
Edge-lit: LED light bulbs are affixed on the edge of TFT LCD display. A light guide plate (acrylic panel typically made from pure PMMA resin) is used to direct light out the front. Edge-lit LCD backlight can be very thin. But its efficiency is low. The thinner edge-lit backlight is, the lesser efficient it will be.
Bottom-lit: As its name states, LED light bulbs are installed evenly across the back of TFT LCD display. The advantages of bottom-lit backlight is its uniformity and brightness. But bottom-lit backlight is thicker than edge-lit, and heating could be a problem.
Driving LED backlight
LED is sensitive to temperature. And it has a non-linear volt-ampere characteristic linked to temperature change.
As temperature rises, forward voltage drop reduces, forward current will increase quite a bit.
Constant Current Driving
LED light bulbs are serially connected. When applying constant current to LED backlight, LED temperature goes up. Since supply current is not changed, voltage applied on LED backlight will drop. And LED backlight will produce even brightness.
Constant Voltage Driving
Because of LED's unique volt-ampere property, when supplying constant voltage to serially connected LED backlight, higher temperature leads to higher current. And higher current brings more heat. Such effect will be magnified for serial circuit. High current speeds up LED's degradation and shortens its life span. Further more, since each LED has slightly different volt-ampere property, individual LED will have different pass-thru current. Thus, LED backlight will have uneven brightness.
Therefore, when using TFT LCD with LED backlight, engineer should apply constant current to LED backlight's power supply, and use PWM function to control brightness.