Automotive Interior Lighting Evolves with LEDs
A new platform to revolutionize in-car lighting control incorporates up to 4,096 LEDs that are individually addressable from a single system controller.
by Robert Isele, Roland Neumann, and Karlheinz Blankenbach
Car interiors have undergone more than 100 years of development, and now constitute an unmistakably modern, technologically rich environment, sometimes also referred to as the driver’s workplace. Over this long period, even over the past few decades, we have been witness to several truly revolutionary milestones, with entertainment systems, driver assistance, displays, and lighting raising the expectations of both drivers and passengers. Today, developments in car sharing, electrification, and autonomous driving are putting greater focus on the car’s interior. Modern materials, interactive displays, and ambient lighting are all a part of this development.
Interior lighting inside today’s motor
vehicles is becoming an increasingly important selling point. Ambient light inside a car has the power to elicit both positive and negative emotional responses from drivers and passengers. It can also play a key role in shaping a potential customer’s initial reaction when seated inside a vehicle in a showroom. First impressions count! So, logically, carmakers are seeking to create a lighting experience “like no other” that will make their brand stand out from the competition by creating a unique in-vehicle atmosphere that enhances users’ emotional connection with the vehicle.
LED Control Issues
In recent years, LEDs have become an accepted standard for high-end, in-car lighting; nevertheless, there exist significant limitations to the way they are controlled. Present state-of-the-art “ambient light” technology typically consists of a multicolor LED feeding an optical fiber (Fig. 1).
Fig. 1: Most automotive interior color lighting uses multicolor LEDs that feed into optical fibers. Source: BMW
The flexibility and visual impact of this existing technology are severely limited, and its effect is often somewhat mediocre, due to the fact that the luminance and color performance of these LEDs can, as a rule, only be configured en masse or in relatively large groups. A central controller manages the LEDs via a local interconnect network (LIN), a serial network protocol that has become popular in many automotive applications as a lower-cost alternative to the controller area network (CAN) bus variant.
A more innovative approach is to mount 10 to 30 LEDs on a flexible light strip, where a controller converts commands into current pulses for each RGB LED individually, thereby enabling various colors and lighting effects (Fig. 2). Unfortunately, this solution involves significant drawbacks, as wavelength and luminance tolerance variations lead to non-uniform brightness and color perception. These effects are further exacerbated over a vehicle’s lifetime due to aging effects and varying temperature conditions along the length of the LED strip. Another flaw of this approach is circuit and wiring complexity (see below; Fig. 4 vs. Fig. 5), increased weight, and reduced flexibility.
Fig. 2: Multiple RGB LEDs addressed by a system controller enable various colors and lighting effects. Source: BMW
This aging effect manifests itself in declining luminance caused by falling efficiency, which is strongly affected by the LEDs’ temperature, and in particular, high temperatures of long duration. Furthermore, temperatures can vary widely throughout a car’s interior, and in some cases temperatures exhibit noticeable divergence even along a single LED strip. Another unfortunate detail is that the temperature-related characteristics, i.e. luminance and color coordinate shifts of green and blue LEDs, differ substantially from the characteristics of red LEDs.
These limitations notwithstanding, car manufacturers continue to demand further LED functionality in order to support innovative features and improve daytime LED visibility. While in 2010, a typical high-end vehicle might have been fitted with fewer than 50 LEDs, by year 2021 it is expected that more than 300 LEDs will be on board (Fig. 3). This growth is primarily in RGB-type LEDs, where three LEDs are integrated into a single light emitter. A controller is then required to set the individual color by means of RGB pulse-width modulations (PWMs), thus adding a further unwelcome layer of complexity and creating a system significantly more complex than the white and monochrome LED platforms that had dominated the market only a few years ago.
Fig. 3: The number of LEDs used in automotive interior lighting is rapidly increasing. Source: BMW
Requirements for In-car LED Control
To meet their objectives, carmakers require a sophisticated LED control platform that meets a lengthy list of challenging requirements. First, each RGB LED on the color strip must be individually controllable in terms of luminance and color, and second, it must enable calibration. An example is the cross-fading of adjacent LEDs from one color into another, where no brightness and color differences should be noticeable. Next, it must be possible to compensate for the effects of temperature and the non-uniform aging of the LEDs. Control must be possible over various architectures, and it must be straightforward to handle and manage the entire system. Such systems must also meet and ideally even exceed the rigorous automotive quality standards for technology, production, and testing, and must provide advanced diagnostic capabilities. Furthermore, LED control electronics must meet the safety and risk requirements as defined by the Automotive Safety Integrity Level (ASIL) scheme and the International Organization for Standardization (ISO) 26262 functional safety standard. Specific safety risks include, for example, spontaneous switching on of LED lighting, which could disturb the driver. It is also vital to ensure that warnings by LEDs are definitively provided (e.g. ensure the LEDs are really lighting up by checking the current through the LED at time of activation).
