introduction
There are some key applications that have led to explosive growth in LEDs used as thin-film transistor (TFT) liquid crystal display (LCD) backlights, including high-definition (HD) televisions, portable tablets, automotive infotainment displays, and a host of handheld communications. device. However, in order to maintain this phenomenal growth rate, LEDs must provide higher reliability, lower power consumption, and a more compact form factor, while achieving higher contrast and color accuracy. In addition, in automotive, avionics, and marine displays, while all of these parameters must be improved, the LEDs must accommodate a variety of ambient lighting conditions, from daylight to night without moon.
These TFT-LCD applications include infotainment systems, meters, and a variety of instrument displays. Of course, backlighting these displays with LEDs creates some unique LED driver IC design challenges because of the readability of the display under a variety of lighting conditions. LED drivers need to offer a very wide dimming range and high efficiency conversion, while also adapting to harsh automotive electrical and physical environments. Needless to say, the height of these solutions must be very flat, the footprint is very compact, and the overall price/performance ratio is also increased.
What are the factors driving LEDs to grow in automotive displays?
To support the phenomenal growth in automotive lighting applications, LEDs must have an advantage over incandescent lamps. The advantages of LEDs include: LEDs are 10 times more luminous than incandescent lamps and almost twice as many as fluorescent lamps, including cold cathode fluorescent lamps (CCFLs), thus reducing the amount of light output required (unit: lumens per watt) Electric power. As LEDs continue to evolve, their efficiency or ability to produce light output from power supplies will only continue to increase. In addition, we are now very concerned about environmental protection, and LED lighting does not require the handling, exposure and disposal of toxic mercury vapors commonly found in CCFL lamps. Finally, incandescent lamps usually need to be replaced after about 1,000 hours of operation, while fluorescent lamps can run for up to 10,000 hours. However, these figures are dwarfed by the 100,000-hour lifespan provided by LED lighting.
In most applications, this long operating life allows the LED to be permanently embedded in the final application. This is especially important for backlighting displays for automotive instrumentation and infotainment systems, as these displays are often embedded in automotive dashboards that do not need to be replaced during the life of the car. In addition, LEDs are smaller and more compact than other types of lamps, so LCD panels can be made extremely thin, taking up minimal space inside the car. Also, an unlimited number of colors can be provided by using red, green and blue LED configurations. Not only that, the LED dimming and on/off speeds are much faster than the human eye can perceive, significantly improving the backlighting of LCD displays while enabling extremely high contrast and display on the display. High resolution.
Barriers to LED adoption in automotive applications
However, one of the biggest obstacles facing automotive lighting system designers is how to optimize all the features and benefits offered by the latest generation of LEDs. Since LEDs typically require an accurate and efficient current source and a dimming method, the LED driver IC must be designed to meet these needs under a variety of operating conditions. In addition, power solutions must be very efficient, must be rugged and reliable, and be extremely compact and cost effective. It can be said that one of the most demanding applications for driving LEDs is the backlighting of automotive infotainment and instrumentation TFT-LCDs, which are required to compensate for a variety of ambient lighting in harsh automotive electrical environments. The conditions change and must be adapted to situations where the available space is very limited. At the same time, such applications must have an attractive cost structure.
Many newer car designs use a single lighting panel to backlight all meter displays for driver control. LED backlighting for dashboards is also often used in infotainment systems, resulting in an easy-to-read, integrated control panel. Similarly, many cars, trains, and airplanes offer LCD displays that provide entertainment for movies, video games, and more. In the past, these displays have been using CCFL backlighting, but it is now more common to replace these designs with relatively large illuminators with very flat white LED arrays to provide more accurate and adjustable backlighting and extended use. life.
Automotive LED lighting design requirements
To ensure optimum performance and long operating life, LEDs require an efficient drive circuit. Such driver ICs must be able to operate on the most demanding automotive power bus, and must also be cost effective and have high space efficiency. In order to maintain a long working life, it must also be ensured that the current and temperature limits of the LED are not exceeded.
