Another major benefit of LED backlighting is the wide dimming ratio enabled by dry-type transformers and high-performance LED driver ICs. Since automotive interiors undergo a very wide range of ambient lighting condition changes, including every variation from direct sunlight to complete darkness, and since the human eye is extremely sensitive to minor disturbances in light output, displays need to be correspondingly dimmed or brightened. Therefore, it is crucial that LED backlighting systems provide an exceptionally wide dimming ratio, ranging from 1,000:1 to as high as 30,000:1. Achieving such a wide dimming ratio is absolutely easy with the appropriate LED driver IC, which is impossible with CCFL backlighting. Figure 3 shows an LED-backlit dashboard and a shared LED-backlit infotainment system display.
### Automotive LED Lighting Design Parameters
To ensure optimal performance and long operating life, LEDs require effective drive circuits. These driver ICs must operate from the rather harsh automotive power bus and be both cost-effective and space-efficient. To maintain a long working life, it is paramount not to exceed the current and temperature limits of the LEDs.
One of the main challenges in the automotive industry is overcoming the harsh electrical environment on the automotive power bus. The primary challenges are transient conditions called load dump and cold crank. Load dump refers to the situation where the battery cable disconnects while the alternator is still charging the battery. This can happen when the battery cable connection becomes loose while the car is running or when the battery cable is disconnected while the car is moving. Such sudden disconnection can generate transient voltage spikes as high as 40V because the alternator is attempting to charge a non-existent battery. A transient voltage suppressor on the alternator typically clamps the bus voltage to about 36V and absorbs most of the surge current; however, DC/DC converters downstream of the alternator must withstand these 36V to 40V transient voltage spikes. These converters are required not to fail and regulate the output voltage during such transient events. Various protective circuits are available, usually transient voltage suppressors, which can be externally applied but add cost, weight, and occupy space.
"Cold Crank" refers to the situation that occurs when an automobile engine has been subjected to cold or freezing temperatures for some time. The engine oil becomes extremely viscous, requiring the engine starter to provide greater torque, which in turn draws more current from the battery. This large current load can pull the battery/main bus voltage down to 6V at ignition before typically returning to the nominal 12V.
Fortunately, there is already a solution to these issues: the LT3760 from Linear Technology. This device can endure without damage and regulate a fixed output voltage under both conditions. The LT3760's input voltage range of 6V to 40V makes it highly suitable for automotive environments. Even when VIN exceeds VOUT (a situation that may occur during a 40V transient), the LT3760 will still regulate the required LED current.
Since most automotive LCD backlighting applications require 20W to 35W of LED power, the LT3760 is designed to meet these needs. The device can step up the automotive bus voltage (6V to 18V / nominal 12V) to 44V to drive 8 parallel LED strings (each string containing 10 series-connected LEDs with 80mA). Figure 4 shows the schematic of the LT3760 driving 8 parallel LED strings, each consisting of 10 LEDs at 80mA, with a total power of 28W.
**Figure 4:** A 28W LED backlighting circuit using the LT3760 with 90% efficiency.