Another major benefit of LED backlighting is that high-performance LED driver ICs enable a wide dimming ratio. Since the interior of a car experiences a very wide range of ambient lighting conditions, the new generation of automotive circuit maintenance plans includes every change from direct sunlight to complete darkness, and the human eye is very sensitive to slight disturbances in light output. Therefore, it is important for displays to be correspondingly dimmed or brightened. Accordingly, LED backlighting systems need to provide an exceptionally wide dimming ratio ranging from 1,000:1 to as high as 30,000:1. Using appropriate LED driver ICs, this wide dimming ratio is relatively easy to achieve, which would be 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 reliably from the quite harsh automotive power bus and be cost-effective and space-efficient. To maintain a long working life, it is crucial 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 known as load dump and cold cranking. Load dump refers to the situation where the battery cable is disconnected while the alternator is still charging the battery. This can occur when the battery cable connection becomes loose while the car is running or when the battery cable is disconnected while the car is in motion. Such sudden disconnection can generate transient voltage spikes up to 40V because the alternator is trying 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 current surge. However, DC/DC converters downstream of the alternator have to withstand these transient voltage spikes between 36V and 40V. These converters are required not to be damaged 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 take up space.
"Cold Cranking" refers to the situation when the car engine has been exposed to cold or freezing temperatures for some time. The engine oil becomes extremely viscous, requiring the engine starter to provide more torque, which in turn draws more current from the battery. This large current load at ignition can pull the battery/main bus voltage down to 6V, after which it typically returns to the nominal 12V.
Fortunately, there is already a solution for these issues — Linear Technology's LT3760. This device can withstand both situations without being damaged and regulate a fixed output voltage. The LT3760's input voltage range of 6V to 40V makes it highly suitable for automotive environments. Even when VIN exceeds VOUT (which can happen during a 40V transient), the LT3760 will still regulate the required LED current.
Since most automotive LCD backlighting applications require LED power ranging from 20W to 35W, 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 containing 10 series-connected LEDs at 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:** 28W LED backlighting circuit using the LT3760 with 90% efficiency.