Only by ticking all these boxes can vehicle manufacturers be assured of achieving their goals of daytime visibility, fine control over luminance and color, and homogenous lighting with consistent output over the long term. These requirements simply cannot be met utilizing today’s LED control technology – a new approach is needed.
At the present point on the technology timeline, a microcontroller containing LED-specific data utilizes current-mode drivers to individually control each RGB LED. This solution typically proves to be too cumbersome and expensive to be viable, plagued by the high number of integrated circuits and extensive wiring needed. This system design (see Fig. 4) requires high-speed, one-way communication to the LEDs and sub-controllers, which impacts electromagnetic interference (EMI) robustness, hampers useful diagnostics, and leads to latencies. To make matters worse, it is not feasible to capture individual LED parameters such as functionality and temperature degradation. In general, solutions similar to those used for LED video walls are not suitable for advanced high-quality automotive interior lighting.
Fig. 4: This traditional system concept for RGB LED lighting is derived from LED video walls. Source: Inova Semiconductors
A New LED Control Concept
A new industry body – the Open ISELED (Intelligent Smart Embedded LED) Alliance – was formed to meet the challenge of providing reliable, modern in-car LED lighting. Its purpose is to provide a full and comprehensive ecosystem for a new digital LED concept. The founding partners are Inova Semiconductors, Dominant Opto Technologies, NXP, TE Connectivity, and Pforzheim University. Working together, and in close cooperation with BMW, these organizations have taken a new approach to interior automotive LED lighting, which is now set to redefine the world of automotive lighting. ISELED technology enables true “digital” LEDs to be integrated into a motor vehicle, without the complications of today’s “analog”-based LED concepts.
Inova Semiconductors unveiled this revolutionary digital ISELED concept for automotive applications at the recent Electronica exhibition in Germany. It addresses the need for precise control of interior lighting within the automotive temperature range and lifetime, respecting multiple automotive quality and robustness requirements and inherent cost implications. It represents a completely new technical architecture for high-speed LED control.
This new concept (see Fig. 5) is built around a smart digital LED controller that is embedded in a tiny 3-mm × 4-mm RGB LED package. The scalability of this approach will enable substantial cost savings and open up new market opportunities. The upcoming generation of interior car lighting will typically consist of 10 to 30 LEDs mounted on a flexible light strip. Each group of one red, one green, and one blue LED will form a “pixel,” which at 24-bit resolution (3 × 8 bit) can be set to more than 16 million colors.
The smart digital embedded LED controller from Inova provides sophisticated calibration features, with no need for binning classes or bar coding. This ensures that every LED renders the same color and luminance over the full temperature range, thereby guaranteeing automotive-level illumination consistency, even accepting greater LED manufacturing tolerances than are
currently attainable. The actual emission of each of the three LEDs is optically measured during the final test in real time, thus enabling the delta between measured and target value to be stored in the integrated LED controller. The LED controller then utilizes this information to correctly set the LED drivers, thus ensuring that color and luminance correspond to the RGB settings.
Leveraging the experience gained during the development of its automotive pixel link (APIX) display interface standard, Inova has built a high-speed communications protocol that allows every LED to be individually addressed. By supporting data rates of up to 2 Mbit/s, the new protocol enables fast, dynamic lighting effects.
A single microcontroller (Fig. 5, blue box on the left), acting as the system controller, can now manage an LED strip containing up to 4,096 LEDs, and each RGB LED module now has its own smart digital embedded LED controller (Fig. 6). Bi-directional communication between the system microcontroller and each LED module is achieved with very low latencies, and coupled with a 2 Mbit/s communication capability (without a dedicated clock), this ensures a reliable EMI-robust design. Bandwidth is utilized very efficiently due to individual addressing of each LED – while all LEDs can be addressed together via broadcasting, if required.
The system interface is provided by a microcontroller from NXP, which acts as the lighting controller (see “µC” in Fig. 5). The ISELED concept is ideally aligned with NXP’s recently announced S32K microcontroller product line. The S32K provides performance of up to 112 MHz, with a FlexIO configurable serial communication interface. Devices in the family provide from 8K to 2M Flash, all with an ARM Cortex M core. NXP is taking this new concept to market as a complete solution including hardware, software, and a fully developed ecosystem.
