White light and color light intelligent lighting system solutions

High-brightness LEDs have brought continuous changes to the lighting industry, adding more flexibility and intelligence to various lighting systems, including white light and color light designs. These lighting systems allow designers to dynamically control color temperature while maintaining a high color rendering index (CRI) in white light applications. In addition, these systems can produce a wide range of high-precision color spectra. Although white light and color light appear to be very different, most LED smart lighting applications are designed and produced using basic components such as mixed-signal controllers, constant current drivers, and high-brightness LEDs. Multiple LED channels are usually used in white light and color light designs, so all LED design solutions need to address issues such as device sorting, temperature effects, aging, and overall color accuracy. The use of mixed-signal controllers is indeed a powerful and effective method that can intelligently handle the above issues while ensuring the realization of high-precision white light or color light. For many designers who have switched from traditional lighting (incandescent lamps, fluorescent lamps) to LED lighting, how to make good use of mixed-signal controllers has become a huge challenge.

This article will explore the similarities and differences between designing for white light and colored light applications, the challenges of LED system design, and some powerful, off-the-shelf solutions that can help designers address these issues (some without requiring any coding).

Smart lighting

High-brightness LEDs (HB-LEDs) represent the future of lighting technology, and people have been paying more and more attention to HB-LED technology in recent years. It is not surprising that people do this, considering the significant improvement in HB-LED performance (lumens/watt) and the sharp decline in cost (lumens/dollars). In addition, the world is currently actively participating in the "Green Action". In this environment, HB-LEDs even pose a strong challenge to the currently popular, cost-effective but less eco-friendly mercury-containing fluorescent lamps. Although the high efficiency and environmental advantages of HB-LEDs are the focus of publicity, the "smart lighting" function will become an important force to promote the further development of HB-LED technology.

The application range of intelligent lighting technology is quite wide, and the only limit is our imagination. This article will focus on an important application area in intelligent lighting - dimming function. In the past, dimming mainly refers to adjusting the brightness of light or manipulating the scattering pattern of light through optical devices. In the case of HB-LED, dimming means manipulating different characteristics of light. First, designers must consider what type of light to generate: white light, colored light, or both. If it is white light, designers can adjust the color temperature and color rendering index (CRI). If it is colored light, designers can mix the colors of the entire spectrum from the same fixed LED channel group according to the number of LED color channels used in the system. By mixing colored light, white light and colored light can also be generated on the same lighting device. This flexibility does lead to increased complexity and trade-offs between each system. Fortunately, although white light systems and colored light systems look very different, their design methods are actually basically the same.

HB-LED System Design

Every smart lighting system consists of the following basic building blocks (Figure 1): HB-LEDs, some type of power topology (this article discusses only switch-mode regulators), and mixed-signal controllers. The first challenge facing designers is to choose LEDs. The main suppliers of LEDs include Lumileds, Cree, Nichia, and Osram, and their products vary in power and current ratings, scattering patterns, colors, efficiency, form factors, thermal characteristics, bins, and the number of LEDs per package. These parameters are the same for white light and colored light, but white light also needs to consider color temperature and color rendering index CRI.

Figure 1: Smart lighting system block diagram.

Advanced industrial design constraints and market demands usually help narrow the selection of most LED characteristic parameters. In most cases, designers should focus on the thermal characteristics of the LED, especially for miniaturized devices or applications with limited space and cannot use large heat sinks. Similarly, optical technology can help alleviate the problem of poor scattering patterns, while mixed-signal controllers can greatly reduce the limitations of temperature and device binning.

The first step in narrowing down the types of power topologies that are suitable for smart lighting systems is to decide whether to use discrete components or integrated circuits. Discrete implementations are lower cost and more flexible because they can be tuned to a specific system, but they take up more board space and require specialized design techniques. Power management ICs offer a compact solution that, while more expensive, takes up less board space and is easier to design.

Secondly, depending on the efficiency requirements of the lighting system, designers need to choose between linear or switching topologies. Efficiency is important in two ways. First, the more efficient the power conversion, the less power is wasted. Second, less power waste means less heat is generated in the system. Linear regulators are simpler and less expensive, but they are generally less efficient.

Switching regulators are more complex and usually more expensive due to the need for an inductor, but they are more efficient, regardless of the input and output voltages of the regulator. Linear and switching regulators can be designed using either monolithic ICs or discrete components. Depending on the supply voltage of the lighting system, designers should choose to use buck, boost, or buck-boost switching topologies accordingly. Another disadvantage of linear topologies is that they cannot boost voltage.

Third, designers must choose a mixed-signal controller for the smart lighting system. Most of the intelligence and flexibility of the HB-LED system is implemented by this device, and it can even solve some of the technical challenges brought about by HB-LED dimming. Therefore, it is important to choose a mixed-signal controller with the highest possible flexibility and as many useful peripherals as possible. Typically, an 8-bit MCU core is sufficient to provide enough processing power for most lighting applications, as well as enough RAM or flash memory.

Designers should pay special attention to the digital and analog peripherals on the MCU device. For digital peripherals, the number of dedicated hardware dimming channels, their resolution, and the ability to implement different communication interfaces are very important. The dimming channel is used to drive the buck regulator. Although software counters can also be used to achieve this function, the software dimming channel will consume valuable processing power, making it difficult for the device to perform other functions.

Smart lighting systems typically use at least 8-bit resolution to achieve high color accuracy. If the system quality requirements are extremely high, resolutions up to 16 bits can be used. However, for most applications, 8-bit resolution is sufficient to achieve the required accuracy, and designers usually use higher resolution to achieve better dimming linearity at low output levels. Some designers turn to smarter interpolation methods to solve the problem of output variations at low levels.

Common communication interfaces include SPI, UART, and I2C, but it is also important that mixed-signal controllers support important lighting interfaces such as DALI, DMX512, RF communication, and even power line communication. In terms of analog peripherals, designers should pay attention to ADCs, PGAs, and comparators. ADCs can support temperature feedback by reading temperature sensor values, and can also enable intelligent interaction between the lighting system and various physical (analog) aspects of the surrounding environment. Comparators and PGAs can simplify the implementation of power topologies.

Most MCU vendors offer some or all of these peripherals in their controllers, but designers may soon find that as system requirements change, the variety of peripherals needed will also change accordingly. It is indeed a huge challenge to design systems to take into account the future innovation of technology, especially considering that HB-LED lighting systems themselves are still a new thing. If the system requires ultra-high performance, then FPGAs will be a good value-for-money solution. Controllers with configurable peripherals and routable I/Os provide the greatest flexibility.