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DRV8701EVM: Queries on the output current limit

Amirul Kamaruzaman
Prodigy 40 points
Part Number: DRV8701EVM
Other Parts Discussed in Thread: DRV8701, CSD18532Q5B

Tool/software:

Hi,

From the DRV8701EVM, the continuous current is 15A and peak current 20A.

Looking at the schematic of DRV8701EVM, the Vref is left floating, thus the current chop is calculated to be 0A?

Then for the Vdsocp for DRV8701 is fixed to be 1V, and mosfet used in the EV kit is CSD18532Q5B with 2.5mohm Rdson, the OCP is calculated to be 400A.

Can you please advise what am I missing in this case and how do you achieve the claimed 15A continuous and peak current 20A as described in the features?

Best Regards,

Amirul

  • 0
    TI__Mastermind 29655 points

    Hi Amirul,

    Thank you for posting in this forum.

    Amirul Kamaruzaman said:
    Looking at the schematic of DRV8701EVM, the Vref is left floating, thus the current chop is calculated to be 0A?
    Amirul Kamaruzaman said:
    Can you please advise what am I missing in this case and how do you achieve the claimed 15A continuous and peak current 20A as described in the features?

    The VREF has optional resistor divider using R6 and R8 which is DNP in the EVM. The VREF is also connected to VREF_MCU which is a variable DC voltage output DAC based on PWM which is controlled by the GUI 0 to 3.3 V to set the output current. The sense resistor in the EVM is 10 mΩ. When VREF is set to 3.3 V using the GUI the current will be chopped/regulated at 16.25 A.

    Amirul Kamaruzaman said:
    Then for the Vdsocp for DRV8701 is fixed to be 1V, and mosfet used in the EV kit is CSD18532Q5B with 2.5mohm Rdson, the OCP is calculated to be 400A.

    The VDSOCP is specific for over current protection OCP. The DRV8701 has two ways of OCP. For very low Rdson FETs such as the CSD18532Q5B VDS OCP MIN of 0.8 V translates to minimum of 320 A or 1 V translates to 400 A like you mentioned. This may not be a practical current limit to protect the FETs in case of an output short. However the second protection in the DRV8701 is based on the Rsense voltage. In the EVM Rsense is 10 mΩ. So VSP OCP MIN 0.8 V translates to 80 A minimum trip point for OCP. 

    Regards, Murugavel

     

  • 0 in reply to Murugavel4637
    Prodigy 40 points

    Hi Murugavel,

    Thank you for the response.

    So Ichop is 16.25A and OCP is 80A based on EVM design, but how does the feature state to be 20A peak H-bridge output current?

    I have designed using a DRV8702 for more selection of Vds ocp, schematic as below. If possible, can you quick review and highlight if there are any issues?

    Motor Driver circuit.pdf

    Best Regards,

    Amirul

  • +1 in reply to Amirul Kamaruzaman
    TI__Mastermind 29655 points

    Hi Amirul,

    Amirul Kamaruzaman said:
    So Ichop is 16.25A and OCP is 80A based on EVM design, but how does the feature state to be 20A peak H-bridge output current?

    Like I mentioned, the FET's in the EVM can support 20A continuous (let alone peak) output current as per its datasheet - see below. If you'd want 20A continuous you have to modify just the sense resistor value accordingly.

    The DRV8702H has programmable VDS options hence better control on OCP settings compared to the DRV8701. Thanks for sharing the DRV8702 schematic. I review it and it looks fine. Thank you.

    Regards, Murugavel