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MCUs from Renesas simplify BLDC motor control designs

Posted: 29 May 2015 ?? ?Print Version ?Bookmark and Share

Keywords:Renesas Electronics? BLDC motor? MCU? home appliance? programmable gain amplifier?

Renesas Electronics Corp. has expanded its low-power RL78 family of MCUs with the release of 20 additional devices. According to the company, the RL78/G1F group of multi-function MCUs boast improved peripheral functions and compatibility across the RL78/G1x series of MCUs. The MCUs also claim to simplify sensor-less brushless DC motor (BLDC motor) control and deliver precision operation at faster rotational speeds with high accuracy for energy-efficient home appliances and electric power tools, added the company.

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 sensor-less BLDC motor systems. The RL78/G1F MCUs integrate fast 3-phase timer and dedicated comparators and operational amplifiers to ensure high-speed sensor-less control. The peripheral functions required for sensor-less control, including high-speed timers and comparators, contributing to 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 pulse width modulation (PWM) output timer supporting operation at 64MHz (max.) 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 30K rpm under 120-degree conducting control. This makes it possible to realise sophisticated motor control from the start-up to high-speed rotation at low cost.

RL78/G1F MCUs

In addition, the on-chip programmable gain amplifier (PGA) has a high slew rate of over 3V/s (min.) and a variable amplification factor from 4x to 32x, 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.

Renesas also said the RL78/G1F integrates a two-channel high-speed comparator with a response time of 70ns (typ.), 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.

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