ISELED – The LED goes Digital

Technology News | October 25, 2017

By Christoph Hammerschmidt

The LED – a highly sensitive creature
Without a doubt, the LED is currently revolutionizing the entire lightning technology segment. With the bulb of conventional incandescent and energy saving lamps flickering away into the past, this semiconductor-based light source has paved the way for a virtually unlimited number of designs; inspiring designers and architects alike to conceive new “luminaire” ideas. Besides that, its high efficiency makes the LED the most energy-efficient light source relative to all conventional lighting technologies. If we think about it, a LED is nothing other than a diode – a simple P-N semiconductor junction – that emits light. While a “normal” diode generally uses silicon as an “indirect” semiconductor, the LED diode relies on a gallium compound as a “direct” semiconductor. Without having to explain all the physics in detail, during the transition of the electron from the conduction to the valence band a photon is produced, which is emitted in the form of visible light. By altering the material used and its chemical composition, it is possible to affect the distance between these two bands, i.e. the band gap, and thereby determine the light’s frequency and wavelength. Accordingly, it is also possible to have LEDs light up in virtually any color of the visible and even the invisible spectrum (ultraviolet, infrared).

On the downside, however, the LED’s variety of colors, which is literally a product of alchemy, is also the reason for its peculiarly fastidious behavior. Depending on the used chemical compound and the resulting color, there will be changes in the forward voltage, which varies from around 2V for red to up to 3.5V for blue LEDs, whilst also impacting brightness and ageing behavior. While this effect is hardly noticeable on a single signal LED, the side-by-side installation of three monochrome LEDs – normally red, green and blue – in a single housing, i.e. “RGB LED”, can present an almost insurmountable challenge by producing a large number of blended colors caused by the varying brightness of the individual LEDs. It becomes even more complex when these RGB LEDs – mounted in large numbers on copper strips – must meet strict requirements in the vehicle with regard to color fidelity and brightness stability under fluctuating temperatures and over the effective lifetime of the vehicle.

Limitations of today’s concepts

Presently, preparing LEDs for high light conformity requires substantial effort from manufacturers. This preparation process involves the measurement of the wavelength and brightness of the individual LEDs and then sorting them into categories – a process known as binning. This information is then included in the form of a barcode, a type of summary profile for each individual LED. The characteristics of each LED used are then stored in a system controller. From there, the data is transmitted to sub-controllers, to which groups of usually four RGB-LEDs are linked. Considering the large volumes of data that need to be transmitted in this manner, a fast SPI bus with a clock frequency of up to 50 MHz is normally used – resulting in further EMC-related challenges. It thus comes as no surprise that the substantial expenditures in terms of components, wiring, power supplies, EMC measures, etc. and the final calibration of the LED strip at the supplier dramatically raises the cost of such a “workaround”.

This does not bode well for the vehicle manufacturers’ vision, which expects that up to 300 color (RGB) LEDs or more will be used per vehicle by 2021. There are further unpleasant restrictions: the data is received, read and then transmitted by each sub-controller according to the “shift register principle”. Consequently, the data transfer rate to the individual LEDs declines with the increasing number of LEDs mounted on the LED strip. This in effect makes attempting the implementation of sophisticated light effects in video speed for all practical intents and purposes quite unrealistic. Add to this the expectation that car interior lighting not only create a pleasant atmosphere (ambient light) but also perform functional tasks: for instance, to inform the driver of a self-driving car, whose eyes are not on the road or the display, that he/she must reassume control of the vehicle. Another functional task may be for the vehicle to signal to a pedestrian: “Yes – I have seen you and registered that you are about to cross the street.” This means that these LEDs will also have to meet stringent functional safety requirements whilst being diagnostic-capable – a feat well beyond the capabilities of today’s LED lighting systems.

“Digital” LED: all functions for addressing and diagnosing the three color LEDs,
including the calibration and control of all optical parameters, are directly
embedded in the RGB LED on a tiny controller chip. The LEDs are addressed
via a super-efficient protocol, practically interference-free at a
transmission rate of just 2 Mbit/s.

