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DRV8872: Fault pin query

Shinu.V.Mathew
Genius 10330 points
Part Number: DRV8872

Hi Team,

In the datasheet of DRV8872 the over-current condition, nFAULT pin of the IC (pin #4) will go low and the driver will try to recover in t_retry time.  Customer tested this and found that the fault pin is not going low however the retry is happening attached is screenshot of the over-current test.

 

Dark Blue: Fault pin voltage

Light Blue: Output Current (5 A as measured in overcurrent condition)

Green: Output Voltage of one of the output pins

Can you please let me know why the pin is not going in fault.

Regards, Shinu Mathew.

  • 0
    TI__Guru** 114905 points

    Hi Shinu,

    This is current regulation shown in the image. It is not overcurrent in the image.

    Current regulation limits the current through the load by setting both outputs low for approximately 25us (toff). Please refer to section 7.3.3 and Figure 5 of the datasheet for details.

    The outputs are disabled for 3ms (tretry) when an overcurrent event occurs, and the nFAULT will be asserted. Please refer to section 7.3.5.2 and Table 2 of the datasheet for details.

  • 0 in reply to Rick Duncan
    TI__Genius 10330 points

    Hi Rick,

    Thanks for your inputs.

    That means it looks like I_TRIP is being triggered over here rather than overcurrent. The I_OCP value in the datasheet is given between 3.7 A to 6.4 A, with a typical value of 4.5 A. As you can see in the graph, the load is taking a current of 5 A, which is well above the typical value of I_OCP. Does this mean that in such cases only I_TRIP will be triggered and not overcurrent condition? Or do it have to touch a 6.4 A and above to trigger overcurrent?

    Regards, Shinu Mathew. 

  • 0 in reply to Shinu.V.Mathew
    TI__Guru** 114905 points

    Hi Shinu,

    Yes, current regulation (I_TRIP) is being triggered.

    As the device begins driving the current increases during the blanking time (typically 2us). The blanking time allows for inrush currents of capacitors on the outputs or across the motor windings.

    At the end to the blanking time, the device looks at the current and determines the next step.

    If the current is above I_OCP max (6.4A) for more than typically 1.5us, an overcurrent event should occur.

    If the current is above the I_TRIP limit and above I_OCP min (3.7A) for more than typically 1.5us, either current regulation or an over current event may occur. This will vary from device to device.

    If the current is above the I_TRIP limit and below I_OCP min, current regulation should occur.