Tony Armstrong, Linear Technology
Some of the key applications which drove the explosive growth of LEDs used to backlight thin film transistor (TFT) liquid-crystal displays (LCDs) included high definition (HD) TVs, portable tablet PCs, automotive infotainment displays and the myriad of handheld communication devices. However, in order to maintain this impressive growth rate LEDs must not only offer enhanced reliability, reduced power consumption and more compact form factors, but they must also provide improvements in contrast ratios and color accuracy. Furthermore, in automotive, avionic and marine displays all of these improvements must be optimized while simultaneously being subjected to a wide array of ambient lighting conditions ranging from bright sunlight to moonless nights.
These TFT-LCD applications range from infotainment systems, gauge clusters and a wide array of instrument displays. Of course, backlighting these displays with LEDs creates some unique LED driver IC design challenges in order to optimize display readability across a myriad of lighting conditions. This requires LED drivers to offer very wide dimming ratios and high efficiency conversion while also withstanding the rigors of the demanding automotive electrical and physical environment. It goes without saying that these solutions must offer very low profile, compact footprints while simultaneously enhancing overall cost-effectiveness (see Figure 1).
Click image to enlarge
Figure 1: Display solutions must offer very low profile, compact footprints while simultaneously enhancing overall cost-effectiveness
What is driving LED growth in automotive displays?
In order to support the impressive growth in automotive lighting applications, LEDs must offer a value proposition over their incandescent bulb counterparts. These include; ten times more efficient at producing light than incandescent bulbs and almost twice as efficient as fluorescent lamps, including cold cathode fluorescent lamps (CCFL); thereby reducing the amount of electrical power required to deliver a given amount of light output (measured in lumens per watt). As LEDs are further developed, their efficacy, or ability to produce lumens of light output from an electrical power source, will only continue to rise.
Secondly, in this environmentally conscious world of ours, LED lighting does not require the handling, exposure and disposal of the toxic mercury vapor commonly found in CCFL bulbs. Finally, incandescent bulbs are usually required to be replaced after approximately 1,000 hours of operation while fluorescent bulbs can last as long as 10,000 hours. However, these figures are dwarfed in comparison to the 100,000 hour plus lifetimes afforded by LED lighting.
In most applications, this extended operating lifetime allows for LEDs to be permanently embedded into the end-application. This is especially important for the backlighting of automotive clusters, instrumentation and infotainment panels - which are often embedded into a vehicle’s dashboard, since they will not require replacement during the working life of the car.
Additionally, LEDs are orders of magnitude smaller and more compact than their counterparts so the LCD panels can be made extremely thin, thereby requiring minimal volumetric space in a vehicle’s interior. Also, by using a configuration of red, green and blue LEDs, an infinite number of colors can be delivered. Furthermore, LEDs also have the ability to dim and turn on/off much faster than the human eye can detect, enabling significant improvements in backlighting of LCD displays while simultaneously allowing dramatic contrast ratios and a higher resolution picture.
Obstacles to adoption for automotive applications
Nevertheless, one of the biggest obstacles facing automotive lighting systems designers is how to optimize all of the features and benefits provided by this newest generation of LEDs. Since LEDs generally require an accurate and efficient current source and a means for dimming them, a LED driver IC must be designed to address these requirements under a wide variety of operating conditions.
Further, their power supply solutions must be highly efficient, rugged and reliable while also being very compact and cost effective. Arguably, one of the most demanding applications for driving LEDs will be found in the backlighting of automotive infotainment and instrument TFT-LCDs as they are subjected to the rigors of the automotive electrical environment where they must compensate for a wide variation of ambient lighting conditions and must fit in a very space constrained areas. And all the while, they must have an attractive cost structure.
Many emerging automotive designs use a single panel to backlight all of the display gauges for driver control. Often, the LED backlighting for the instrument panel is shared with the infotainment system, creating an easy to read all-in-one control panel. Similarly, many vehicles including cars, trains and airplanes also have LCD displays that entertain passengers in the rearward seat(s) with movies, video games and so forth. Historically, these displays have used CCFL backlighting; however, it is becoming more common to replace these relatively large bulb designs with very low-profile arrays of white LEDs to provide more precise and adjustable backlighting as well as an extended service life.
