Electronic Design

Designing Multichannel HBLED Systems

High-brightness LEDs (HBLEDs) are increasingly becoming the light source of choice in both general and specialty lighting applications. Advances in LED technology have led to higher lumens per watt. Improvements are also being made in package size, color options, color rendering index (CRI) ratings, binning, and temperature stability. LEDs bring flexibility, efficiency, and intelligence to any lighting application.

A typical application of such a system is general household lighting, where users can create multiple shades of white or colored light with just one fixture. Such flexible lighting fixtures enrich the end-user experience by providing control over light that consumers have never experienced before.

Two very important aspects enable the flexibility of these features: the possible color gamut of the fixture and the number of unique mixed colors the fixture can create, which is known as color resolution. For some applications, users desire as large a potential color gamut as possible. This enables the fixture to create more vibrant colors that don’t appear to be washed out.

Color gamut and color resolution can vary for any given system (Fig. 1). The black triangle and “+” symbols represent a lighting fixture with a limited gamut and smaller set of unique mixable colors. The white shape and “+” symbols represent a fixture with a larger color gamut and a higher color resolution. Fixtures with a larger color gamut and higher color resolution are more desirable.

One other key aspect of a white light fixture is its CRI rating, which grades how well colors and objects look when they’re illuminated with the fixture. CRI is better when a light fixture emanates more unique wavelengths of light for a given mixed color.

Each of the three aspects—gamut, color resolution, and CRI— can be optimized by a single design factor: the number of uniquely colored LED channels in the system. Larger numbers of LED color channels increase the possible color resolution exponentially with each channel added. They also increase the color gamut, since the different LED colors will cover a greater area of the color space. Finally, using more wavelengths of light to mix colors boosts the CRI rating of the fixture.

Four-channel color combinations often work well for LED color mixing fixtures. Two common combinations are RGBA and RGBW. (“A” is for amber, and “W” is for white.) The RGBA combination provides a larger gamut than RGB or RGBW, and it generally creates light with a good CRI. RGBW doesn’t have a larger color gamut, but it has a good CRI and more of the primary mixed color that’s desired—white. The number of channels needn’t stop at four. LED lighting fixtures with five, six, or even seven channels are sometimes necessary for very high-performance or high-end systems.

While having more independent LED channels presents clear advantages, there are drawbacks, too. These include the obvious need for more hardware (LEDs and drivers) and an embedded microcontroller with more complex firmware. In such a system, the controller continuously calculates the appropriate dimming levels needed for each LED color channel. The output for each channel must be finely adjusted to mix to the proper color.

In a multichannel system, the processor in the embedded microcontroller receives the request for a particular mixed color from a data network or some other interface (Fig. 2). The processor must then calculate the dimming values needed for each LED channel to create the mixed color. This calculation process should only be as complex as needed so it doesn’t burden the processor.

The system requirements will determine the optimal calculation process. One question needs to be answered—how many unique colors must the fixture be able to create? Put another way, can any color in the gamut be requested, or only a small subset of colors? For instance, a lighting fixture that just creates various shades of white may only need to create 100 or so unique colors ranging from warm white to cool white.

In this case, it’s advantageous to design the microcontroller’s firmware to calculate the dimming values with a lookup table (LUT). For each unique mixed color, a set of dimming values is predefined and stored in flash memory. When the processor receives a mixed color input, it looks up the appropriate values in the LUT. This fast and simple method allows very complex calculations to be preprocessed so the microcontroller doesn’t have to make timeconsuming calculations on the fly.

A LUT method is best if it’s feasible. It becomes unfeasible when many unpredictable, unique mixed colors are required. For example, a color mixing system that has four LED channels with an 8-bit dimming resolution for each can create more than 4 billion unique colors. For each of these unique colors, the LUT would need to store one 8-bit dimming value for every dimming channel, requiring more than 16 Gbytes of memory. In general, a fixture with firmware using a LUT for color mixing can only support as many unique mixed colors as the memory size allows.

With color-mixing algorithms, the firmware can generate a large number of unique color outputs without using a gigantic LUT. An algorithm can take any color request input and calculate dimming values that will create that mixed color. A general algorithm doesn’t use an excessive amount of memory, but it’s more complex to develop and takes a longer amount of CPU processing time to generate dimming value outputs.

For an embedded color-mixing system, choosing the right microcontroller is essential. It’s best to use more LED color channels in a colormixing system when light quality or flexibility is important. When designing the firmware for such a system, choose a LUT method or a dimming value calculation algorithm appropriate to the requirements of the fixture.

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