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DRV8842: Confirmation with Isense pin of DRV8842

Jonathan Geronga
Expert 7930 points
Part Number: DRV8842

Hi team,

I would like to get your input with the customer inquiry below:

In the customer design, With Rsense=100 mohms, , he has a max of 1.2A chopping current with the driver OUT1/2 (voltages) being chopped when supplied with a VM=24V. But if he reduce the Rsense to 50mohms, they see a constant current of 1.2A with VM=24V, no voltage drops noticed at OUT1/2 pins. But if calculate Ichop max, as per the datasheet with Vref=3.3V and Rsense=50mohm, they get a Ichop of 13.2A max : which is confusing. So they question is what is the min current sense resistor I can use, and what is the advantage/disadvantage going with a real low value( of Rsense like 25mohms )?

I will send the schematic via email if you want to check it.

Thanks,

Jonathan

  • 0
    TI__Guru 56650 points

    Jonathan,

    DRV8842 OCP current is 6A which is independent of the ISENSE resistor value or VREF voltage. So, the Ichop equation is only good up to 6A.

    " they see a constant current of 1.2A with VM=24V, no voltage drops noticed at OUT1/2 pins"

    With 100mohm current sensing resistor; 3.3VREF, the Ichop setting is higher than 1.2V. I would think the load resistance is too high, the current cannot reach the regulation point, even the FET is fully on.

  • 0 in reply to Wang5577
    TI__Expert 7930 points

    Hello Wang Li,

    Thank you for supporting this thread.

    After sharing your information with our customer, here's the response we got:

    Could you please explain this below- how is Ichop calculated as 1.2V ?
    "With 100mohm current sensing resistor; 3.3VREF, the Ichop setting is higher than 1.2V."

    After testing, we do see that 100mohm load resistance is too high, but is there a lower limit value for external current sense resistor? I mean, with respect to the motor internal resistance which comes in series with the current sense resistor, what would be the ideal current sense resistor value for maximum power transfer to the motor?

    Regards,

    Jonathan

  • 0 in reply to Jonathan Geronga
    TI__Guru 56650 points

    Jonathan,

    1.2V is a typo, should be 1.2A as mentioned in your first post.

    To fully use the full 3.3VREF range, the current sense resistor would be VREF_max/ (5*Iout_max).

  • 0 in reply to Wang5577
    TI__Expert 7930 points

    Hi Wang Li,

    Thank you for this correction.

    I have follow up question from the customer below:

    From the formula VREF_max/ (5*Iout_max): with Vref=3.3 & Iout_max of 5A, the resistor value comes to 132mohms. But when they use the 100 mohms, they see that the driver is not able to provide more than 1.2A without chopping the out1/out2 signals (with a peak of 24V) . But if reduce the Rsense to 50mohms, they able to obtain a peak voltage of 24V with 1.2A, throughout the motor drive cycle.
    So that brings back the original question, what is the ideal resistor value for max Iout with a Vref of 3.3V?

    Regards,

    Joanathan

  • 0 in reply to Jonathan Geronga
    TI__Guru 56650 points

    Joanathan,

    Please check the current with a scope. It can help them to understand the issue. The current regulation controls the peak current. When they say 1.2A, is it peak current or average current? If it is 1.2A peak, please upload the waveform to here to check.

  • 0 in reply to Jonathan Geronga
    TI__Guru 54645 points

    Jonathan,

    Something isn't making sense here.  With Vref = 3.3V, and Rsense = 100mohm, the calculated full scale chopping current should be 6.6A.  But, you are saying the device is chopping at 1.2A.  

    So, now we need to check the state of the I0-I4 pins that scale this full scale chopping current to a lower value.  This might explain things better.  See table 4.  

    Ideally, you want to limit the amount of power loss across the resistor so you use a smaller, lower power resistor.  This is described in section 8.2.2.2 of the datasheet.  But, if resistor is too small, it starts to impact the signal to noise ratio.  I would choose a resistor that results in at least 500mV of signal at the desired trip point.