Application of LED backlight in automotive display design

introduction

According to Strategies Unlimited (see Figure 1), the market for high-brightness (HB) LEDs is expected to reach $12 billion by 2012 and grow to $20.2 billion by 2015, a compound annual growth rate of 30.6%. LEDs used to backlight displays are currently the primary driver of this unprecedented growth. Applications include high-definition televisions (HDTVs), automotive displays, and a wide range of handheld devices. To maintain this impressive growth rate, LEDs must not only provide enhanced reliability, lower power consumption, and smaller/flatter form factors, but must also achieve significant improvements in contrast, picture clarity, and color accuracy. Furthermore, displays for automotive, avionics, and marine electronics must maximize all of these improvements while accommodating a wide range of ambient lighting conditions, from bright sunlight to moonless nights. These display applications, enabled by transistor-liquid crystal displays (TFT-LCDs), include infotainment systems, gauges, and a wide variety of instrument displays. Backlighting these displays with LEDs poses some unique LED IC driver design challenges, as the readability of the display must be optimized under a variety of ambient lighting conditions. This requires LED drivers to provide very wide dimming ratios and high-efficiency conversion while withstanding the rigors of the relatively harsh electrical and physical environment in an automobile. Finally, these solutions must provide a very low-profile, compact footprint while offering cost-effectiveness.

Figure 1: High-brightness LED market forecast

What is supporting such amazing growth potential in the automotive lighting market? First, LEDs are 10 times more efficient than incandescent lamps and nearly twice as efficient as fluorescent lamps, including cold cathode fluorescent lamps (CCFLs), at producing light, thus reducing the electrical power required to provide a certain amount of light output (measured in lumens). As LEDs are further developed, their effectiveness, or ability to produce lumens of light from electrical power, will only continue to increase. Second, in an environmentally conscious world, LED lighting does not require the handling, exposure, and disposal of toxic mercury vapors commonly found in CCFL/fluorescent lamps. Finally, while incandescent lamps need to be replaced approximately every 1,000 hours, and fluorescent lamps last for 10,000 hours, LEDs can last for more than 100,000 hours. In most applications, this allows LEDs to be permanently embedded in the end application. This is particularly important in the case of backlighting of automotive instrument/navigation/infotainment system control panels, which are embedded in the interior of the car, as they will never need to be replaced during the life of the car. In addition, LEDs are several orders of magnitude smaller and flatter than other lighting, so LCD flat panel displays can be very thin, requiring minimal space inside the car. Also, by using a red, green, and blue LED configuration, an infinite number of colors of light can be provided. LEDs can also be dimmed and turned on/off much faster than the human eye can detect, allowing for significant improvements in LCD display backlighting while allowing extremely high contrast and high resolution.

One of the biggest challenges facing automotive lighting system designers is how to optimize all the benefits of the latest generation of LEDs. Because LEDs generally require an accurate and efficient DC current source and dimming method, LED driver ICs must be designed to meet these requirements under a wide range of conditions. The power solution must be efficient and reliable in terms of functionality and reliability, while being very compact and affordable. Logically, one of the most demanding applications in terms of driving LEDs will be automotive infotainment and instrument panel TFT-LCD backlighting applications, as they are located in the harsh automotive electrical environment, must compensate for a wide range of ambient lighting conditions, and must fit into very constrained spaces, all while maintaining an attractive cost structure.

Automotive LED Backlighting

Advantages such as small size, extremely long life, low power consumption, and enhanced dimming capabilities have led to the widespread adoption of LED TFT-LCD backlighting in today's cars, trucks, trains, airplanes, and ships. LED backlighting started primarily with infotainment systems, which typically have an LCD display mounted somewhere in the center of the dashboard so that the driver and passengers can easily see where they are, perform audio tuning, and other various tasks. Many emerging automotive designs utilize a single instrument panel to backlight all of the display instruments that facilitate driver control, as shown in Figure 2. The LED backlighting of the dashboard is often shared with the infotainment system, making the numbers on the control panel very easy to read. Similarly, many vehicles, including cars and trains, and also airplanes, have LCD displays mounted behind the seats for passengers to watch movies, play video games, and so on. Historically, these displays have been backlit by CCFLs, but it is becoming more common to replace relatively large CCFL lamps with very flat arrays of white light LEDs to provide more accurate and adjustable backlighting and an operating life that can exceed the life of the vehicle or aircraft.

There are several positive implications of using LEDs in this type of environment. First, LEDs never need to be replaced because they have a reliable lifespan of more than 100,000 hours (11.5 years of service life), which exceeds the life of the vehicle. This allows automakers to permanently embed LEDs into the interior backlighting system without having to leave room for replacement. Because LED lighting systems do not require as much depth or area as CCFL lamps, styling can also change significantly. In addition, LEDs are generally more efficient than incandescent lamps in providing light output (measured in lumens) from input electrical power. This has two positive effects. First, LEDs leak less electrical power from the vehicle bus, and just as importantly, LEDs reduce the heat that needs to be dissipated in the display, eliminating the need for bulky, expensive heat sinks.

Another important benefit of LED backlighting is the wide dimming ratio capability provided by high-performance LED driver ICs. Since the interior of a car is subject to a very wide range of ambient lighting conditions, including everything from direct sunlight to complete darkness, and the human eye is very sensitive to small perturbations in light output, the display needs to be dimmed or brightened accordingly, so it is of primary importance that the LED backlighting system can provide a very wide dimming ratio from 1,000:1 to as high as 30,000:1. With the right LED driver IC, this type of wide dimming ratio is relatively easy to achieve, which is impossible with CCFL backlighting. Figure 3 shows an LED backlit instrument panel and infotainment system display sharing the LED backlight.