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Adding a Personal Touch
to Automotive Interior Lighting via LIN
By Michael Bender, Melexis
Solid state lighting is now becoming commonplace
in automotive designs, replacing more costly, less
energy efficientand not as reliable incandescent
lamps Though it is only in recent times that it is
has started to be considered for the parking lights,
brake lights, side lights and front lights on the
car’s exterior, it has been used for
illuminationpurposes in some car models for over
a decade. Now, by employing more sophisticated, highly integrated semiconductor
solutions, car manufacturers have the possibility to make their interior lighting systems far
more exciting for vehicle occupants and thus enhance the whole driving experience.
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. They are now looking to utilize it in many of their mid-range models.
The only real issue that needs to be overcome is how to implement such lighting systems
while ensuring that the expense involved is not too great.
Rudimentary ambient lighting mechanisms first started to be included in vehicles about four
years ago. At this stage, however, they werevery limited in terms of the functionalitythat
could be supported, plus theyoften proved fairly expensive to implement. The degree of
customer adjustability needed to be increased markedly if such a feature was toever have
serious market appeal. This has led to a move from the passive approach of these early
implementationsto 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, altering this whenever they wish, as well asbeing able to set a number of specific
lighting profiles to suit their individual tastes. They will be able to setspecific 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 - being located 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 performancebut costly controller area network (CAN).As each RGB
LED module can beconnected 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, itis capable of
driving up to 4 LEDs (red, green, blue and additional whiteif necessary) directly thanks to
built-in constant current sources. It incorporates multiple functions, including a physicallayer 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 16bit microcontroller (with 24/32kByes of Flash, 512Bytesof RAM, 128Bytes of EEPROM
memory). Through the microcontroller each of the 4 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 1014different colors can
be generated – for an expansive user selection. Key supporting functions like 16-bit current
control and constant current high voltage capable outputsare 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 capabilityprovides aging and temperature compensation for the LED emitters’
changing color valueswithout 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 already
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 andthe driver electronics all contained
on a highly compact board.
The high degree of built-in functionality in this function-packed singlechip 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, moving forwards, be of major benefit to car manufacturers, allowing greater
personalization of their models through more compelling and entertainingambient 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.
MLX81106 Application Schematic