Strategies for Implementing Automotive LED Lighting Systems

Fionn Sheerin
Microchip Technology Inc.

New vehicle designs contain more lighting than ever before: dome lights, display backlights, turn signals, low & high beams, fog lights, cornering lights, tail lights, accent lights and even infrared lighting for automated driving assistance. Inside the cabin, LED lighting has been replacing incandescent bulbs, primarily due to size, cost, efficiency and longevity.

Recent technological advances in LEDs and drive circuits for high-brightness LEDs have accelerated that trend, replacing exterior lighting for improved safety, reliability, and aesthetics. Swapping incandescent dome lighting and display backlighting with LEDs that will last for decades is an easy switch, but replacing critical, legislatively mandated exterior lighting required significant technological advances in both the diodes and the control circuits (and, in some cases, it still requires legislative changes). Novel, intelligent control schemes are able to do completely new things with LEDs, and this is rapidly revolutionizing automotive exterior lighting.

This trend started with the LED daytime running lamps on the 2004 Audi A8, which expanded to full LED headlamps on the 2007 Audi R8, and now complete LED exterior front lighting is available on a wide range of production vehicles around the world (including Cadillac, Audi, BMW, Mercedes-Benz, Toyota, Jaguar and Volkswagen, to name a few). OSRAM recently announced that it expects one in five headlights to be LED-based by 2020, which is a very rapid industry shift by automotive standards.

There are advantages to the efficiency and power consumption of LED lighting compared to Halogen and HID lighting, the price of high-brightness LEDs is falling rapidly, and the reliability of the diodes is unmatched. However, the primary driver of LED adoption is the controllability; and this requires intelligent LED drive circuits. Lastly, and perhaps the most important element for many consumers, are the aesthetic effects of good lighting. Lighting is an important vehicle design element; users are easily frustrated by poor lighting, and can directly appreciate good lighting.

Drivers ask for it, and in many cases are willing to pay extra for it. Premium headlights are a common, up-market vehicle option for new cars and aftermarket modifications. They affect the vehicle style, perception, ease of driving and safety. Car manufacturers recognize this, and lighting systems are the focus of increasing efforts by automobile and component manufacturers.

HELLA, Automotive Lighting (Magneti Marelli), Koito and Valeo have all issued press releases related to lighting systems in the last six months, illustrating the focus on this area. Through safety, reliability and curb appeal, good LED-based lighting designs can add significant value to vehicles, improving sales volumes and sale prices.

Administrators and legislators are particularly interested in vehicle exterior lighting.In most parts of the world, the number, brightness and color of exterior lights are mandated.Lights that are too bright for the road conditions cause glare problems, while dim or failed exterior lights pose a safety hazard.

Commonly, jurisdictions have legislated acceptable brightness ranges for daytime-running, low-beam, high-beam, turn-signal, cornering and fog lights on the front of the vehicle; with similar requirements for rear vehicle lighting. In some cases, there are also rules about which lights can be used in what conditions; including whether lights need to auto-level to compensate for the road angles, and the speeds at which cornering lights can turn on.

This is a nightmare of design requirements, which would necessitate a multitude of traditional halogen and HID fixtures, potentially adding mechanical motors or shutters. However, arrays of LED lights can address many of these requirements, if well designed constant-current regulators and intelligent architectures are employed. Good LED drive circuits are reusable, and the electronics can be replicated into multiple designs.

Microcontrollers packaged into the headlamps can use information from light and temperature sensors to compensate LED drive current; maintaining consistent light output, or deliberately refocusing the beam or adjusting brightness to changing driving conditions. Different individual diodes within the headlamp can be switched on and off or dimmed, to light around corners or to avoid blinding oncoming cars - without any mechanical components.

In addition, brake, hazard and turn-indication lights can light up in patterns or sequences to make them easier to notice. Last, but not least, properly implemented LED lighting can turn on or off nearly instantaneously (compared to the 250 ms delay with a standard incandescent bulb, to full brightness).

Admittedly, it remains to be seen how effective these adaptations will be at reducing vehicular accidents, long term, but it looks very promising. The catch is that all of these features require both high-quality LED arrays and high-accuracy addressable drive circuits with intelligence. Microchip’s dsPIC® digital signal controller (DSC) chips fit these needs very well. The quality of the circuit implementations, algorithms and component selection will separate the winning lighting products from the rest.

