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DRV8320: Supply (pin 3) = VM - diode drop

Nicholas Layshot4
Prodigy 220 points
Part Number: DRV8320

I have an application where I need to be able to OR two supplies (VM = 16-36V and VD = 16-36V) to the DRV8320 and to the Half-bridges but must limit the current to the bridges for the VD supply and not have any voltage drop to the half-bridge from the VM supply. See picture below. What affects would this have? Would the only affect be that VGS (high side FET) = VGS (normal) - diode drop?

Thanks, Nick

  • 0
    TI__Guru** 114905 points
    Hi Nick,

    Will the VD and VM voltages be the same? If not, what will be the largest variation (lowest VD and highest VM, and highest VD and lowest VM) the system will experience when enabled?
  • 0 in reply to Rick Duncan
    TI__Guru** 114905 points
    Hi Nick,

    I forgot to ask: What is connected to VDRAIN?
  • 0 in reply to Rick Duncan
    Prodigy 220 points

    Hi Rick,

    By the way: the part I'm using is the DRV8320SRTVR. I'm using it in independent mode (full control over the gates) but will be adjusting IDrive via software.

    Both VD and VM will have a range of 16-36 volts and are independent. Meaning one could be at 16V while the other is at 36V. If VD is present VM might or might not be connected (open). If VM is present VD will be present.

    ** I need to check to make sure VM will not be turned on after it's connected. In this case VD would already be there (16-36V) when VM starts at 0V and rises to 16-36V.. I will check and update this. What would happen in this case... Would this cause any issues?

    VDRAIN is connected directly to the Drain of the Top FET as shown below.

    Thanks,

    Nick

  • 0 in reply to Nicholas Layshot4
    TI__Guru** 114905 points
    Hi Nick,

    Assuming the motor will operate under the conditions you described, there a few combinations to be concerned about:

    1) One could be at 16V while the other is at 36V.
    If VD=36V and VM=16V, this will probably violate the VGS of the high side FETs as the VM voltage drops to 16V. The assumption is that VM is VD - a forward drop until the motor begins running. At that point, the current is limited by the 332 Ohm resistor. This can create a VGS approaching 32V ((VD - VM) + 12).

    2) If VD is present VM might or might not be connected (open).
    If VM is not connected, this will probably violate the VGS of the high side FETs. Also the motor will probably not operate since the current is limited by the 332 Ohm resistor.

    3) If VM is present VD will be present.
    This condition should work with reduced VGS by a diode drop on the high side FETs.