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Can DRV8825 connect two independent Vm by Vma and Vmb

Felix Wei
Prodigy 310 points
Other Parts Discussed in Thread: DRV8825, DRV8843

Hi,

 

It shows that pin Vma and pin Vmb is independent in datasheet for DRV8825. But the datasheet also tell us that Vma and Vmb should be connected to the same supply.

Could you please explain why?

And BTW, is H-bridge for DRV8825 synchronous rectification?

 

Thanks for replying.

 

  • 0
    TI__Expert 7455 points

    Hi Felix,

    VMA and VMB are the bipolar stepper motors VM power supply voltage input and are not independent. They are labeled with A and B just to show their proximity to the respective H bridge, but in essence it makes no sense to connect them to different power sources. They should be connected together to the same VM source.

    Yes, the DRV8825 utilizes Synchronous Current Regulation by utilizing opposing FETs as alternate current path during current the current regulation cycle.

    Best regards,

    Jose Quinones

  • 0 in reply to Jose Quinones
    TI__Prodigy 310 points

    Hi, Jose

     

    Thank you for your time.

    I am still curious that why not connect VMA and VMB internally and just derive ONE pin VM outside?

    And for DRV8843 which can drive tow DC motor, it also said that VMA and VMB should connected to the same supply. Could I use DRV8843 to drive two independent DC motor with different supply?

     

    Regards,

    Felix Wei 

  • 0 in reply to Felix Wei
    TI__Expert 7455 points

    Hi Felix,

    Connecting the two pins into one is definitely way easier said than done. If you do that, then a total of 5A would have to flow through a single pin, instead of 2.5A per pin. It is just good practice not to try and pass too much current through a single node, especially when the sizes are so small. If it was a 10 gauge wire, then sure! But it is not a 10 gauge wire what connects the pin to the die.

    Same applies for the DRV8843. The two H Bridges are independent with regards to functionality, but the voltage must be the same. If you want the motors to see different voltages, you can always PWM the control lines and the motors will see a voltage directly proportional to the PWM's duty cycle.

    The number of applications where you need two voltages to drive two different motors is rather small. And at the same time, these are often massive applications where the cost of using a chip to drive at high voltage and a second chip to drive at lower voltage is quite negligible when compared to the cost of the entire application.

    Hope the info helps. Best regards,

    Jose Quinones