One requirement especially in automotive applications is the diagnosis of failures in functions and systems. Therefore light functions realized with LEDs like break light, daytime running light, low and high beam may require a diagnostics function. This application note describes some items which have to be taken into account, when a diagnostic function for a LED string or a multi LED module has to be realized.

In order to guarantee constant brightness for LED illumination systems with long product cycle times, the availability of LEDs with constant brightness within the product cycle is often still required or expected. This application note is intended to show that in spite of the continuous further development of LED technology, the issue can be avoided or completely eliminated with a simple solution.

The Golden DRAGON LED is OSRAM Opto Semiconductors’ high performance LED requiring special considerations in thermal management and electrical implementation.

This application note is intended to help the design engineer with the special electrical considerations of the Golden DRAGON LED. With a higher current there is higher power, and therefore more heat to dissipate. The Golden DRAGON LED package is optimized for removing this heat efficiently. With an integrated heat slug (also known as a heat spreader) the thermal performance is far superior to standard LEDs.

This application note describes the procedure for adjusting the brightness of light emitting diodes (LEDs) in applications by means of resistors. For better repeatability, the calculation of the required resistance values is shown by means of an example.

In recent years, Light Emitting Diodes (LEDs) have become a viable alternative to conventional light sources. The overriding advantages long life, high efficiency, small size and short reaction time have lead to the displacement, in ever increasing numbers, of incandescent bulbs. One of the markets where this change has become most evident is Automotive, where LEDs are used now not only for backlighting dashboards and switches, but also for exterior illumination in Center High Mounted Stop Lights (CHMSL), Rear Combination Lamps (RCL), turn signals and puddle lighting.

Despite the long life and low failure rates of LEDs, cars can be found, on occasion, with failed LEDs in their CHMSL. Most often this is due to a flawed circuit design wherein the LEDs were allowed to be overdriven. It is with that supposition in mind that this application note is written: to identify, characterize and comment on LED behavior and failure modes in serial and matrix circuits.

LEDs are currently used in many application areas. In the automobile sector, nearly all dashboards utilize LEDs for backlighting. For new application areas which require a higher light output, even more efficient and powerful LEDs are needed. Brake lights, turn signals and fog lights, for example, require powerful LEDs in order to fulfil statutory regulations.

Every application requires the selection of an appropriate LED. The Advanced Power TOPLED (APT) serves to complement the Power TOPLED and Golden DRAGON power components. With a maximum power consumption of approximately 0.5W and high optical efficiency, this package fills the light output gap between the Power TOPLED and the Golden DRAGON.

This application note describes the electrical characteristics of the Advanced Power TOPLED along with various electrical simulations.

Nowadays, applications increasingly make use of LEDs as a replacement for traditional light bulbs. For example, LEDs are frequently used in the design of automobile tail lights, signal lights, traffic signals, variable message signs, ...

LEDs provide several advantages over traditional light bulbs, such as smaller size and longer life. In many applications, the LEDs must be driven with intelligent control circuitry. According to the task at hand, this control circuitry must be able to fulfill various functions and tasks. In the following pages, solutions are provided for various application areas. These solutions are principal suggestions, not a concept ready for series production.

One possible task for control circuitry is regulation of intensity, in case the LED brightness must be set to various levels. A solution is described in the section “Dimming“. In addition, the specified brightness should be maintained at a constant level. Fluctuations in the supply voltage, for example, could lead to significant variations in current. In this case, one must insure that the current through theLEDs and thus the brightness is maintained at a constant level. This problem is covered in more detail in the section “CurrentRegulation“.

Another task for control circuitry is failure recognition. Modules consist of individual LEDs which can be tested for total failure. Additional information can be found in thesection “Failure Recognition“. A particular characteristic of LEDs is their strong temperature dependency. Since LED brightness is strongly dependent on temperature, the driver circuitry can carry out temperature compensation. Two possible approaches are described in thesection “Temperature Compensation“.

Furthermore, it may be necessary to adapt the driver for LEDs in different brightness groups by means of hardware selection. This is described in the section “Adjusting for Different Brightness Groups“. In the following applications, a PICmicrocontroller is used as a controlling unit.

The first true ancestors to the Indium Gallium Nitride (InGaN) LED evolved last decade. These took the form of blue LEDs utilizing Silicon Carbide (SiC) as the active, light-emitting material. These early LEDs were characterized by very low light output, less than 2cd/m². The next generation of blue LEDs relied upon SiC as a base layer only and employed Gallium Nitride (GaN), grown directly on the SiC substrate, as the active, light-emitting epitaxial layer. This process initially increased light output by a factor of eight. The final iteration saw the introduction of Indium (In) to the epitaxial layer to form InGaN. This development further boosted light output by a factor of five - a full 1300% increase in intensity over the first SiC LEDs. Today, through advances in process, packaging and thermal transfer technologies, light output continues to evolve.

Besides increasing the intensity of blue, and by extension, white LEDs (since all white LEDs use a blue chip in conjunction with a light converter, or phosphor), the InGaN process has replicated two new colors: verde and true green. These unique colors, alongside InGaN’s high intensity and inherent reliability, has greatly increased their proliferation into applications once reserved solely for incandescent lighting: traffic signals, realcolor displays, message boards, moving signs, dashboard backlighting, battery flashlights and toys.

While the InGaN process produces the brightest light output across blue, verde, true green and white, it is important to understand that the wavelength of the light emitted is strongly dependent upon the forward current driven through the device, and that in order to avoid shifts in color, careful consideration must be paid to dimming strategies. This application note, then, will examine methods for dimming InGaN LEDs with little or no effect on wavelength.

In the design of a driving circuit for LEDs, the dimming behaviour is an important topic to fulfil the end customer requirements. The intend of this application note is to describe the behaviour of LEDs in respect to brightness by varying the current and to suggest solutions for avoiding negative influence for the application.

Following items point out some topics where it is necessary to adjust the brightness of LEDs, if the customer specification is tight:

- LED brightness at the grouping current doesn’t fit to the specified brightness in the application, hence the LED grouping current doesn’t correlate to the application current,

- different brightness groups of the used LEDs,

- dimming of brightness in the application,

- different switches or displays to backlight.

Some years ago, the color range of Light Emitting Diodes (LEDs) on the market was limited to the red to green spectrum. Then, blue LEDs were developed and introduced into the market. These blue devices made it possible to build so called “single-chip white“ LEDs, using a yellow converter material in combination with a blue die. Most of the blue and white LEDs use Indium Gallium Nitrite (InGaN) as an epitaxial layer. The wavelength (chromaticity coordinates) of the generated light of these InGaN-based LEDs shows a strong dependency on the driving current. This special property of InGaN based LEDs must be considered well in advance for new application solutions. This application Note is intended to enable the reader to avoid some common design mistakes when using InGaN-LEDs.