The LED turns “digital”

The history dates back to the summer of 2002, where an extremely fruitful “brainstorming” session with BMW generated the “APIX” idea (Automotive Pixel Link), which has now surpassed 70 million installed automotive nodes and has become the undisputed and de-facto in-vehicle communications standard. In the summer of 2017, Inova launched the third-generation APIX3, enabling a bandwidth of 12 Gbit/s. The “digital LED” clearly carries many of the genes of APIX, especially its highly effective communications protocol.

Against this background, in the fall of 2015, Inova Semiconductor joined forces with BMW and started investigating alternative LED lighting solutions. The core concept behind the new “digital LED” is to do away with the present overly complex and expensive external processing which is required for delivering consistent brightness and color parameters, by embedding these process in the RGB LED itself. This essentially “digitalizes” the LED, which can now be addressed via a “lean” protocol – just like any other digital component – by simply defining the target parameters for color and brightness.

The centerpiece of this new digital LED is a tiny 1 mm² controller chip from Inova Semiconductors, which together with three LEDs – a red, a green and a blue one – is embedded by Malaysia-based Dominant Opto Technologies in a compact housing measuring only 3 x 4 x 0.6 mm. The controller chip provides the required driver stages for addressing the LEDs, and also includes all the devices required for calibrating all three individual LEDs precisely to the color and brightness reference values during the final test of the LED module at Dominant. And all this without the need for the usual binning and barcoding of the LEDs. The characteristics are stored in the controller chip’s memory and subsequently used as corrective values when addressing the LEDs. In addition, a temperature sensor is integrated and calibrated during chip testing, which determines the current temperature of the LEDs and adjusts their brightness accordingly.

The ISELED concept (bottom) compared with conventional solutions (top)
for addressing large numbers of RGB LEDs.

This eliminates the substantial “data overhead” of all LED characteristics communicated in today’s solutions between the system controller and the LEDs. The only remaining task of the communications protocol is thus to transmit the actual light control commands, all in a differential and highly EMC friendly manner, at a speed of only 2 Mbit/s via an unshielded two-wire line woven into the vehicle’s cable harness.

Despite this seemingly low data rate, it is theoretically possible to accommodate up to 4,096 LEDs. The control commands are virtually present quasi-simultaneously at each RGB module, which forwards the data stream to the next module in the chain, with a delay of only 2 clock cycles, i.e. ~ 1 µs. It is thus possible to address all RGB LEDs in video speed and to use these as individual pixels on large LED screens or displays with 24 (3*8) bit resolution. It is, of course, no coincidence that the maximum of 4,096 LEDs per line is equivalent to the horizontal resolution of the UHD (Ultra High Definition) format. The possible applications of this innovative LED concept can thus go far beyond the vehicle; in fact, interest has already been forthcoming from many diverse areas, ranging from facade illumination and aircraft cabins through to passenger trains and cruise liners.

The ISELED Alliance.

Inova’s goal behind the launch of ISELED was more than just to create an innovative LED concept. In the fall of 2016, the ISELED Alliance was established with the founding members: Dominant Opto Technologies (LED manufacturer), NXP (system controllers), TE Connectivity (connectivity), Pforzheim University of Applied Sciences (optical measuring systems) and, of course, Inova Semiconductor to offer customers an all-inclusive system solution with compatible hardware components and corresponding software. This software includes a matching driver for the NXP controller; and ISELED now comes with the first user software designed by LucieLabs, a French IoT start-up and a member of the alliance since March. The software allows for the design of individual light scenarios and the convenient control of these via a smartphone. In August, French automotive supplier Valeo also joined the alliance. With its strong focus on in-car LED lighting, it will contribute to the Alliance important aspects from the user’s perspective.

Since August 2017, the first customers have been receiving ISELED demo kits (Picture 3). At the new “Lighting Technology” exhibition, held in Essen October 10-12, 2017, Inova Semiconductor, together with the other alliance partners, did not not merely present the ISELED concept, but more importantly demonstrate it running “in action”.