Most brushless DC motor controllers operate sort of like the gas pedal of a car. If the pedal is pushed down some amount, the car will travel at different speeds depending on if it’s climbing a hill, going downhill, or plowing through snow.
A standard PWM input for a motor is like that pedal: it goes from 0% to 100% and directly affects rotational speed, but the actual speed varies with load torque, motor characteristics, temperature, and mechanical wear.
Today, TI solves this problem with the release of the DRV8308 chip, which drives BLDC motors to specific RPMs!
With the help of an input clock (which can be generated by an MSP430), the device matches motor speed to the applied frequency, independent of mechanical factors. The fully integrated closed-loop control system needs no processor, and it’s tunable by a handful of register values that allow it to work well in many types of motor systems.
In addition to the precise speed control, the DRV8308 is packed with other highly valuable features:
- Configurable predriver output strength that lets you control the VGS slew rate on the external FETs. This provides a simple way to minimize half-H bridge switching noise and EMI.
- Automatic commutation from the Hall sensors, with a configurable timing shift that can make commutation occur at exactly the right time for optimal power efficiency.
- Sine wave current drive mode that silences audible motor noise.
- Configurable motor current limiter, which prevents supply voltage droop (especially during startup).
- Integrated non-volatile memory that stores device settings.
- Classic PWM mode that can work from a register value instead of a clock.
- Regulated 5V output for powering Hall sensors.
- Low-power sleep mode.
- Multiple protection circuits and fault reporting.
Here’s the device block diagram:
How could TI make this any better? By providing a simple evaluation platform, of course! The DRV8308EVM includes a PCB to connect to your motor, a comprehensive user’s guide, and a graphical user interface that lets you control the device from your computer. It also uses TI’s brand new 60V dual FETS, the CSD88537ND. Here’s a typical bench system diagram:
I hope to review your next design with the DRV8308!
To learn more on BLDC motors, check out my colleague’s Engineer It video on ‘Understanding basic sensored BLDC motor operation.’