Microcontroller-based LED drivers inspire new designs
The lighting market trend today is defined by LED technology which offers significant advantages over traditional light sources. LEDs are known for their low power consumption and long life cycle. Technological advances in the past decade have continually improved the luminous efficacy (Lumen/Watt) of LEDs and their productions costs are getting lower. These technical features, catalyzed by economic, ecologic, and political constraints for the industry, are boosting the spread of LEDs in the lighting industry at a rapid rate.
For Renesas Electronics, the lighting segment is one of the key markets with an increasing focus worldwide. Renesas’ commitment to offering powerful and state of the art lighting products (intelligent LED drivers) to the industry substantiate this, enabling it to design and produce lighting end-products to meet the present day economic and ecologic demands.
Renesas offers a wide range of dedicated, advanced semiconductor components for lighting applications. The spectrum ranges from its latest RL78/I1A MCU ASSP family through to discrete LED driver ICs like the R2A20134 and the R2A20135, and include a wide variety of MOSFETs and optocouplers supplemented by lighting specialized communication solutions. These products cover all aspects of the design of LED lighting, replacing incandescent bulbs and fluorescent tubes not only in commercial indoor and outdoor event lighting but also for architectural, street, and domestic lighting applications.
This article outlines briefly RL78/I1A MCU ASSP features specifically for lighting applications, its development tools, and the technology outlook. The discrete LED driver ICs R2A20134, R2A20135, and other components are not discussed further here.
RL78/I1A MCU ASSP for lighting applications
RL78/I1A is an ASSP member of Renesas’ latest RL78 series of microcontrollers which combines the best features of its previous industry standard microcontroller cores, the 78K0 and the R8C, and offers higher computing performance.
This new 16-bit microcontroller generation boasts the lowest power consumption in its class in the world. It offers two different on-chip flash ROM configurations, and – depending upon the ambient temperature – can optionally operate at 24 or 32MHz. The MCU is available in 20, 30, 32, and 38 pin plastic (TSSOP, SSOP, and VQFN) package variants.
The RL78/I1A integrates all the necessary functions and resources required for a typical LED lighting application on board and it thus represents a single chip low power, low cost solution without sacrificing efficiency.
Lighting application specific functions like DALI (Digital Addressable Lighting Interface), DMX512 (Digital MultipleX), RDM (Remote Device Management), ZigBee, and Power Line Communications (PLC) are integrated on-chip. The on-chip extended DALI transceivers are augmented by other major standard interfaces like UART, SPI, and I2C.
The integrated on-chip functionality and intelligence reduces the need for additional external components (BoM). The reduced BoM in turn simplifies the parts procurement, system tests, production process, and increases the overall system reliability; it is thus a compelling argument for many LED drivers and lighting ballast designs where space is at a premium.
Figure 1: Block diagram of the RL78/I1A MCU
The RL78/I1A offers multi-channel LED control. The designer has the option of selecting 10-bit analog or 16-bit PWM function for dimming each channel individually. In addition, the integrated high-precision (±1%) on-chip oscillator facilitates PWM timer frequencies of up to 64 MHz required for finer light dimming function and improved “color-temperature” adjustments. Higher PWM frequency in turn enables the use of small size inductors (space saving) and provides more bandwidth for constant current control required for proper LED operation.
Other RL78/I1A enables the PFC (Power Factor Correction) control function supporting the power stage control, and the different operation modes including CRM (Critical Conduction Mode) and DCM (Discontinuous Conduction Mode). Similarly, the MCU supports market demands for features like higher CRI (Color Rendering Index) and CCT (Correlated Color Temperature) control as well as the automatic detection and compensation of LED lumen power degradation.
The microcontroller draws a substantial performance boost from its smart internal architecture where on-chip comparators and ADCs are interlocked with PWM channels and the op-amps are directly linked to the ADCs internally. This translates to reduced computing load for the CPU and smaller program code size, while maintaining automatic system control including driving LEDs, PFC control, DALI communication, sensor data handling, over-current protection, and soft start.
The integrated soft-start circuit allows for minimizing the in-rush current through LEDs, thus providing protection for the LEDs and the power supply circuit. In addition, the function contributes to a reduction of the electro-magnetic emissions and helps to reduce the code size on the application level as well. The LED system designers would want to mimic the operation of an incandescent lamp where they don’t want the LEDs to instantly turn on and off, they want them to gradually turn on and gradually turn off, and this function helps achieve that on the hardware level.
New features, like the Snooze mode implemented in the RL78/I1A, enable ultra-low current consumption (0.22µA) while still ensuring robust DALI communications, A/D conversions, as well as data flash management, which operates in the background while the CPU is busy with other tasks. It is important to note that no information is lost when the DALI interrupt wakes up the CPU from the sleep mode.
Figure 2: Block diagram of µC RL78/I1A and its typical integration as an LED driver. For full resolution click here.
The internal architecture of RL78 family microcontrollers enables the majority of the required application functions to be realized in hardware and a highly optimized program code size allows the memory footprint to be kept to a minimum. This not only keeps the system costs low, but a smaller program code size also means faster application testing and less debugging efforts. Again, this makes a positive contribution towards overall system costs optimization for the designer.
Such a set of technical features is very valuable in the decision making process for selecting the right MCU for a lighting application design. Renesas’ benchmarking has revealed that compared to competition solutions, the RL78/I1A is potentially capable of achieving 20% BOM cost savings in a typical LED driver.
