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DRV8701: Drive/Coast Application

Aarogya Pandey
Prodigy 130 points
Part Number: DRV8701

Tool/software:

Hello,

My customer uses TI motor drivers to drive their motors in a Drive/Coast fashion. They mentioned that there are no problems for a single motor but face steering issues for multiple applications (like two wheels) due to coasting getting pulled by along by the other side. Specifically, they tried using the DRV8701 as a H-bridge driver topology but they seem to be getting Drive/Brake instead of Drive/Coast. Do you have any useful tips for driving the DRV8701 in a H-bridge topology or using it for multiple motors? Additionally, are there any useful white papers or apps note that the customer can use for reference for their motor driver design?

Thank you!

  • 0
    TI__Genius 10760 points

    Hi Aarogya,

    To switch between Drive/Coast mode the inputs should match the control table below. 

    IN2 should be set to 0, and IN1 should be PWM signal. 

    If this is already how they are driving the motor, then can you send a picture of the layout and schematic. Not sure what you mean by driving "two" motors.

    For more information on current recirculation and decay modes, see the following app note:

    Current Recirculation and Decay Modes (Rev. A) 

    Best,

    David

  • 0
    Prodigy 160 points

    Customer here.

    My question was not so much a DRV8701 question, but more of a general DC motor driving question.

    One method is to sink one side of the motor and turn on and off a sourcing FET on the other side of the DC motor. I would call this Drive/Coast for that lack of a better term. The motor is switching between one side sourcing and the other side grounded, and one side floating (flyback diodes in play) and the other side grounded.

    The other method I have seen to drive DC motors is to ground/sink one side of the motor and then use a DRV8701 or other driver chips that switch between sourcing and sinking the other leg of the motor. Here the motor is switching between one side sourcing and the other side grounded, and both sides grounded.

    My question is, why use one method or the other. Seems like if you alternate in grounding both sides, you would be applying the brakes essentially, and making the drive less powerful. On the other hand, if you are running two motors with two drivers where you want to control the relative speeds, I can see that you would have more speed control if you use the braking to keep the slower motor from being pulled along by the other one.

    Is it as simple as: Drive/Float is for maximum power, Drive/Brake is for optimal speed control?

  • 0 in reply to Tim Harvey84
    TI__Genius 10760 points

    Hi Tim,

    It would depend on the application. In simple applications where only minimal control and only one direction is necessary, using a single FET as a switch to turn on the motor would be ok. 

    Using a motor driver like the DRV8701 allows for much more control. This allows for slew rate control by controlling how and when the FETs source or sink current. Controlling the VDS slew rate is necessary for controlling things like power dissipation, radiated emissions, diode recovery and inductive voltage spikes, and dV/dt parasitic turn-on. Also, TI motor gate drivers can help with detecting short to battery or ground during certain conditions. 

    Typically, slower slew rates lead to improved performance with radiated emissions, voltage spikes, and parasitic coupling; however, it will also increase power dissipation. So, the design choice is dependent on the application. 

    See the following application note for more on TI's Smart Gate Drive and how to detect shorts with TI motor gate drivers.

    Understanding Smart Gate Drive (Rev. D) (ti.com)

    Detecting Short to Battery and Ground Conditions with TI Motor Gate Drivers

    Best,

    David