Project Lighting - April 2008 - (Page 20)

FeAtURe one of the main aDVantageS of the leD technology—their eXtremely long oPerating life—cauSeS an imPortant iSSue: color Drift coating on top of a blue or an ultraviolet emitter. The actual phosphorous material composition, its thickness, and its placement are currently the subject of extensive studies by all the major LED manufacturers. The results can be observed by reading the stream of announcements, as competitors keep touting newer and everhigher efficiency devices. Additionally, the quality of the light produced keeps improving. What our eyes perceive as quality is best measured as the correlated color temperature (CCT), which is the temperature of a black body that most closely matches the lamp’s perceived color. 20 | Project lighting | aPr 08 aLL thE prEtty coLors There is another way to obtain white light from LEDs—by accurately mixing the light from three color emitters: red, green and blue (RGB) in the right proportion to obtain not only a white light, but also the desired color temperature. The simple application shown in Figure 1 uses a tricolor RGB LED controlled by an 8-pin 8-bit microcontroller. Using a simple algorithm, the relative intensity of the three emitters can be controlled with about 6 bits of resolution (64 intensity levels) each, sufficient to provide control of the color output and CCT. The choice of the PIC12HV615 flash microcontroller, incorporating a shunt voltage regulator, an oscillator providing an 8-MHz clock, a reset circuit, and an A/D converter allows us to offer a simple flexible single-chip solution. The incircuit programmability of the flash device allows color calibration to be performed at manufacturing. thE Long LifE of thE LED value (compared to 1,500 hours to sudden death for an incandescent lamp). Unfortunately, during those 50,000 hours the CCT of a white LED will change—drifting toward the higher temperatures (toward blue) as the phosphors age. An RGB LED solution will suffer a similar malady, as the three-color emitters age at a different rate along different curves. Using a microcontroller, we can imagine several techniques that could be used to compensate for the aging of the components, either with predictive algorithms or closed-loop control systems. Color photosensors are available from several manufacturers and, while they add to cost, when combined with a simple PID algorithm they can solve the color drift problem once and for all. Since the change is extremely slow, there is no need for high Contents Viewpoint Selecting HBLED Drivers for Lighting Apps Simulation Technique for Optimizing White-LED Design An Analytical Approach to Cooling HBLEDs Lighting News The solution presented in Figure 1 has many applications—but one important drawbacks is low efficiency. It’s a linear solution, and power is dissipated in the ballast resistors. Additionally, more problems could arise over the life of the application. One of the main advantages of the LED technology—their extremely long operating life—causes an important issue: color drift. LEDs can operate for 50,000 hours or more and their luminous output will decline gradually to 70% of its nominal Home Page Product Training Module Online Ordering Suppliers Catalog http://digikey.com/ http://ad.doubleclick.net/clk;198717884;26055153;t http://ad.doubleclick.net/clk;198717989;26055167;e http://ad.doubleclick.net/clk;198717988;26055168;e http://ad.doubleclick.net/clk;198717986;26055170;v http://ad.doubleclick.net/clk;198717985;26055171;v

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Project Lighting - April 2008 Contents Viewpoint About Project Lighting Sponsor Simulation Technique for Optimizing White-LED Design An Analytical Approach to Cooling HBLEDs LED Lighting Needs Smarts to be Competitive Lighting News Enter to Win an iPhone Refernce Materials Lighting Quick Links

Project Lighting - April 2008

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