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Original question:

TPS24751 IMON Current

TPS24751: IMON does not appear to function as described in the datasheet

Brian Eldridge
Prodigy 80 points
Part Number: TPS24751
Other Parts Discussed in Thread: TPS24750, TPS24750EVM-546

Similar to the thread originally titled "TPS24751 IMON Current", as opened by user named Alex Dukhovny1, I am not seeing behavior of the IMON voltage relative to measured current through the device that matches what is described in the datasheet.  On Nov 10th, 2015, Alex Dukhovny1 posted an Excel spreadsheet with his measurement results illustrating the problem.  My measurement results do not match his exactly, but its a very similar behavior, with the voltage across the IMON resistor of anywhere between 1.6 and 2.0 X the expected value.  It seems to be somewhat proportional to the current (the larger the current, the worst the reading).

I found the same behavior both with my own PCB design AS WELL AS the TPS24750 EVAL board.... it simply does not appear to perform as documented.

I do not see any resolution to Alex Dukhovny's original question - can we get some feedback on his posted data and the opinion of TI on this behavior?

Does TI have any measurement data and/or measurement setup documented that validates the datasheet formula for the relationship between V-IMON and I-OUT?  As it stands right now it does not appear to be reliable or usable without calibration on a device by device basis, but perhaps I am overlooking something.

Thanks,

Brian Eldridge

  • 0
    TI__Guru** 105611 points
    Hi Brian Eldridge,

    Welcome to E2E!
    It is sad that it was not resolved in that particular thread. I will make sure to get resolution for this issue. Please allow couple of days time.

    Best Regards,
    Rakesh
  • 0 in reply to Rakesh
    TI__Guru** 105611 points

    Hi Brian Eldridge,

    Please measure offset voltage at IMON pin i.e., IMON voltage at no-load. Then subtract this voltage from the IMON value measured at other load currents before multiplying with GAIN.

    Please refer the attached excel sheet with values captured on my EVM. In my case, the IMON offset is 29mV. Considering this, we will be close to actual load current.

    Imon linearity.xlsx

    Best Regards,

    Rakesh

  • 0 in reply to Rakesh
    Prodigy 80 points
    Thank you for your response, but unfortunately this did not resolve my issue. I have 4 samples of TPS24751 on my custom PCB, plus I was also comparing against a TPS 24750 eval board. I measure the no load offset of my 4 samples to be 14mV, 18mV, 25mV and 33mV respectively and that of my TPS24750 eval board to be 28mV. For one, these are highly inconsistent results, and for another it unfortunately does not eliminate the error I am seeing across all these devices, however, this technique DOES pull the error across my 4 devices more into line with one another, so it does appear to improve the situation. Before applying your technique, I was seeing V-IMON measurements which were approximately 90% too high on samples 1,2, & 3, 120% too high on sample 4, and 45% too high on the EVAL board. After applying your technique, I see V-IMON measurements which are approximately 60% too high on all 4 of my samples (so this is more consistent) and 25% too high on the EVAL board.... but clearly this technique does not correct the problem even with the eval board.

    I see from your data that your measurements do fall into line with the technique you propose, but I'm wondering how consistent this is across multiple boards as the range of values is really not consistent across my 5 samples at all. Also, I am using a standard Fluke DMM to take the voltage measurements and have used a combination of methods to apply the loads, but have only measured in the 700-3000 mA range of load current - can you elaborate on your measurement technique so that I am not overlooking anything.

    Thanks again,
    Brian Eldridge
  • 0 in reply to Brian Eldridge
    TI__Guru** 105611 points
    Hi Brian Eldridge,

    I have used Agilent 6 1/2 benchtop digit multi-meter for IMON voltage and load current measurements.
    Can you share your results for one sample to discuss with my design team. Is there any circuit or capacitance at IMON pin.? Please share your schematic also.

    Best Regards,
    Rakesh
  • 0 in reply to Rakesh
    Prodigy 80 points

    Thank you Rakesh.  I only have a few datapoints because I did not have great granular control over my load current but I was able to measure it accurately.

    To simplify things AND address your request for a schematic, I will simply share the data I collected from your TPS24750EVM-546 reference board, schematic can be found in your own datasheet , ie:

    http://www.farnell.com/datasheets/1777886.pdf?_ga=2.64026540.227346666.1548097370-454358170.1540412336

    The data I collected measuring with a Fluke DMM monitoring DC voltage to ground at schematic reference designator TP16 is as follows:

    LOAD CURRENT MEASURED IMON VOLTAGE MEASURED IMON VOLTAGE UNDER NO LOAD CURRENT CALCULATED BY METHOD DESCRIBED ABOVE ERROR
    0.77A 0.0782V 0.028V 0.81A 5%
    2.197A 0.1961V 0.028V 2.72A 24%

    As you can see, the method you suggest does work fairly well at the 0.77A range (only 5% error) but at 2.2A the error grows to 24%.  Granted this is only two datapoints, and I do have a lot more datapoints with my OTHER samples of device but the results are similar and I thought it more instructive to use the values specifically observed on your reference board since you could potentially replicate them exactly.  If I had confidence in the reference board and a methodology for arriving at the correct current, I'd be happy to deal with the discrepancies of my own design myself.  But right now I can't even get your reference board to behave as advertised.

