Display Technology
Automotive displays demand brighter backlighting
The market for (HB) LEDs is booming, especially in the automotive sector. Jeff Gruetter looks at how efficient LED design can best support this impressive record of growth
B
y 2012, the market size for high brightness (HB) LEDs is expected to reach $12 billion and grow to $20.2 billion by 2015 according to Strategies Unlimited. LEDs used to backlight displays are currently the main driver with applications ranging from HDTVs to automotive displays to a myriad of handheld devices. In order to maintain this impressive growth rate LEDs must not only offer enhanced reliability, reduced power consumption and smaller/flatter form factors, but they must also offer obvious improvements in contrast ratios, picture clarity and colour accuracy. Additionally, automotive, avionic and marine displays must maximize all of these improvements while being subjected to a wide array of ambient lighting conditions ranging from bright sunlight to moonless nights. These Transistor-Liquid Crystal Display (TFT-LCD) display applications range from infotainment systems, gauge clusters and a wide array of instrument displays. Backlighting these displays with LEDs
create some unique LED IC driver design challenges in order to optimise display readability under a wide range of ambient lighting conditions. How can such an impressive growth potential in automotive lighting be supported? First of all, LEDs are ten times more efficient at producing light than incandescent bulbs and almost twice as efficient as fluorescent lamps, including cold cathode fluorescent lamps (CCFL). As LEDs are further developed, their efficacy, or ability to produce lumens of light from electrical power, will only continue to increase. Secondly, LED lighting does not require the handling, exposure and disposal of the toxic mercury vapor commonly found in CCFL/fluorescent bulbs. Finally, incandescent bulbs need to be replaced about every 1,000 hours, while fluorescent bulbs last up to 10,000 hours compared to a 100,000+ hour lifetime for LEDs. In most applications, this allows the LEDs to be permanently embedded into the final application. Additionally, LEDs are orders of magnitude smaller and flatter then their counterparts so LCD panels can be very thin, and by using a configuration of red, green and blue LEDs, an infinite number of colours are available.
12 October 2011
One of the biggest challenges for automotive lighting systems designers is how to optimise all the benefits of the latest generation of LEDs. As LEDs generally require an accurate and efficient DC current source and a means for dimming, the LED driver IC must be designed to address these requirements under a wide variety of conditions. Power solutions must be highly efficient, robust and reliable while being compact and cost effective. Arguably, one of the most demanding applications for driving LEDs will be found in automotive infotainment and instrument panel TFT-LCD backlighting applications as they are subjected to the rigors of the automotive electrical environment, must compensate for a wide variation of ambient lighting conditions and must fit in a very space constrained footprint, all while maintaining an attractive cost structure.
Auto LED backlighting Their benefits have triggered a wide-spread adoption of LED TFT-LCD backlighting in today’s automobiles, trucks, trains, planes and boats. LED backlighting started primarily in infotainment systems and many emerging automotive designs use a single panel to backlight all of the display gauges for driver control as seen in Figure 1. Often, the LED backlighting for the instrument panel is shared with the infotainment system, creating an easy to read control panel. Similarly, many vehicles also have LCD displays in the rearward seat(s) with movies, video games and so forth. Historically, these displays have used CCFL backlighting, but it’s becoming more common to replace these relatively large bulbs with very low-profile arrays of white LEDs to provide more precise and adjustable backlighting as well as a service life that can out live the vehicles.
The benefits of using LEDs have several positive implications. First, they never need to be replaced, since their solid state longetivity of up to 100K+ hours (11.5 service years) surpasses the life of the vehicle. This allows automobile manufactures to permanently embed them into “in cabin” backlighting without requiring accessibility for replacement. Styling can also be dramatically changed as LED lighting systems do not require the depth or area as do CCFL bulbs. LEDs are also generally more efficient than fluorescent bulbs at delivering light output (in lumens) from the input electrical power. This has two positive effects. First, it drains less electrical power from the automotive bus and it reduces the amount of heat that needs to be dissipated, eliminating the need for bulky and expensive heat sinking. Another important benefit of LED backlighting is the wide dimming ratio capability provided by a high performance LED driver IC. As the interior of a car is subjected to a very wide variation of ambient lighting conditions it is imperative that the LED backlighting system is capable of very wide dimming ratios, from 1,000:1 to as high as
30,000:1 since the human eye is very sensitive to minor
perturbations in light output, the screens need to dim or brighten correspondingly. With the proper LED driver IC, these wide dimming ratios are relatively easy to attain, which are not possible with CCFL backlighting.
life, the LEDs current and temperature limits should not be exceeded. One of the automotive industry’s major challenges is overcoming the harsh electrical environment found on the car’s power bus. The major challenges are transient conditions known as load dump and cold crank. Load-dump is a condition where the battery cables are disconnected while the alternator is still charging the battery. Such an abrupt disconnection of the battery cable can produce transient voltage spikes up to 40V as the alternator is attempting to full-charge of an absent battery. Transorbs on the alternator usually clamp the bus voltage to approximately 36V and absorb the majority of the current surge; however DC/DC converters down stream of the alternator are subjected to these 36V to 40V transient voltage spikes. These converters are expected to survive and regulate an output voltage during this transient event. There are various alternative protection circuits, usually transorbs, which can be implemented externally; however, they add cost, weight and take up space.
Figure 2: LED backlit instrument panel and navigation display Figure 1: LED backlit instrument panel Components in Electronics
Design parameters LEDs require an effective drive circuit and these ICs must be capable of operating from the caustic automotive power bus and also be both cost and space effective. In order to maintain their long operating
“Cold Crank” is a condition that occurs when a car’s engine is subjected to cold or freezing temperatures for a period of time. The engine oil becomes extremely viscous and requires the starter motor to deliver more torque, which in turn, draws more current from the battery. This large current load can pull the battery/primary bus
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