[Design View / Design Solution]
MCUs Provide Power Control And Intelligence For Lighting Applications
Using an MCU or DSC can foster efficient lighting in any setting, as well as decrease complexity and increase flexibility.
Whatever Color You Want The world’s longest-lasting light bulb, now 105 years old, is installed in a firehouse in Livermore, Calif. Unfortunately, most modern-day light bulbs don’t last as long as this one. When lifetime, efficacy, and durability are important, power LEDs have the most to offer.
Present power LED technology provides efficacy values that rival fluorescent technology. In addition, the power-LED industry expects to double present efficacy values in the next few years. The lifetime of LEDs can exceed 50,000 hours, which is a huge benefit in commercial applications where the cost of changing the light bulb isn’t free.
Although LEDs have tremendous efficiency and lifetime advantages, the CRI of white LEDs can be very low. Many white LEDs have a bluish color; white light is produced by covering a blue or infrared emitter with yellow phosphorus to shift the light to the desired wavelength. The LED’s final color spectrum is limited by the emitter’s initial wavelength and by how the phosphorous spreads the light energy across the visible spectrum.
One emerging application for power LEDs is LCD video-display backlighting. Many current LCD-panel designs use fluorescent technology to provide the backlighting. LED technology improves LCD image quality by using separate red, green, and blue (RGB) emitters. The use of separate emitters allows for much greater control of the produced color spectrum over white LED technology. Within a range defined by the three component colors, any color can be generated.
RGB LEDs allow the LCD panel to produce a broader range of colors than a typical fluorescent design. In addition, the LEDs can be modulated on and off using the video scan information. LEDs have instant on and off times when compared with other lighting technologies. The scan modulation allows the LCD panel to produce a sharper image.
Figure 6 shows an RGB LED-control circuit that could be used for an LCD panel application or even a general-illumination application. A source of constant current must drive the LEDs. When multiple LEDs are employed, they’re typically connected in series so each LED receives the same amount of current. The choice of current-drive level will be a tradeoff between the amount of light produced, efficacy, and possibly thermal limitations.
Three MCP1630 switching devices are used to implement the constant-current drivers. These devices are MCU peripherals that contain the analog components necessary for an SMPS control loop. The MCU provides a clock signal to set the switching frequency and limit the maximum duty cycle. A buck or boost topology could be implemented in this circuit, depending on the available input voltage and the forward voltage across the string of LEDs.
The PIC18F1330 MCU was selected because it has three 14-bit PWM channels. These PWM channels are used to modulate the outputs of the three constant-current drivers and set the light intensity of each color. The high PWM resolution is required so there’s accurate color control over a wide range of brightness levels.
The wavelength and light intensity of LEDs can change with variations in manufacturing process, age, and drive-current level. In most backlighting applications, active color control is needed to ensure a consistent color, and brightness is produced. An RGB sensor is used to detect each component of the light output. The MCU or DSC calibrates the output of the sensor, determines the amount of color error, and calculates three PID routines that set the R, G, and B component levels.
Efficient lighting applications require power-circuit control and intelligence. You can integrate both of these functions with a MCU or DSC, decreasing circuit complexity and increasing flexibility.
Reprints
