Low-power MCUs simplify sensorless BLDC motor control

28th May 2015
Barney Scott

Renesas Electronics has announced the RL78/G1F Group of multi-function MCUs, adding 20 members to its low-power RL78 family. Featuring enhanced peripheral functions and compatibility across the RL78/G1x Series of MCUs, the devices simplify sensorless BLDC motor control and deliver precision operation at faster rotational speeds with high accuracy for energy-efficient home appliances and electric power tools.

The need for more efficiency and reduced system costs for motor-driven appliances, such as vacuum cleaners, dishwashers and refrigerators, as well as electric power tools are driving a shift from traditional motors with built-in position sensors to sensorless BLDC motor systems. The RL78/G1F MCUs integrate fast 3-phase timer and dedicated comparators and op amps to ensure high-speed sensorless control.

The peripheral functions required for sensorless control, including high-speed timers and comparators, contribute to a reduction of up to 20 external components and lower development costs. In addition, Renesas has added an auxiliary timer designated for motor control to enable better control at a wider range of speed.

In addition to the PWM output timer supporting operation at 64MHz (maximum) offered on Renesas’ existing MCUs, the RL78/G1F has an auxiliary timer for motor control that operates at the same clock frequency. By combining this timer and the high-speed comparator, the MCUs enable position sensing without a sensor even when the rotor is stationary, and at high rotational speeds of over 30,000rpm under 120° conducting control. This makes it possible to realise sophisticated motor control from the start-up to high-speed rotation at low cost.

The on-chip PGA has a high slew rate of over 3.0V/μs (minimum) and a variable amplification factor from 4 to 32 times, selectable by software. This eliminates the need for an external amplifier for voltage detection or overcurrent detection used in BLDC motor control. The PGA is designed with input and feedback resistor ground pins that enable amplification while reducing high-current noise to the MCU.

The RL78/G1F integrates a two-channel high-speed comparator with a response time of 70ns (typical), eliminating the need for an external comparator. One channel can use the PGA output as its input signal, enabling implementation of overcurrent detection for the inverter circuit during brushless motor control. Reliable overcurrent detection can be achieved from built-in functionality (PWMOPA) that uses the comparator output directly to cut off the output signal to the inverter circuit.

The other comparator channel allows the user to select any combination of positive and negative inputs among four input pins to implement rotor position detection for sensorless BLDC motor control, basically to manage the motor back EMF signals. Both comparator channels allow selection of external pin input, the internal reference voltage, or the dedicated generation DAC output as the negative input.

The RL78/G1F integrates a one- or two-channel DAC, a feature not previously available on comparable products with 64kB of flash memory or less. Renesas also added IrDA support to the serial communication functionality, broadening the range of possible applications, such as simplifying diagnostics, monitoring, e-locks, or handy terminals.

The product lineup includes 24-pin HWQFN and 36-pin WFLGA compact 4mm2 square packages. The 36-pin WFLGA products are suited for more space-constrained devices. While they have a low pin count, the MCUs support dual power supplies, allowing the CPU to operate at 3 or 5V while supporting communication at 1.8V with external devices without the need for an external level shifter. Either an internally generated subclock or one input externally can be used, allowing low-power operation or intermittent operation over extended intervals to cut power consumption and extend battery life.

The RL78/G1F lineup comprises 20 product versions with pin counts from 24 to 64 pins and flash memory capacities of 32 or 64kB. Samples are available now. Mass production is scheduled to begin in July 2015 and is expected to reach a scale of 1m units per month in December 2016.

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