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DRV8306: Setting VDS_OCP value and understanding VDS values in the datasheet

Tomer Bruchiel
Intellectual 620 points
Part Number: DRV8306

Hi Experts,

I am designing a new motor driver based on the DRB8306.

I read the data sheet and the EVM guid and have some questions.

1. Bellow is the section from the data sheet with the respected VDS_OCP values:

In section 8 there is a description of how to calculate the required VDS_OCP:

At the end, the minimum VDS_OCP is calculated to be 0.441V (Which is correct, I followed the calulation) right after that, it says that the selected value for the VDS_PCP is selected to be 0.51V but I can not find anywhere on how to set it to that value.

Can you please direct me on how I can set it to 0.51V?
Based on the table I can set it to 0.6 using a 500K ohm resistor but no 0.51.

Am I missing something?

Also there is another section in the table which I do not understand it context. (Don't know if it is related)

Can you explain it to me please? (See below).

Thanks,

Tomer

  • 0 in reply to Tomer Bruchiel
    TI__Expert 8401 points

    Hi Tomer,  

    Thanks for your question!  

    It appears likely that the example used in this datasheet may not be specifically for this device,  as many of our devices use the same VCD_OCP methodology. I will discuss this further with my team. Stillc setting it to 0.6V would be the best action in this example for the DRV8306.

    Tomer Bruchiel said:

    Also there is another section in the table which I do not understand it context. (Don't know if it is related)

    Can you explain it to me please? (See below).

    This section actually refers to the same 7-level hardware (resistor) setting configuration.  These are the expected voltages the IDRIVE/VDS pins will register when the resistor is properly connected.  

    If you were to use an external supply to apply one of these voltages to the pins (not recommended),  their corresponding setting should be set as if the resistor had been connected. 

    Hope this information has been helpful! 

    Best Regards, 

    -Joshua

  • 0 in reply to Joshua Maize
    Intellectual 620 points

    Thank you Joshua,

    That's explains it.

    But, there is still something I do not understand:
    Shouldn't we set the VDS_OCP to be lower than the calculated value?

    It says in the datasheet the the worst case scenario will be when the MOSFET junction is at 175 degrees so the Rds_on will be 8.82mOhm instead of 4.9mOhm

    Based on that (8.82) we calculated the VDS_OCP to be 0.441
    We want to limit the system to 50A but in a lower junction temperature the Rds_on will be lower, let's say 4.9mOhm.
    Solving backward (0.441/4.9m) for that we will get that the trip will happen in 90A

    Am I missing something?

    I don't know how to internal cooperator work but if it is build backward the TI solution should work.

    Can you explain? Or just confirm that setting the VDS_OCP to above the worst case scenario is the correct thing to do in all cases and that we will not exceed 50A (in this example).

    -------------------

    Another question please as I could not find a reference to it in the datasheet.

    How to calculate R_ISEN?

    In the EVM it is set to 0.007Ohm but I do not know the pulse by pulse current limit based on it.

    From what I can understand is that Vlimit = 0.25V and and in the datasheet the Winding sense current range is -20A to +20A so R=0.25/40 = 0.00625 so 0.007 was chosen.

    Are my calculations correct?

    Why are we measuring +/- range? The direction of the current in the lower side should always be positive.

    Thanks,

    Tomer

  • +1 in reply to Tomer Bruchiel
    TI__Expert 8401 points

    Hey Tomer, 

    Glad that helped to explain your initial inquiries. 

    Tomer Bruchiel said:
    Based on that (8.82) we calculated the VDS_OCP to be 0.441
    We want to limit the system to 50A but in a lower junction temperature the Rds_on will be lower, let's say 4.9mOhm.
    Solving backward (0.441/4.9m) for that we will get that the trip will happen in 90A

    I believe the factor of temperature is what is important, as an increase in current that would signal an OCP event would also significantly increase the junction temperature, which is why factoring in a room temperature junction may not be the most accurate representation for the calculation for normal conditions. 

    And the reason we specify to set the VDS trip level higher than the worst case scenario but as low as possible is to allow for full operational range and spikes, but shut off for protection as current/temperature greatly increases past safe range. Therefore, setting the trip level at the nearest higher setting should work to protect the device, but if sacrificing max range is acceptable, a slightly lower setting may be used if the margin is too large (although I would still be careful to ensure transients and other operational conditions do not trigger a false OCP event).

    I hope this clears things up further.

    And for your secondary question we can gladly help to answer, but first can you please create another post ("Ask a related question") to help separate the topic and allow others user to potentially find this the thread more easily?

    Thank you and best regards,

    -Joshua