The LED driver output can be controlled using different methods. First, the MCU can generate an analog reference voltage through a digital-to-analog converter (DAC) or a digital potentiometer. The reference voltage can be the driver output changing from zero to maximum current. The MCU can also provide a PWM signal for modulating the driver output. The PWM signal can be used to enable/disable the driver itself, or to control the switch that disconnects the LED from the driver output. If PWM control is used, the PWM frequency selected is high enough so that no human eye can detect any sparkle.
The designer must determine the degree of control resolution required by the color control system in order to select the MCU with the appropriate peripherals. The measurement resolution of the ADC on the optical voltage sensor MCU is very important. Optical-to-frequency sensors require an MCU time base that is incremented by an external clock. Optical to digital sensors require corresponding serial communication interface peripherals.
MCUs with multiple PWM peripherals can be used to control individual LED drivers. In high resolution color control systems, PWM peripherals with 16-bit or higher control resolution are preferred. Serial communication peripherals (such as UART, SPI, 12C, LIN, USB, etc.) support input/output and display functions.
For color control systems, MCU devices such as the PIC24FJ16GA002 (see Figure 2) are a good choice. The PIC24 device is available in a 28-pin, small form factor package with a program memory range of 16 to 64 KB and provides a serial communication interface, 10-bit ADC, and five PWM channels in a single device. The 16-bit MCU core can easily handle arithmetic operations related to sensor calibration and color control.
The sensor data output must be calibrated based on the reference voltage to provide consistent results. The calibration process uses a colorimeter to mathematically correlate the output of the different colored LEDs with the spectral response and the sensitivity of the light sensor in a standard colorimetric coordinate system. The calibration process results in a matrix of coefficients that must be stored with the lighting system in non-volatile memory and used to determine the difference between the correlation and the desired output in each control of the control system.
After the calibration is completed, the MCU can compare the sensor data with the ideal CIE (International Commission of Illumination) chromaticity diagram coordinates and adjust the output channel until the desired CCT is achieved. The PID control algorithm for each output channel uses the calibration value to adjust the sensor data, determine the difference from the target set point, and then adjust the output channel. In order to reduce the error, the PID will continue to run until the output CCT matches the setpoint CCT. The PID coefficients can be fine tuned to maximize the system response, but the speed at which the PID algorithm converges to the target CCT is also a function of the MCU's efficiency in the arithmetic operations.
Some color control systems may require faster processing and response speeds than other systems. For example, the general lighting system requirements are lower than the local dimming systems of HDTV panels.
Systems with adjustable light sources or high CR have a range of user control requirements. Medical devices with a graphical LED display may have adjustable LED backlighting (which requires the MCU to communicate with the LCD via SPI), and a touch screen interface for adjusting the CCT and brightness. General-purpose lighting for commercial display devices may require control via a center panel or computer to automatically adjust brightness, CCT, and on/off depending on the time of day.
Communication between these devices can be implemented using hardwired serial bus protocols (such as DALI or DMX512), while other devices may require custom interfaces implemented over USB or Ethernet. In a completed building, installing a hard-wired infrastructure may not be feasible and needs to be controlled through wireless communications and protocols such as ZigBee. For such lighting applications, MCUs with flexible external designs are ideal for implementing communications and user interfaces.
Light source technologies such as candles, kerosene lamps, and incandescent lamps replaced their previous technologies and improved people’s quality of life. The use of LEDs as a light source is just around the corner and it is expected that it will enrich our lives better than all other light source technologies. LEDs have the advantages of high energy efficiency, small size, portability, durability and long life.
Multi-color LEDs controlled by a small MCU can adjust the light output to provide comfortable lighting suitable for the lighting space. The MCU can intelligently control the driver circuit (maximizing energy efficiency), monitor some conditions, and maximize energy efficiency and average life. The MCU color control LED lighting system allows people to see the world with different eye “lights”.
