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TPS3704: TPS37044 accuracy calculation

Wouter Kragt
Intellectual 1820 points
Part Number: TPS3704

Hello team,

For our project we want to monitor the output voltages of a PMIC (LP7745-Q1).

We are looking into the supply voltage supervisor: TPS37044,. We would like to calculate the accuracy of the device for 4 different rails 1V, 1.2V, 1.8V and 3.3V that come from the PMIC.

To do this calculation we are using the excel sheet from the following webpage: TPS3704-ADJ-CALC Calculation tool | TI.com

  • In the excel sheet you need to full in the Isense multiplier. What is a good value for this?

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  • Further, is the answer from this excel sheet the values on which I can base my accuracy assessment?

Thank you.


Kind regards,

Wouter Kragt

  • 0
    TI__Mastermind 22260 points

    Hi Wouter,

    The calculator is OK to use for the accuracy calculation. An Isense multiplier of 150 should be within the OK range. A minimum multiplier of 50 is recommended. The lower the multiplier is, the more accurate the device will be at the cost of power efficiency. This application note explains in detail: https://www.ti.com/lit/an/slva450b/slva450b.pd

    Best regards,

    Ridge Lahti

  • 0 in reply to Ridge Lahti
    TI__Intellectual 1820 points

    Hello Ridge,


    Thank you for your feedback.

    There is are a couple of points that I do not fully understand in the document:  https://www.ti.com/lit/an/slva450b/slva450b.pd

    1. The leakage current.

    In the document, this is stated in equation 3, that I (please see the figure below):

    • Where do I get the value of the leakage current?
    • Is my assumption correct that this is Isense_ADJ from the datasheet of the TPS3704? (please see figure below)

    2. I think there is an error in the document. In the calculation of I_R1 (point 12, see figure below).

    The 187kOhm is used for numerator for R1, but this should be 1.18MOhm, could you please verify this?

    3. Regarding  the figure above, I recalculated point 11 and I do not get 2.974V. My answer is 2.9606.

    I assume this is a small error?

    4. Calculating the inaccuracy due to the resistor accuracy, please see figure below.

    Do you use the V_IT that is previously calculated in point 11 of the document or do you use the ideal V_IT value?

    5. Lastly, I did the full calculation and if I understand it correctly, all the inaccuracies together are:

    1. Resistor accuracy: TOL_R, 
    2. Accuracy of the Vsense node, where TOL_Vref is the the V_IT accuracy on the DS
    3. The accuracy due to the leakage current

    Is this correct or am I missing something? 

     

    Thank you.

    Kind regards,

    Wouter Kragt

     

  • 0 in reply to Wouter Kragt
    TI__Mastermind 22260 points

    Hi Wouter,

    Please find my answers labeled for the corresponding questions:

    Q1: Isense is the leakage current referenced in the application note.

    Q2 and Q3: I agree with your assessment. Those appear to be errors in the document. Follow the equations written with the variables before the numbers are plugged in.

    Q4: Use the typical Vit listed in the datasheet.

    Q5: That is the correct understanding.

    Best regards,

    Ridge Lahti