The more forward-thinking automobile brands have already started to recognize the value of ambient lighting as a way to facilitate greater differentiation in what is an increasingly competitive market. Solid state lighting is now becoming commonplace in automotive designs, replacing more costly, less energy efficient and less reliable incandescent lamps.

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Issue: Dec 2013

Light adds personal touch to automotive interiors

by Michael Bender, Melexis

The more forward-thinking automobile brands have already started to recognize the value of ambient lighting as a way to facilitate greater differentiation in what is an increasingly competitive market. Solid state lighting is now becoming commonplace in automotive designs, replacing more costly, less energy efficient and less reliable incandescent lamps.

Though the technology has only recently been considered for the parking lights, brake lights, side lights and front lights on the car’s exterior, it has been used for illumination purposes in some car models for over a decade. Now, by employing more sophisticated, highly integrated semiconductor solutions, car manufacturers can make their interior lighting systems far more exciting for vehicle occupants and thus enhance the whole driving experience.

Some OEMs are now looking to utilize solid state lighting in their mid-range models. The only real issue how to implement such lighting systems while controlling costs. Rudimentary ambient lighting mechanisms first started to be included in vehicles about four years ago. They were, however, very limited in terms of the functionality that could be supported, plus they often proved fairly expensive to implement. The degree of customer adjustability needed to be increased markedly if such a feature was to ever have serious market appeal. This has led to a move from a passive approach to smarter ambient lighting systems with a much greater depth of features.

Through the development of fully adjustable ambient lighting, vehicle occupants will, in the near future, have the ability to adjust the color of the interior to anything they chose, whenever they wish. They will be able to set specific lighting characteristics for different parts of the vehicle interior, create a particular start up lighting sequence, change the mood (perhaps when music by a particular artist is playing on the CD/radio for example). It also means that the driver (or passengers) can adapt the lighting system in response to diurnal and seasonal alterations in external illumination to maximize the overall lighting effect.

Ambient lighting implementation For ambient lighting to have real impact on the in-vehicle environment and add to passenger/driver comfort, a large number of solid state light sources need to be deployed throughout the cabin interior - in the foot wells, along the dashboard, across the underside of the roof, the door panels, etc, as well as illuminating certain mechanical parts (such as the interior door handles, cup holders and such like). Progress already made in the field of optoelectronics enables the combining of red green blue (RGB) light from single package emitter devices dispersed throughout the vehicle cabin - making it is possible for a broad spectrum of colors to be produced without the need for separate LEDS for each primary color.

Nevertheless, the cost associated with providing interconnections to all of these RGB LEDS has the potential to seriously restrict ambient lighting implementation, with the effect being that it might only be considered for luxury cars. This would be unfortunate as there are clear opportunities to jazz up the interiors of mid-range and entry level car models too if the price is right. Control through LIN With the curbing of any financial outlay required for implementation deemed to be of paramount importance, utilization of the local interface network (LIN) has emerged as a way to control the arrays of LEDs in ambient lighting systems - thereby making use of an existing bus infrastructure.

The lightweight, single wire LIN bus supplements the communication carried out by the high performance but costly controller area network (CAN). As each RGB LED module can be connected to the LIN network there is no need for any addition to the car’s cabling. As a result the overall system costs are kept to minimal levels, plus the vehicle’s overall weight is not increased - thus avoiding the fuel consumption penalty that would be observed if current point-to-point wired arrays were used. To respond to the very real demand now being witnessed from car manufacturers for a way to implement ambient lighting cost effectively, Melexis has recently introduced the MLX81106. Constructed on 0.18μm high voltage CMOS technology, this is a highly integrated, freely-programmable LIN RGB LED slave interface IC that contains all the necessary internal driver technology. Taking up just 9mm x 9mm of PCB, it is capable of driving up to 4 LEDs (red, green, blue and additional white if necessary) directly thanks to built-in constant current sources. It incorporates multiple functions, including a physical-layer LIN transceiver, a 19.2kbit/s LIN controller (supporting LIN 2.x and SAE J2602), a 10-bit analogue-to-digital converter (ADC) with integrated pre-divider, a voltage regulator and 16-bit microcontroller (with 24/32kByes of Flash, 512Bytes of RAM, 128Bytes of EEPROM memory).

Through the microcontroller each of the four outputs can be programmed to give up to 35mA (with internal boost function up to 48mA) of drive current. If an optional white channel needs more output current it can be supported with just one external component thanks to the IC’s high voltage capability on its IOs. A total of 2.7 x 1014 different colors can be generated – for an expansive user selection. Key supporting functions like 16-bit current control and constant current high voltage capable outputs are included too. LEDs show a difference in light intensity caused by the production process used. This leads to a change in output color mix. To compensate the EEPROM of the IC can be used to calibrate the LEDs within the end-of-line (EOL) module production process. The integrated LED threshold monitoring capability provides aging and temperature compensation for the LED emitters’ changing color values without the need for any external components. As there is the need for a large number of solid state light sources to be deployed throughout the cabin interior, it is vital that all the modules are the same (in terms of hardware and software) for logistic reasons. To simplify the car production process together with the need to individual control a single module it is necessary to implement a system to detect the position of a single module in the car. This is done with the help of the field proven auto-configuration mechanism, based on bus-shunt (already used in climate systems). This implemented system allows the position of the modules to be detected during the EOL process of the car. As a result, though all modules within the car are physically the same, they can be addressed via LIN on an individual basis. Melexis offers a simple-to-use reference design with the LED emitter and the driver electronics all contained on a highly compact board. The high degree of built-in functionality in this function-packed single chip solution enables the creation of streamlined highly optimized ambient lighting systems without the need for inclusion of large number of external components. It means that RGB ambient modules can be interconnected with the vehicle’s LIN system without requiring long drawn out development processes or heavy use of engineering resources.

Advanced technology of this kind will be of major benefit to car manufacturers, allowing greater personalization of their models through more compelling and entertaining ambient light features so that they can stand out from those of their rivals. This functionality will no longer be confined to high end automobiles, but will potential be applicable across the board

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