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DRV8308EVM - Brake function

Tobias Schellenberger
Prodigy 70 points
Other Parts Discussed in Thread: DRV8308

Hello,

ich have the eval-board for the DRV8308 BLDC-Motor-Driver.

After a few start- ups the motor running as I want to.
In our application of the motor at a standstill must keep a force. I thought that this with the brake is possible.
Unfortunately I have not managed it yet, that the engine holds the power.

Can you help me there? The braking function is not well described.


Thanks, best regards

Tobias

  • 0
    TI__Guru** 114905 points
    Hi Tobias,

    Our expert has been contacted and should reply soon.
  • 0 in reply to Rick Duncan
    TI__Expert 4650 points
    Hi Tobias,

    I apologize the braking function isn't well defined. When the DRV8308 BRAKE pin is asserted while ENABLE is held high, the device will turn on all of the low-side MOSFETs to short the motor phases together and remove the high-side drive.

    When BRAKE is asserted and ENABLE is high, the driver will hold the low-side MOSFETs high until BRAKE is de-asserted.

    If ENABLE is taken low the driver will hi-z all of the MOSFETs and the BRAKE pin is ignored.

    BRAKE functionality in the DRV8308 works by removing the supply voltage from the motor and creating a low-impedance decay path for the back-emf caused by the rotor. The back-emf creates a torque opposing the direction of the rotors rotation and proportional to the rotors velocity. With no supply voltage to counter the back-emf, this is the only electrical force acting on the rotor, so it will rapidly slow down.

    This method works well to quickly stop the motor, but while the rotor is not spinning or turning slowly, the back-emf is small or non-existent, meaning the opposing torque is not very large or only slightly larger than the detent torque of the motor.

    To achieve a significant holding torque in a brushless motor two phases must be energized to create a magnetic field strong enough to keep the rotor in a constant position.