One of the major challenges in the automotive industry is to cope with the harsh automotive power bus electrical environment. The main challenge is to deal with transient conditions called “disloading†and “cold car launchingâ€. Dismantling means that the battery cable is disconnected and the alternator is still charging the battery. This may occur when the battery cable is not securely connected while the car is running, or when the battery cable is broken and the car is running. This sudden disconnection of the battery cable can produce transient voltage spikes of up to 40V because the alternator attempts to fully charge the unconnected battery. Surge suppressors on alternators typically clamp the bus voltage to approximately 36V and absorb most of the current surges, however DC/DC converters downstream of the alternator must withstand these 36V to 40V transient voltage spikes. It is expected that these converters will not be damaged and can regulate the output voltage when such transient events occur. There are now a variety of different protection circuits, usually with a surge suppressor, because the surge suppressor can be implemented externally. However, surge suppressors increase cost, weight and space.
When the car engine is in a very cold or freezing temperature for a period of time, cold car launching will occur. At this point the engine oil becomes extremely viscous and requires an engine starter to provide more torque, which draws more current from the battery. This high current load can pull the battery/main bus voltage down to 4.0V at the moment of ignition and then typically return to the nominal 12V.
A new solution to these dilemmas is Linear Technology's LT3599, which can withstand both conditions without being damaged and provides a stable fixed output voltage. With an input voltage range of 3V to 30V and transient protection to 40V, the device is ideal for automotive environments. Even when VIN is above VOUT (which can happen with a 36V transient), the LT3599 still provides a stable desired output voltage.
Most LCD backlighting applications require 10W to 15W of LED power, and the LT3599 is designed to meet this application. The device boosts the automotive bus voltage (nominally 12V) to 44V to drive up to four parallel LED strings (each containing 10 100mA series LEDs). Figure 2 shows a schematic of the LT3599 driving four parallel LED strings, each consisting of 10 80mA LEDs, providing a total of 12W.
Figure 1: 12W LED backlight circuit with 90% efficiency using LT3599
The LT3599 uses an adaptive feedback loop design that adjusts the output voltage slightly above the maximum voltage of the LED string. This minimizes the power lost by the ballast circuit to optimize efficiency. Figure 3 illustrates the efficiency of the LT3599 with an efficiency of up to 90%. This is important because it does not require any heat sinks, resulting in a flat solution with a very compact footprint. It is also important to drive the LED array to provide accurate current matching to ensure consistent backlight brightness across the entire display panel. The LT3599 ensures LED current changes of less than 2% over the -40°C to 125°C temperature range.
Current matching with temperature 
Efficiency varies with LED current
Figure 2: LED current matching and efficiency of the LT3599 in Figure 1.
The LT3599 uses a fixed frequency, constant current boost converter topology. Its internal 44V, 2A switch is capable of driving 4 LED strings (each string includes up to 10 100mA series LEDs). Its switching frequency is programmable and synchronizable from 200kHz to 2.5MHz, allowing the device to maintain switching frequencies outside the AM radio band while minimizing external component size. The device is also designed to drive from 1 to 4 LED strings. If fewer LED strings are used, then each string can have a larger LED current. Each LED string can use the same number of LEDs or a different number of LEDs to operate in an asymmetrical fashion.
The LT3599 can dim the LEDs with the True Color PWMTM dimming method or dim the LEDs with analog dimming via control pins. True Color PWM provides a dimming ratio of up to 3,000:1, which is often required for automotive applications. By PWM dimming the LED at full current, any color shift of the LED light can be eliminated, and because the frequency is very high, it is not noticeable to the human eye. Analog dimming provides a very convenient way to achieve a dimming ratio of up to 20:1 by changing the value of the CNTRL pin voltage. The effect of this dimming method depends on the changes in the ambient light of the LCD panel. Finally, the LT3599 integrates protection features including open and short circuit protection and an alarm pin.
in conclusion
High-brightness (HB) LEDs have great growth potential for automotive applications, and it is undoubtedly a significant requirement for the LED itself and the driver ICs needed to drive the LEDs compared to today's adoption rates. increase. Linear Technology has developed a complete line of high current LED drivers for automotive applications for a wide range of applications from LCD backlighting to turn signals to headlights. Automotive lighting system designers now have a simple, efficient source of LED drivers for the most challenging LED lighting designs.
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