This application note provides insight into the universally deployable and flexibly mountable light source of the PointLED product family.

A fundamental overview of the LED construction as well as the optical and electrical characteristics and performance of the LED are presented. The unique and versatile mounting capability of the PointLED is examined in detail.

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.