Using a small MCU to achieve color control of LED lighting

LEDs are now ready for general lighting. LEDs offer many advantages in general lighting systems, such as longer life and higher efficiency. However, LED technology faces several challenges. One of these challenges is producing high quality white light. White LEDs consist of a blue LED and a phosphor that shifts the light output to other wavelengths of the spectrum. Many white LEDs do not produce a high color rendering index (CRI), which is a measure of the ability of a light source to reproduce colors faithfully.

Higher quality white light systems can be achieved by mixing two or more colors of LED light. In these multicolor systems, the light output of each color source drifts over time and temperature. Light sensors and small microcontrollers (MCUs) can be used to maintain a specific color and correlated color temperature (CCT). In this article, we will take a closer look at sensors, the required MCU resources, and software.

There are many small, affordable light sensors available on the market today that can provide information to the MCU for processing. Typically, the sensor has some optional color filters to measure red, green, blue, or white light (no filter). The light sensor output interface can be connected to the MCU in a variety of ways. Light-to-voltage sensors interface with an analog-to-digital converter (ADC) via their output voltage. Light-to-frequency sensors provide a variable frequency output that is proportional to the amount of light. The pulse outputs of these sensors can be accumulated in an MCU timer to determine the light level. Light-to-digital sensors typically have a serial digital interface, such as I2C. Each type of sensor interface has unique benefits and requires different MCU resources. The system block diagram in Figure 1 shows a variety of MCU peripherals that are useful in color-tunable LED lighting designs.

In a complete closed-loop color control system, the MCU must read the color content from the light sensor, calibrate the light sensor output, and adjust the output of each LED driver to achieve the desired color. The LEDs require a constant current driver to maintain consistent light output. This can be achieved using a variety of driver technologies, including linear and switch-mode solutions. The final choice depends on factors such as efficiency requirements, input voltage range, and the number of LEDs used.

The 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 vary the driver output between zero and 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 a switch that disconnects the LED from the driver output. If PWM control is used, the PWM frequency should be selected to be high enough so that the human eye cannot detect any flicker.

Designers must determine what level of control resolution is required for the color control system in order to select an MCU with the appropriate peripherals. For light-to-voltage sensors, the measurement resolution of the ADC on the MCU is important. Light-to-frequency sensors require an MCU time base that is incremented by an external clock. Light-to-digital sensors require corresponding serial communication interface peripherals.

An MCU 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, I2C, LIN, and USB support input/output control and display functions.

MCU devices such as the PIC24FJ16GA002 (see Figure 2) are good choices for color control systems. PIC24 devices have a small 28-pin package, a program memory range of 16 to 64 KB, and provide a serial communication interface, a 10-bit ADC, and five PWM channels in a single device. The 16-bit MCU core can easily handle the arithmetic operations related to sensor calibration and color control.

The sensor data output must be calibrated to a reference voltage to provide consistent results. The calibration process uses a colorimeter to mathematically relate the output of different color LEDs to the spectral response and sensitivity of the light sensor in a standard chromaticity coordinate system. The calibration process generates a coefficient matrix that must be stored in nonvolatile memory with the lighting system and used to determine the difference between the correlation and the desired output at each control of the control system.

After calibration, the MCU can compare the sensor data with the ideal CIE (International Commission on Illumination) chromaticity diagram coordinates and adjust the output channel until the ideal CCT is achieved. The PID control algorithm for each output channel adjusts the sensor data using the calibration value, finds the difference from the target set point, and then adjusts the output channel. To minimize the error, the PID will continue to run until the output CCT matches the set point CCT. The PID coefficients can be fine-tuned to optimize the system response, but how quickly the PID algorithm converges to the target CCT is also a function of how efficiently the MCU can process the arithmetic operations. Some color control systems may require faster processing speed and response than others. For example, general lighting systems have lower requirements than local dimming systems for HDTV panels.

Systems with dimmable light sources or high CRI have a range of user control requirements. Medical equipment with graphic LCD displays may have dimmable LED backlights (which require the MCU to communicate with the LCD via SPI), as well as a touchscreen interface for adjusting CCT and brightness. General lighting for commercial display equipment may require control from a central panel or computer to automatically adjust brightness, CCT, and on/off according to the time of day. Communication between these devices can be achieved using hardwired serial bus protocols such as DALI or DMX512, while some other devices may require the use of custom interfaces implemented over USB or Ethernet. In completed buildings, installing a hardwired infrastructure may not be feasible, and control through wireless communication and protocols such as ZigBee is required. For such lighting applications, MCUs with flexible peripherals are ideal for implementing communications and user interfaces.

Light source technologies such as candles, kerosene lamps, and incandescent lamps have replaced their predecessors and improved people's quality of life. The adoption of LEDs as light sources is just around the corner and is expected to enrich our lives better than all other light source technologies. LEDs have advantages such as high energy efficiency, small size, portability, durability, and long life. Multi-color LEDs controlled by small MCUs can adjust light output to provide comfortable lighting suitable for the illuminated space. The MCU can intelligently control the driver circuit (maximize energy efficiency), monitor some conditions, and maximize energy efficiency and average life. MCU color-controlled LED lighting systems will enable people to see the world in a different light.