    Thanks

    Brian

  • 0 in reply to Brian Eldridge
    TI__Guru** 105611 points
    Hi Brian,

    I have been asked to get more data from you to look into this. Can you please manage to get data at two more current values preferably at higher currents say 4A, 8A.

    Best Regards,
    Rakesh
  • 0 in reply to Rakesh
    Prodigy 80 points

    Thanks for the reply Rakesh.  I will try to collect more data early next week.

  • 0 in reply to Brian Eldridge
    TI__Guru** 105611 points
    Thank you! Brian.
  • 0 in reply to Rakesh
    TI__Guru** 105611 points
    Hi Brian,

    Can you please provide data to proceed further on this.

    Best Regards,
    Rakesh
  • 0 in reply to Rakesh
    Prodigy 80 points

    Hi Rakesh - sorry for the delay - I did collect the data today from 0.7A - 6.2A or so (could not go higher than that in my application) - about 7 data points in that range, but I will need to review it tomorrow before I can post it.  Please expect it by end of day tomorrow or Wednesday at the latest.

    Thanks,
    Brian

  • 0 in reply to Brian Eldridge
    Prodigy 80 points

    Hi Rakesh - I have collected the data on the TPs2475x IMON now to the best of my ability.  Here are the details:

    Setup:  I have a 12V feed on my PCB through a TPS24751.  I placed this in SERIES with a TPS24750EVM-546 and a Fluke DMM and a set of purely resistive loads that I had at my disposal and I measured the current through the DMM, and across the IMON resistor of each of the TPS devices (the '24751 on my board as well as the '24750 on the EVM board.

    Markings on the TPS device on my board:

    TPS24751
    TI   81I
    AX92 G4

    Markings on the TPS on the EVM:

    TPS24750
    TI  66I
    C0G0  G4

    Resistor design on both boards is identical:

    R-SET = 51.1
    R-IMON = 1580
    R-SENSE = 0.002
    THEREFORE the scaling factor should be 16.17 on each board

    The NO LOAD V-IMON on the EVM is measured to be 28.4mV
    The NO LOAD V-IMON on my PCB is measureed to be 16mV (I also have three other samples on my board that measure 29.1, 33.4 and 23.0 mV respectively so there is no consistency across parts.  I have NO other loads on my V-IMON node, my design is the same as the EVM in this regard.

    The data I collect is as follows:

    I-DMM is the actual current measured
    V-IMON is the voltage on the IMON pin for the "EVAL" and for "MY" board under that load condition
    I-CALC is the calculated current based on subtracting the V-IMON-NOLOAD offset and applying the scaling factor
    %ERR is the % ERR between the calcualted value and the measured value

    As you can see, using this method the calcuated value is always 5% off , measured on the EVM and ~ 43% off measured on my PCB.

    Something is not right..... is there a difference in expected behaviour between the TPS24750 and TPS24751?

    Thanks,
    Brian

    I-DMM V-IMON-EVAL V-IMON-MY I-CALC-EVAL I-CALC-MY %ERR-EVAL %ERR-MY
    0 0.0284 0.0160
    0.794 0.0800 0.0945 0.834 1.269 5.09% 59.88%
    1.187 0.1055 0.1200 1.247 1.682 5.04% 41.68%
    2.332 0.1800 0.2235 2.452 3.355 5.12% 43.89%
    4.48 0.3205 0.4142 4.724 6.439 5.44% 43.73%
    5.35 0.3760 0.4897 5.621 7.660 5.07% 43.18%
    5.92 0.4120 0.5393 6.203 8.462 4.78% 42.94%
    6.21 0.4336 0.5682 6.552 8.930 5.51% 43.79%
  • 0 in reply to Brian Eldridge
    TI__Guru** 105611 points
    Hi Brian,

    I appreciate your help in getting the data. But the high %error on your PCB is not what can be expected from the device. The device be it TPS24750 or TPS24751 provides current limit, current monitoring accuracy less than +/-10% across the temperature and process variation.

    Did you thought of device swapping for confirmation? i.e., device from your PCB to EVM board for cross verification?

    Best Regards,
    Rakesh
  • 0 in reply to Rakesh
    Prodigy 80 points

    Ok, at this point I'm not sure I will do that or spend more time on this - I can just abandon the feature, but I will keep that in mind if I need to dig deeper.

    Thanks for your help.

    Brian

  • 0 in reply to Brian Eldridge
    TI__Guru** 105611 points
    Hi Brian,

    Please let me know if there is any other way I can help on this.
    I hope, you followed kelvin current sensing and used 1% tolerance resistors on your board. If you wish, please send me your PCB layout, BOM details for review.

    Best Regards,
    Rakesh
  • 0 in reply to Rakesh
    Prodigy 80 points

    Thanks Rakesh.  We certainly used 1% resistors, but it is true that the layout was not perfect with respect to kelvin sensing due to time constraints..... having said that I would not expect improper kelvin sensing to make THIS large of a difference and the footprint for our 4 instances are identical yet perform very differently, so I'm skeptical that this is the root cause of the issue, but we will clean up the kelvin sense routing on the next version of the PCB and see if it makes a difference.  In the meantime we will likely abandon this feature as it is not critical to our implementation.

    Thanks for your help.  Feel free to close the case.

  • 0 in reply to Brian Eldridge
    TI__Guru** 105611 points
    Hi Brian,

    Thank you!
    Please click "This resolved my issue" button to close this thread.

    Best Regards,
    Rakesh