Designing a closed-loop control system for RGB colour mixing

by Kai Klimkiewicz and Günther Herrmann, Product Marketing Engineers, FLS Division, Future Electronics (Germany)

READ THIS TO FIND OUT ABOUT: The technical requirements of aesthetically pleasing LED lighting. The relative merits of thermal- and optical-feedback solutions for closed-loop RGB colour mixing.

The latest generation of high-brightness LEDs is being widely used in place of traditional incandescent lamps. There is one major challenge, however, that must be overcome to design LED arrays: achieving specific and uniform colour-mixing from different LEDs. Kai Klimkiewicz and Günther Herrmann, Product Marketing Engineers, FLS Division, Future Electronics (Germany) explain.

The output from each LED device is subject to production variations that are an inherent part of the manufacturing process. LED manufacturers group batches of LEDs into groups known as bins. Each bin has its own colour point, which is a set of co-ordinates on the internationally recognised CIE colour chart precisely defining the shade.

Bin differences are not the only variable which designers must manage. An LED’s light intensity and wavelength also vary over time and are affected by operating temperature, which is in turn affected by the ambient temperature and by the cooling scheme implemented.

A practical example of this would be an illuminated sign carrying a company’s logo. Such signs must be able to accurately and consistently render colours such as Coca-Cola’s red or IBM’s blue through the hottest summer and the coldest winter, over a period of years.

The designer of the LED control circuit must compensate for temperature change, ageing and bin variation in order to produce an effective product. Two products have recently been introduced that provide a simple way to compensate for these variables, and thus dramatically reduce design time.

These two solutions both implement a closed-loop system, but work with two different types of feedback: optical and thermal.

 

 

Optical feedback

Avago Technologies offers a highly-integrated optical-feedback closed-loop controller. The mixed-signal ADJD-J883 IC features integrated RGB photosensors, an ADC, a colour-data processor and a 12-bit PWM generator. It works by sensing the LED array’s light output, comparing it with a given target colour, and adjusting the array until these values are matched (see Figure 1).

The colour-data processor requires simple inputs: CIE coordinates for the target colour and a brightness value. These inputs can be provided from the main system controller or from a dedicated lowcost 8-bit microcontroller via a 100kHz serial interface.

This solution can easily be used for stand-alone arrays of three RGB LEDs, such as LCD backlighting applications; but it can also be adapted to tie together strings of arrays in a master-slave configuration. This implementation is useful in, for instance, architectural lighting.

By using real-time optical feedback, the ADJD-J883 can ensure accurate light output in terms of colour and flux, regardless of temperature, age or bin.

The dependence on optical feedback, however, produces a challenge as the sensor must be exposed to the complete RGB array to work effectively, and must sense only the output from the LED array, not sunlight or other ambient light sources.

In some applications, the ADJD-J883 can be placed at the illuminated target itself. In small, enclosed applications (such as LCD backlighting) this can be appropriate. Otherwise, the designer might have to incorporate a means to shield the sensor from external sources of light.

 

Thermal feedback

The thermal-feedback LED controller offered by Cypress Semiconductor avoids the need for optical shielding. The programmable mixed-signal EZ-Color controller integrates a look-up table holding reference data about the output from LEDs.

 

 

By cross-referencing data on the LED’s junction temperature, age and bin against the desired light colour and flux, the EZ-Color controller can drive the correct current to 16 channels of LEDs. In particular, it accurately compensates for the reactions of different LEDs to temperature changes – for instance, blue, green and white are more stable in this regard than red and amber LEDs. In this way, EZ-Color mixes multiple RGB outputs to produce a consistent, accurate light output.

A further benefit of the EZ-Color device is that it provides integrated temperature control. Excessive heat dramatically shortens the life of an LED (see Figure 2). LED manufacturers carefully specify the correlation between junction temperature, drive current and effective lifetime. By using EZ-Color, product designers can choose the appropriate trade-offs between heatsink size, cooling mechanisms, drive current and product lifetime.

 

Relative advantages of optical and thermal feedback

The optical feedback solution implemented by the Avago device provides highly-accurate light output under all environmental conditions. Correctly mounted on the board, its accuracy is such that the human eye cannot detect the difference between a colour reference point and the actual output from a system controlled by the IC.

It also enables colour duplication, where one device controls a reference colour and slave devices mimic the output. This is a simple way to achieve completely consistent light output across multiple light sources, which is ideal for coloured lighting of large structures such as buildings or bridges.

This solution can complicate board layout, however, as it requires the positioning of the 5mm² Avago device in a location that is exposed to the light output of the complete LED array, but that is not directly exposed to other strong light sources. On the other hand, this solution does provide a very accurate means of managing LED ageing.

In addition, the output from the Avago device is in the form of a PWM signal, so the circuit designer must specify LED drivers that can interface to a PWM input.

Although the EZ-Color device’s output is derived from manufacturers’ data combined with thermal feedback, rather than from real-time optical feedback, in practice it accurately produces desired colours across the colour spectrum.

It is also easy to design in for two reasons. First, the device can be placed anywhere on the board, with the only constraint being the relatively easy problem of positioning of the temperature sensor. Second, the PSoC Express design software for EZ-Color (available for download free from www.cypress.com) provides an easy-to-follow design flow, with drop-down menus that allow the user to specify from a range of temperature sensors, LED devices and bins, and to specify a colour output using an intuitive graphical user interface. The designer then specifies a colour output, either by inputting CIE colour co-ordinates or by clicking on the CIE 1931 chart displayed to the user by the software.

However, the Avago device also provides a board and control software which allow for calibration of the board and the sensor, with data being stored in an EEPROM for future use.

 

Conclusion

While the light output of incandescent lamps is relatively stable over time and temperature, the output of high-power LEDs is affected by complex and interrelated variables.

The two solutions from Avago and Cypress presented above provide a highly effective means of eliminating the effect of these variables, ensuring an accurate colour output with a minimum of design input.

 

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