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CCS/LM25066A: LM25066A external Rs limitation

user6106884
Prodigy 40 points
Part Number: LM25066A
Other Parts Discussed in Thread: LM25066

Tool/software: Code Composer Studio

Hi Guys,

Is there any Rs limitation for LM25066A input sense?

I found that the SPM_GUI couldn't show the right value if we used 0.15mohm Rs at P12V for 100A maximum current.

I found that the telemetry always show 2.7A  when the real loading from 0A to 10A.

But I reworked the Rs from 0.15 mohm to 0.3mohm, the telemetry seems work well.

Although it still has 1.5A offset at 0 A, the reporting accuracy is accepted when the real loading over 20A.

So, I need your confirm  whether there is some limitation for Rs value?or how to optimized the accuracy?

  • 0
    TI__Guru 57737 points

    Hi,

    Can you please confirm which software you are using to communicate with LM25066 ?

    PI-Commander EVM GUI is the recommended software to be used for  TI Hotswaps.

    In PI- commander, you will have an option to set the Rsense resistor that you have used on your board as shown below.

    Also, the images you have shared are not uploaded properly. Can you please upload them as an attachment in .png format.

  • 0 in reply to user6106884
    Prodigy 40 points

    Hi,

    I uploaded the PNG file at attached.

    And the IIN reporting result is stiil not linear, as below test table.

    　 Thermal Recorder Multimeter DC load PMBUS register Ratio Result
    Loading Temp Vout Iout Temp Vout Iout Temp Ratio Vout Ratio Iout Ratio
    10.00% 29.9 11.98 7.1 28.9 12.007 2.684 96.66% 100.23% 37.80% 　
    20.00% 29.9 11.98 14.2 29.1 12.003 7.1 97.32% 100.19% 50.00% 　
    30.00% 29.9 11.97 21.3 29.4 11.99 14.2 98.33% 100.17% 66.67% 　
    40.00% 30.5 11.96 28.4 30.3 11.985 21.6 99.34% 100.21% 76.06% 　
    50.00% 31.1 11.95 35.5 30.9 11.976 29.3 99.36% 100.22% 82.54% 　
    60.00% 32.5 11.94 42.6 32 11.96 36.7 98.46% 100.17% 86.15% 　
    70.00% 34.1 11.93 49.7 33.4 11.96 44.2 97.95% 100.25% 88.93% 　
    80.00% 34.5 11.92 56.8 35.1 11.948 51.7 101.74% 100.23% 91.02% 　
    90.00% 36.7 11.91 63.9 36.4 11.948 59.3 99.18% 100.32% 92.80% 　
    100.00% 37.1 11.91 71 36.2 11.939 66.7 97.57% 100.24% 93.94% 　
  • 0 in reply to user6106884
    TI__Guru 57737 points

    Hi,

     There are three reasons for current sensing errors in LM25066,

    1. Offset error of the internal current sense amplifier. For LM25066, the offset voltage can be around 4mV (including margin). This will result in poor accuracy at lower currents and the accuracy degrades with lower shunt resistor values.
    2. Rshunt Not Kelvin Sensed. Proper kelvin connections are needed to avoid errors in sensing.

    3. The current flowing through VIN can be 8mA. If the Sense+ trace is long, there would be voltage drop on this trace (8mA x Trace resistance) which will cause offset error to your current sense measurement.