The advantage of using microcontrollers against hard wired control ICs lies in the fact that microcontrollers are flexible in use, versatile in terms of features and, last but not least, very adaptable in terms of software. The possibility of the reconfiguration of microcontrollers simply by reprogramming lighting application firmware allows LED driver designers to develop one hardware platform with options to adapt it to local needs and to different illumination tasks. This not only saves development time and costs but also reduces time to market. Further, handling system updates and maintenance tasks as well as monitoring is easier.
Development tool support for RL78/I1A lighting MCU
For application design and development tasks, Renesas offers a wide spectrum of state of the art hard and software development tools, application notes and on-site tools training (by appointment). In addition, 3rd parties in Europe and other continents offer development tools for Renesas products.
Hardware development tools
The hardware development tools range from the entry level evaluation boards, low cost serial interface Emulator (E1 Emulator) right up to full real time In-Circuit Emulators (IECUBE).
Evaluation board: The RL78/I1A family demonstration kit can be used by customers for an initial product “get to know” task. The kit comprises the RL78/I1A based board with 3 LED channels, DALI and DMX connectors, allowing the customer to evaluate the dedicated features of the ASSP and the dedicated lighting GUI tools included in the kit. It is a comprehensive tool, which allows customers to get an initial feel for the MCU features without having to make the effort to design their own hardware and software.
E1 Emulator: The low cost serial interface on-chip debugger (E1 Emulator) is a powerful development and debug tool featuring on-board programming, program execution, memory and register manipulation etc. The E1 Emulator has a simple 4-pin interface between the host and the device. The actual serial mode debug communication is carried out over just a single pin of the device. The E1 Emulator supports standard software tools such as the C-Compiler and Assembler, and has a powerful GUI.
In-Circuit Emulator: The In-Circuit Emulator (IECUBE: QB-RL78I1A-TxxSP) is a powerful full-fledged real time emulation and debugging tool with comprehensive break and real time trace functions and user friendly GUI. The IECUBE supports both Renesas’ proprietary software tools and 3rd party software tools – for example, those from IAR Systems® (Sweden).
All hardware tools use a standard IBM® compatible PC as host.
Software development tools
Complementary to the above mentioned hardware tools, Renesas and 3rd party language simulation and debugging software tools are available. In Europe, the IAR Embedded Workbench® (EWRL78-FULL-EE) from IAR Systems® is promoted.
The IAR Embedded Workbench®, an IDE with a powerful GUI, supports application development. The debugging suite consists of IAR proprietary C-Compiler, Assembler, and C-Spy debuggers and interfaces with Renesas IECUBE and E1 emulators.
For initial device configuration and evaluation purposes, Renesas also offers a free of charge GUI based development tool called AppliletEZ. This tool assists the user in configuring on-chip functions for the required lighting application on the RL78/I1A MCU without the customer needing to know each MCU register structure at bit level. Out comes a pre-compiled C-code at the flick of a switch. This is a boon for customers to start with Renesas MCUs, especially if they are not familiar with them.
To further ease the development/debugging of lighting applications, a comprehensive set of GUI tools for DALI and DMX communications is available from Renesas.
Outlook
It is being observed that the market is moving towards LED drivers operating directly from the mains. The RL78/I1A MCU with its on-chip PFC controller would have a definite advantage in terms of system costs and space for lighting ballasts above the 25W rating. On the output side, though, the higher voltage levels required will depend very much upon what the designer aims to achieve with the application. Here the customers will have to make a compromise between targeted application goals and the driver output specifications at hand. However, with the flexibility of available and forthcoming lighting solutions, Renesas lighting products would be able to cover most of the future application requirements.
One potential factor for efficiency enhancement is to consider special design techniques and features for LED drivers coupled with software – in other words, bringing more intelligence into the drivers. Such intelligent drivers could take over communication tasks, manage sensors, steer dimming, handle enhanced controls etc. Renesas considers the development of such intelligent drivers as very important.
In the mid-term, such intelligent drivers would even be instrumental in improving the brightness and the life cycle of the OLEDs. Intensive work is ongoing at Renesas to further improve the degree of degradation compensation of the LED/OLED lumen output over life cycle, the PWM control (finer dimming), and more precise color mixing suitable for dedicated applications.
All these factors are important in public and domestic lighting applications where not only longevity, brightness and dimming functions play a role but also subjective comfort perception of “color temperature” of the light by people is equally important. Thus besides considering costs and power efficiency issues and the LED driver design, engineers will also have to take into account the target application – such as domestic, production floor, public places, or street-lighting – for which the potential LED light source is required.
Renesas engineers are working incessantly on innovative processes to optimize internal architecture, enabling the design of lighting ballast systems with enhanced intelligence and higher functional integration density. The overall goal is to have lighting ballast electronic components with the best technical, cost efficient, and environmental compromise.
References:
1. RL78/I1A User’s Manual – Hardware
2. RL78/I1A In-Circuit Emulator (IECUBE: QB-RL78I1A)
3. RL78/I1A E1 Emulator
4. RL78/I1A Evaluation board (QB-78F7032-TB)
5. Link: https://www.renesas.com/products/mpumcu/ri78/rl78i1x/rl78i1a/index.jsp
About the author: Alex Zaretsky spent 7 years at Embedded Systems, working on the development of a variety of medical and industrial applications. In 2007 he joined NEC Electronics (now Renesas Electronics) and worked as a Senior Applications Engineer for lighting. In 2009, he was promoted to Marketing. Since then, he runs all major lighting segment activities at Renesas Electronics Europe. This also includes defining the roadmap for Renesas dedicated lighting products and the strategy for the key lighting accounts. Mr. Zaretsky has authored more than 20 marketing and technical publications and is an active member of the DALI consortium. Furthermore, he works closely with other known lighting industry organizations such as the LIA.
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