Fig. 5: This new controller concept from Inova features a digital controller (visualized as black boxes) built into each tiny RGB LED package. Source: Inova Semiconductors
For system diagnostics, the temperature, status, and functionality of each LED can also be accessed individually. Characteristics, such as function and power consumption of each LED, are fully retraceable and can be read retrospectively, which is particularly important for automotive ASIL-compliant lighting.
It goes without saying that an RGB LED with an integrated driver (Fig. 6) will cost more than a “conventional” RGB LED. Nevertheless, the challenge has always been to keep this additional cost to a minimum by making the controller chip highly compact and integrated, utilizing sophisticated packaging. The key cost advantage of this new ISELED concept comes from eliminating individual RGB strip calibration tasks and tighter binning for lower tolerances. Dedicated light strips were engineered by TE Connectivity in order to properly mount a higher number of "smart” (or digital) LEDs to fit seamlessly into the car interior.
Fig. 6: At top is an RGB LED with embedded driver and at bottom, a connectivity system concept for up to 4,096 LEDs. Source: Inova Semiconductors
First Product Sees Light
The first samples of a smart LED utilizing this new concept have recently become available from Dominant Opto Technologies. The samples consist of a smart LED controller from Inova with a bidirectional serial interface and daisy-chain capability. Each LED is calibrated to the required white point at the factory and can be used without any further compensation or measurements.
The driver includes three constant current mode (CCM) drivers to control the red, green, and blue LEDs. The color of each RGB LED “trio” can be set with 24-bit resolution (3 × 8 bit) for “display-like” performance in terms of gamma. For temperature and manufacturing tolerance compensation, or so-called binning, the luminance of each individual LED can be controlled with 12-bit resolution including daytime/night-time dimming. The LEDs have precise RGB calibration up to 1-step MacAdam Ellipse, with auto-compensation at high temperature, and dominant wavelength calibration (Fig. 7). A built-in temperature sensor ensures accuracy, while the calibration values of the CCM LED drivers and temperature compensation parameters are securely stored in non-volatile memory, an indispensable prerequisite for safety-related applications.
Fig. 7: Calibrated LED color space. Source: Dominant Opto Technologies
Packed and Ready to Go
The packaging is very compact and provides outstanding corrosion resistance and electrostatic discharge (ESD) protection exceeding 2kV. The extremely low thermal resistance of the LEDs’ housing, 30% below that of comparable products, further reduces power consumption of the LED by delivering improved light efficiency with cooler LEDs (Fig. 8). Further details will be available once the patent application is granted.
Fig. 8: This die-and-package schematic shows state-of-the-art LED packaging. Source: Dominant Opto Technologies
The first applications for the new ISELED concept are in automotive interiors, but there are multiple other potential use cases – including exterior car signaling. The fact that this communications protocol enables a large number of LEDs to individually change color and brightness in real time also makes it a candidate for non-automotive applications like
aircraft and cruise ships.
Trends in the automotive sector will also open up new applications. As the market moves toward autonomously driven vehicles, lighting will become more and more important, and requirements will be more rigorous and challenging. “Take back control” warnings and pleasant, high-quality in-car lighting in autonomous vehicle applications are just a few examples.
Not only will interior lighting play a major role in future design, it is essential that vehicles are able to better communicate with the world around them. The ability of autonomous vehicles to inform pedestrians that they have been recognized is very important from a safety standpoint.
The key concept here is that LED lighting is now undergoing a revolution that will forever transform the way people perceive the car interior. Technology has, in this case, simplified a previously troublesome issue for car manufacturers and promises to help early adopters stand out and gain market share in the premium vehicle segment.
About the ISELED Alliance
The ISELED Alliance develops LED-related products and solutions based on a new in-car LED lighting concept, which integrates a smart LED driver with three color LEDs into a tiny package. This drives down costs, simplifies control, and expands the functionality of LED lighting and display solutions. Founding members are Inova Semiconductors, Dominant Opto Technologies, NXP, TE Connectivity, and Pforzheim University. The last is involved in system design and optical measurements and optimizations. •
Robert Isele, electrics/electronics and driving experience environment, manager ambience light, modular system interior lighting at BMW, has an engineering degree in electronics from FH Munich. Roland Neumann, co-founder and VP of engineering/CTO at Inova Semiconductors, studied
telecommunications at Karlsruhe University. He can be reached at firstname.lastname@example.org. Professor Dr. Karlheinz Blankenbach has been a full professor at Pforzheim University since 1995. He is the founder of the university’s display lab. He holds an M.Sc. (Diploma) in Physics and a Ph.D. degree, both from the University of Ulm. He can be reached at email@example.com.