When building large LED backlight systems, it is important to control the current, minimize power dissipation, reduce electromagnetic interference and achieve local dimming.
Markus Luidolt, austriamicrosystems AG
LEDs have been the standard backlighting source in mobile devices for several years. Now televisions, monitors and notebooks all are starting to move toward LED backlighting.
Although LEDs cost more, they have several benefits over cold-cathode fluorescents as backlights. Their size permits the design of very thin devices, with thicknesses less than 1 inch. In RGB backlighting, a much wider color gamut can be reached. Contrast ratios also can be increased with local LED backlighting. They are robust and offer a better lifetime than other lighting sources and, last but not least, LEDs are environmentally friendly, especially compared with cold-cathode fluorescents.
When building a large LED backlighting system, several important factors must be considered. Accurate current control is needed because LED brightness is determined by the current flowing through it. Thus, a circuit driving any LED must provide enough voltage to bias the LED into conduction and must control the luminous intensity by regulating the current flow.
Because block dimming often requires more than 100 LED channels, it’s extremely important that the variation between the channels be kept to a minimum (Figure 1). Otherwise, unwanted brightness or color variations caused by the backlighting system might be seen on the display. Low variations can be achieved only by very accurate LED drivers — those with good channel-to-channel as well as device-to-device accuracy. A good reference value for both of these parameters is 0.5 percent.
Figure 1. Backlighting a television requires accurate current control and local dimming.
Power dissipation
LEDs are driven with constant current, but their forward voltage varies over their lifetime and with thermal changes, so it is very important to minimize the power dissipation of each LED string. For example, if a string of 10 is driven with 100 mA and each LED has a variation of ±0.2 V, the maximum variation in the required supply voltage is ±0.2 V. Therefore, a 4-V variation on a channel with 100 mA results in 400-mW power dissipation on just one channel; with 100 channels, the maximum power dissipation would be 40 W.
The only way to avoid this power dissipation is to reduce the supply for those LEDs with a lower bias voltage. In an RGB system, because each LED color behaves differently, it is important to have at least three separate power supplies. The AS369x driver series, which was co-developed by austriamicrosystems of Unterpremstaetten, Austria, and by LG Philips LCD of Seoul, South Korea, automatically controls the external power supplies to minimize power dissipation by providing only the voltage output required, resulting in efficient systems.
The feedback lines of several LED drivers can be connected to achieve optimized output voltage (Figure 2).
Figure 2. A large LED backlight system has integrated feedback functionality. Courtesy of austriamicrosystems.
One of the greatest challenges in building a large LED backlighting system is reducing electromagnetic interference. To minimize radiation, the wires connecting to the LEDs should be kept short. In addition, the slew rate of each LED output channel should be programmed at the optimal rate for the system. Driving the LEDs with spread spectrum signals instead of pulse width modulation also helps reduce electromagnetic interference.
To reach highest contrast ratios, the backlight is divided into multiple segments. This method of block dimming turns on only the required segments of the backlight. Bright spots in the television picture end up being really bright, and dark parts are completely black. So in addition to controlling each LED channel individually, which also can be done by some standard LED drivers, these new integrated circuits allow the designer to synchronize the LED drive with the television picture. This is possible by using the Hsync and Vsync signals from the video processor and using additional settings for delay times of each LED channel. The result is a better match between television picture and backlight, with a flicker-free picture without any artifacts. Additionally, a whole string of LED drivers can be controlled easily via an SPI or I2C-interface.
Building a high-quality backlighting system requires minimizing the current variations of all LED strings, reducing power dissipation, reducing electromagnetic interference problems and integrating the highest-quality local dimming functionality. The selection of the right LED driver supporting these key requirements is an important step in reaching the goal of a high-quality LED backlighting system.
Meet the author
Markus Luidolt is the marketing manager of backlighting at austriamicrosystems AG in Unterpremstaetten, Austria; e-mail: [email protected].