MCU-based LED drivers inspire designs

RL78/I1A MCU ASSP for lighting applications - Page 2
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 percent) 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.


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 percent 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.