Automotive LED Lighting Design Requirements
In order to ensure optimal performance and long operating life, LEDs require an effective drive circuit. These driver 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 life, it is imperative that the LEDs current and temperature limits are not exceeded.
One of the automotive industry’s major challenges is overcoming the electrically caustic 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. This can occur when a battery cable is loose while the car is operating, or when a battery cable breaks while the car is running. Such an abrupt disconnection of the battery cable can produce transient voltage spikes up to 40V as the alternator is attempting a full-charge of an absent battery. Surge suppressors on the alternator usually clamp the bus voltage to approximately 36V and absorb the majority of the current surge; however, DC/DC converters downstream 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 surge suppressors, which can be implemented externally. However, they add cost, weight and take up space.
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 voltage below 4.0V upon ignition, after which it typically returns to a nominal 12V.
It is important to find a solution that properly addresses this dilemma. For example, Linear Technology’s LT3599 is capable of both surviving and regulating a fixed output voltage throughout both of these conditions. With an input voltage range from 3V to 30V, and transient protection to 40V, it’s ideal for the automotive environment. Even when VIN is greater than VOUT, which could occur during a 36V transient, the LT3599 will regulate the required output voltage.
As most LCD backlighting applications require between 10 and 15watts of LED power, the LT3599 has been designed to service this application. It can boost the automotive bus voltage (nominal 12V) to as high as 44V to drive up to four parallel strings, each containing ten 100mA LEDs in series. Figure 2 shows a schematic of the LT3599 driving four parallel strings, with each string comprised of ten 80mA LEDs delivering a total of 12W.
Click image to enlarge
Figure 2. 90% Efficient 12 Watt LED Backlighting Circuit Using the LT3599
The LT3599 utilizes an adaptive feedback loop design, which adjusts the output voltage slightly higher than the highest voltage LED string. This minimizes power lost through the ballasting circuitry to optimize the efficiency. Figure 3a & 3b illustrate the LT3599’s efficiency that can be as high as high as 90%. This is important because it eliminates any requirement for heat sinking, enabling a very compact low profile footprint. Equally important for driving LED arrays are to provide accurate current matching to insure that the backlighting brightness remains uniform across the entirety of the panel. The LT3599 is guaranteed to deliver less than 2% LED current variation across its -40˚C to 125˚C temperature range.
Click image to enlarge
Figure 3. LED Current Matching & Efficiency of LT3599
The LT3599 uses a fixed frequency, constant current boost converter topology. Its internal 44V, 2A switch is capable of driving four strings of up to ten 100mA LEDs connected in series. Its switching frequency is programmable and synchronizable between 200kHz and 2.5MHz enabling it to keep switching frequency out of the AM radio band while minimizing the size of the external components. Its design also enables it to run one to four strings of LEDs. If fewer strings are used, each string is capable of delivering additional LED current. Each string of LEDs can use the same number of LEDs or can be run asymmetrically with a different number of LEDs per string.
The LT3599 can dim the LEDs using either True Color PWMTM dimming or analog dimming via the control pin. True Color PWM offers dimming ratios as high as 3,000:1, which are often required in automotive applications. By PWM dimming of the LEDs at full current, any color shifts of the LED light are eliminated and the frequency is so high, it is undetectable by the human eye. Analog dimming offers a very simple means to achieve dimming ratios up to 20:1 by varying the level of CNTRL pin voltage. This means of dimming will be dependent on the variations of ambient light that the LCD panel is subjected to. Finally, the LT3599 has integrated protection features that include open and short circuit protection and alert pins.
There can be no doubt that the growth potential of HB LEDs for use in automotive applications represent a significant increase in demand from today’s deployment rate for both the LEDs themselves and for the LED driver ICs required to drive them. Additionally, Linear Technology has developed an entire family of high current LED driver products aimed specifically at automotive applications ranging from LCD backlighting to turn signals and even headlight applications. Today’s automotive lighting system designers now have an easy and effective LED driver source for their most challenging LED lighting designs.