The inherent reliability of LEDs is also a major advantage over previous lighting generations. With some LED manufacturers claiming device lifetimes exceeding 20 years, it is conceivable that in the future vehicle lights would not require replacement. Factory-installed lights could last as long as the power train, without requiring maintenance. LED-based lighting systems can also be designed with inherent reliability.

Placing multiple independent lighting strings into a brake light or tail light can cost-effectively create redundancy within a single light, such that damage or failure would merely reduce the light output instead of turning it off completely. In addition, more advanced electronics can add fault-reporting capabilities, so the lights’ status can be displayed on the vehicle dash or reported through the diagnostic code reader.

With the lighting systems attached to CAN, LIN or a similar in-vehicle communications bus, the automobile could warn the driver if the lights are not functioning correctly. (Microchip makes a variety of CAN and LIN transceivers and microcontrollers that can be used for this application.) This is not a new concept, as "CANBUS compatible" lighting has been available in certain vehicles for a long time.

But the older systems are only capable of detecting major changes in functionality; efficient, low-power replacement bulbs commonly cause false errors. However, with intelligent LED drive circuitry, it is possible to report more detail than just a gross failure. Lighting systems can cost-effectively report detailed telemetry data for current/voltage shifts, temperature changes, or even input-voltage shifts. The diagnostic data could indicate future failures before they occur, even detecting minor changes such as a single shorted LED in a long string.

In order to properly design these diagnostic features, the LED drive and monitoring circuit must be as reliable as the diodes it controls. Often, LED-based lighting systems contain more components than the legacy bulbs they are replacing. Getting the full lifetime from an LED requires a proper drive circuit - one with good current regulation across all operating conditions. For superb light quality and reliable operation, the drive circuit should compensate for changes in temperature, input voltage and load resistance; maintaining a constant output current in every circumstance. Each extra component in the system introduces another possible failure point.

In order for a taillight to function for 20 years, both the LEDs and the LED drive circuit must last for 20 years. This kind of regulation and longevity is not possible using bias resistors. Lighting systems must use tightly controlled DC-DC regulators, from suppliers with proven track records in automotive quality, in order to achieve long-term reliability.

Microchip manufactures several Digitally Enhanced Power Analog PWM controllers, including the MCP19115, which are capable of industry-leading current regulation and have built-in digital communication interfaces. Designed properly, the lighting system will have all the elements required for a long product lifetime: high-reliability devices, combined with redundancy and monitoring.

Safety and reliability are certainly desired features, and automotive consumers often see deficiencies in this area as a reason not to purchase a vehicle. They will readily avoid brands or models that they perceive as unsafe or unreliable, while selecting from the remaining "acceptable" vehicles based on appeal.

Fortunately, well designed lighting systems add that vehicle-differentiating aesthetic that can lure discriminating buyers.More and more often, cars are being marketed with dramatic videos of car headlights turning on suddenly and moving purposefully through the dark. Well designed headlights have a distinct appeal to both high-end and low-end vehicles.

Most importantly, customers are willing to pay for these features. Aftermarket lights are among the most common additions to a vehicle, whether it is driving lights for an off-road truck (which may never leave the pavement) or fog lamps for a luxury sedan (which may never drive in fog), the visual appeal of exterior lighting is every bit as important as the body or interior styling.

Despite all the safety, reliability, cost and longevity concerns, for many drivers, the vehicle is as much a reflection of their personality as it is a tool for safe transportation. Emotional appeal sells cars, and well placed strings of carefully controlled LEDs can evoke that emotion (even in the dark). Today, some automakers and tier-one suppliers can be concerned about the added cost of the increased semiconductor content in their vehicles; but the reality is that most of those electronic features are adding back more far value than they cost to implement.

Automotive exterior lighting is particularly well suited to the application of LED technology. Older Halogen and HIDsystems do not offer the same flexibility to support modern designs and future safety expectations.

Precise, constant-current DC-DC converters, sometimes combined with microcontroller intelligence, can be used to implement everything from a basic taillight to the most advanced segmented headlamp. Future lighting designs, both with the upcoming LED-based designs and the new laser-based systems, are paving the way for intelligent vehicle systems of the future.

Identifying obstacles to implementing them is one more step down the path of vehicle automation, and an important opportunity for the automotive industry to prove it can add safety, increase reliability, improve aesthetics, and ultimately increase the overall value of vehicles.