Advanced ICs key to automobile LED front lights

BY CHRISTIAM GASPARINI, Applications Engineer,
and PAUL DECLOEDT, Product Marketing Engineer,
ON Semiconductor,
www.onsemi.com

In recent years, the use of LEDs in automotive front lighting has become a booming business, deploying faster than expected.  OEMs are accelerating their efforts to offer LED front lighting solutions in multiple grades.  The challenge now is for suppliers to deliver that increased demand from OEMs while at the same time shortening the development cycles for all of the different front lighting designs that OEMs would like to offer in their vehicles.  Suppliers who are not prepared to meet the increase pace will miss out on significant opportunities.  

This article will discuss an approach that will support these faster design cycle requirements.  

It is now universally recognized that car manufacturers can realize numerous benefits through migration to solid state lighting. These include energy savings, improved reliability levels (leading to longer effective life span), greater operational flexibility and reduced form factors in what can often be space constrained settings. So far though, the vast majority of implementations have been restricted to side lights, brake lights, back lights and interior lighting.  The front lighting aspect is still normally being taken care of by traditional methods, with the exception of daytime running lights (DRL).  The emergence of innovative LED architectures is starting to change all this however, allowing the main objections (such as ensuring compatibility with existing system designs and mitigating the heavy initial monetary investment associated with it) to be tackled head on.  

More elaborate diagnostics are likely to be another important motivation. Incandescent bulbs are dependent on utilization of a single line, through which the current is applied. It is also through this line that any diagnostic mechanisms that are put in place must operate. This has obvious limitations, as very little information can be derived concerning the lamp’s operation – basically just whether it is working or not. Solid state lighting implementations, provided the right semiconductor devices are used to support them, can offer access to a more comprehensive array of data.  

There are economic aspects to be reflected upon too. Though a common perception exists that the upfront investment needed to move to solid state front lighting is very large, this does not in fact represent as big an obstacle as would at first be assumed. It can, to a reasonably high degree, be counterbalanced by the cost advantages offered by basing numerous lighting system designs on one core design. By taking this ‘platform’ approach, engineers can avoid having to start a design completely from scratch for each car model, thereby saving time and engineering overhead. The core design will only have basic functionality, but have provision for support of various LED string configurations, so that its complexity can then be ramped up first to fit the requirements of mid-range and still further to furnish luxury models with more extensive sets of features.

The ability to configure the IC technology at the heart of the system is critical. Through this it is also possible to adjust emitter outputs so that binning is not required – thereby further reducing the costs involved. There is the prospect that deterioration in the LED’s light intensity outputs over the lifespan can be factored out, so the system’s operational effectiveness is prolonged.   

It often happens that during development, the LED configuration needs to be changed to match with the optical and/or style designs from the lighting engineers and car stylists. Configurable LED current and voltage through in-line free-programmable parameters provide the flexibility to develop the electronics in parallel or even before the design of the LED headlight itself.   

Conversely engineers can prepare for future improvements in LED performance. For example, this could mean that though the LEDs in a lighting design created today might need to be run at a certain current level, the next generation LEDs that are incorporated into the same design five years from now may be able to run at a substantially lower current (or the system might be able to cope with fewer LEDs to be employed). A configurable semiconductor technology as offered by certain forward-thinking IC manufacturers will enable changes of this kind to be carried out without any need for redesign work. Another advantage of configurability is a fine-tuned regulation of LED output, which is matched to the photo sensitivity of the human eye. This means that standardization can be achieved and the logarithmic element compensated so that increments in the current then correspond to increments in the emitter output.   

Resources will need to be conserved not only in hardware. Engineers looking to implement front light systems based on solid state technology must have access to semiconductor solutions where only minimal development of software is mandated. This will mean that ICs need to be introduced which have a great deal of functionality directly embedded into them. An additional upshot of this is that just a small quantity of external components should be required, thus reducing overall bill-of-material (BoM) costs to an acceptable point, as well as minimizing utilization of board real estate.   

Taking all of the different technical, financial and logistical issues that have been outlined here into account, ON Semiconductor has developed is a single-chip solution targeted specifically at automotive front lighting applications. Based on a boost-buck topology with integrated diagnostics and control, the NCV78763 is suitable for all manner of different front light tasks – such as high beam, low beam, daytime running lights, turn indicators, fog lights, etc. It incorporates an extremely efficient smart power ballast plus a dual LED driver.

Fig. 1: Functional Block Diagram for ON Semiconductor’s NCV78763

The device delivers a high degree of overall efficiency – the complete input to output figure is typically higher than 90%. Thanks to its pair of internal independent buck switch channel outputs, the device effectively provides a complete IC solution for driving two strings of high current LEDs up to 60 V from a standard 12-V battery, with the assistance of only the minimum of external components. If more than two LED channels are required on one module, then several NCV78763 ICs can be combined together. This means that design is easily scaled up to the number of LED channels necessary.

Fig. 2: Typical NCV78763 application diagram

The boost-buck topology makes it much easier to find a stability setting across a wide range of different load power levels. The booster element provides the required voltage source for the LED string voltages out of the available battery voltage. Its control is slow in nature, offering better stability, while in contrast the buck element reacts much faster so that LED current regulation is enhanced. For each individual LED channel, the output current and voltage can be adjusted to fit particular application requirements. On-chip diagnostics designed specifically for automotive front lighting implementations are provided.   

Fully programmable via its SPI interface, it gives engineering teams the means to exploit a single hardware design that can then be used to support multiple system configurations so that a flexible platform design approach can be realized, thereby curbing the engineering effort involved and the time taken to complete the project. It also supports exciting new dynamic lighting technologies such as ‘Pixel Light’ and ‘Matrix Beam’.  

Though it has been a long time coming circumstances are finally right for solid state lighting to become ubiquitous within the automobile industry. The advent of highly sophisticated ICs that are optimized for automotive implementations, and offer programmability plus diagnostic capabilities, now present engineers with the means to make widespread proliferation of LED technology in vehicle front lights a reality. These ICs will satisfy the need for greater breadth of functionality, while calling for very few external components and minimizing the engineering resources that need to be allocated.

About the authors:

Christiam Gasparini is a System Application Engineer at ON Semiconductor. He joined ON Semiconductor in 2008 as Applications Engineer and member of APG, Body Chassis & Safety BU, has been in front line through the business development and direct customer project support in the Automotive LED Lighting and Motor Control sectors. With a background of 11 years focus in electronic design for hardware, firmware & software, performing full concept-design-to-production processes for modules in the Medical sector, Industrial Automation, Motor Control, Home appliance and Transports signalling electronics, towards either innovative solutions with high added value or low cost / high volumes applications. Christiam has a Master of Science (MSC) degree in Electronic Engineering, with specialization in Automatic Control Systems & Modeling, Sensors and Actuators.
 
Paul Decloedt is a product marketing engineer for the Automotive Products division of ON Semiconductor with more than 14 years experience in the ASIC business.  In addition to his role as product marketing engineer he has worked as a technical marketing engineer and a business development manager, acquiring expertise for the challenging automotive environment.  Paul earned a MSc degree in electronics and microelectronics in 1989 at the Katholieke Industriele Hogeschool in Ostend, Belgium, specializing in telecommunications, networking and microcontrollers.  After his studies, he received an in-depth marketing education at MCE Europe.