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DRV8889-Q1: Motor driver redundant application

Vincent Su
Prodigy 20 points
Part Number: DRV8889-Q1

Tool/software:

Hi Sir,

We have a stepper motor design that requires a redundant system. I would like to ask if it is possible to connect two DRV8889/A-Q1 drivers—one from system A and one from system B—to the same motor.
Systems A and B will not operate at the same time.

What kind of issues might this cause? For example, could the current from system A's DRV8889/A-Q1 backfeed into system B's DRV8889/A-Q1?

thanks

  • 0
    TI__Mastermind 43985 points

    Hi Vincent,

    Thank you for your question.

    Vincent Su said:
    We have a stepper motor design that requires a redundant system. I would like to ask if it is possible to connect two DRV8889/A-Q1 drivers—one from system A and one from system B—to the same motor.
    Systems A and B will not operate at the same time.

    Would the redundant driver be powered with VM voltage and control inputs? 

    Vincent Su said:
    What kind of issues might this cause? For example, could the current from system A's DRV8889/A-Q1 backfeed into system B's DRV8889/A-Q1?

    If the redundant driver is not powered its power FETs body diodes would conduct the motor coils voltages to VM of the driver. VM will rise > VM UVLO threshold and power up the driver and it could potentially conflict with the other driver depending on startup conditions. If this back-fed power violates the absolute max ratings of the device it could be permanently damaged. 

    Regards, Murugavel  

  • 0 in reply to Murugavel4637
    Prodigy 20 points

    Hi Murugavel,

    Thanks for the reply.
    Our design uses a single 28V input voltage (VM), which is split into two paths for the motor drivers. We have implemented a switching mechanism to ensure that the two ICs are not powered on at the same time.
    Each driver is controlled by a separate system, so when System A is active, power is switched to Driver A; when System B is active, power is switched to Driver B.
    However, the motor control outputs of both Driver A and Driver B are connected together at the motor side.

  • 0 in reply to Vincent Su
    TI__Mastermind 43985 points

    Hi Vincent,

    Thank you for the additional details. This would likely pose a problem because the unpowered device will receive the voltage rail from the motor connections and back power the VM and GND via the MOSFET body diodes. How are digital control connections IO control signals and SPI dealt with for the unpowered device? Like what would be the logic levels for the nSLEEP, DRVOFF control pins? Have you tried connecting two DRV8889-Q1EVM together with this type of architecture? 

    Regards, Murugavel 

  • 0 in reply to Murugavel4637
    Prodigy 20 points

    Hi Murugavel,


    This is our current idea, and we are exploring its feasibility. We will only proceed with implementation after confirming that it is viable.
    The unpowered system side will be completely without power. Except for AOUT1/2 and BOUT1/2, all other signals are isolated and will not connect System A and B together.
    So, will the voltage received from the output side cause the driver to power on?
    In our system, the GPIOs connected to the driver's control signals are in a tri-state condition when powered off. On the unpowered system side, DRVOFF has an internal pull-up resistor to DVDD, and nSLEEP has an internal pull-down resistor.
    Therefore, the system should remain in a non-operating (inactive) state.
    Is my understanding correct?

    Thanks,

  • 0 in reply to Vincent Su
    Prodigy 20 points

    Or do we add a N-channel MOSFET between the driver and motor?

    Or is there any more suitable component?

  • 0 in reply to Vincent Su
    TI__Mastermind 43985 points

    Hi Vincent,

    Understood. Unfortunately we cannot recommend doing this because we have not characterized two drivers connected together like you suggested. We do not prevent you from implementing and characterizing this for stability and longevity. 

    Vincent Su said:
    So, will the voltage received from the output side cause the driver to power on?

    Most definitely it will. See below conduction path of the positive voltage via body diodes to VM rail. VM bulk capacitor will charge up to the peak value of the voltage pulses. xOUTx pins are not designed to be driven with external voltage. Although high level block diagram shows only the power FETs there are internal protection components for these power FETs. The behavior of these components or circuits around them is undefined for such use case. 

    Vincent Su said:
    In our system, the GPIOs connected to the driver's control signals are in a tri-state condition when powered off. On the unpowered system side, DRVOFF has an internal pull-up resistor to DVDD, and nSLEEP has an internal pull-down resistor.
    Therefore, the system should remain in a non-operating (inactive) state.
    Is my understanding correct?

    Logically this is correct. 

    Vincent Su said:
    Or do we add a N-channel MOSFET between the driver and motor?

    The current flow to each coil would be bidirectional. So this may not be a viable approach. 

    Vincent Su said:
    Or is there any more suitable component?

    An electromechanical (bistable maybe) relay with four poles and double throw could be a safer and low cost approach. Thank you.

    Regards, Murugavel