OFweek Semiconductor Lighting Network: In the past ten years, automotive electronics products have developed by leaps and bounds. In-vehicle electronic control, in-vehicle information services and entertainment systems have significantly improved both in quantity and in fineness. This article will focus on one of the main components of this growth: the rapid increase in LED lighting usage in current and next-generation vehicles. This new field of lighting presents new challenges for designers and manufacturers of automotive electronics. It is most important to understand these challenges and find viable solutions, as the developments associated with these lighting systems seem to be endless.
LED lighting
Advantages such as small form factor, low power consumption and fast turn-on time have created a situation where high-brightness LEDs are widely used in today's automobiles. The initial application of LEDs in automobiles was the Central Overhead Parking Light (CHMSL); these applications used red LEDs to provide a very flat, light-emitting array that was easy to install and never needed to be replaced.
Traditionally, incandescent bulbs are the most economical source of light and are still used by many cars. However, as the available lighting space shrinks and the life expectancy of lighting sources continues to increase, the color and design of the lights provided by LEDs is rapidly replacing incandescent bulb applications. Even traditional CCFL TFT-LCD backlight applications are gradually being replaced by white LED arrays.
What's more, people are still using an electric "steerable" high-current LED array to develop headlights, which has been dominated by halogen/æ°™ filament design. Almost all automotive lighting applications, including interior/external lighting and backlighting applications, will gradually transition to LED adoption. The benefits of using LEDs have many positive implications. First (and perhaps the most important), it never needs to be replaced because its 100,000-hour solid-state life (service life: 11 and a half years) is longer than the car's life. This allows automakers to permanently embed them in the lighting system in the cabin without the need to have an inlet for replacing the filament bulb as in the past. Since the LED lighting system does not require the installation depth or area required for incandescent bulbs, it can also cause significant changes in the shape of the car. Another advantage of LEDs is their low power consumption, which reduces fuel consumption.
Automotive LED lighting design parameters
To ensure optimum performance and long operating life, LEDs require an efficient drive circuit. These special drive circuits must be able to draw power from a fairly demanding automotive power bus, and they should also be cost and space "efficient." In order to maintain its long working life, the current and temperature limits of the LED must not be exceeded. Table 1 lists the relationship between typical forward voltage and drive current for a high current white LED.
In applications with a single LED to three (series) LEDs, a step-down LED driver (eg, Linear Technology's LT3475) will be required to reduce the automotive bus voltage (nominally 12V) to a further The appropriate LED voltage can vary from 2.68V to 4.88V (per LED) depending on the LED color and brightness requirements of the application. In contrast, in applications such as brake lights that require multiple LED strings consisting of up to eight LEDs in series, the required output voltage is 21V to 39V, so a boost LED driver must be used (eg: Linear Technology's LT3496). All of the LED drivers offered by Linear Technology use a current mode architecture designed to deliver a constant current.
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