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Wrong formula in the Application report

Janani Baskar
Intellectual 330 points

www.ti.com/.../slta055.pdf

Hi!,

I read through the above application report to calculate the input Capacitance for a DC/DC Converter.

I think the calculation mentioned is wrong or my understanding of it is incorrect.

The Capacitance Value does not come to 84uF but 84mF.

Kindly check and clarify this for me.

Thanks!

  • 0
    TI__Intellectual 2910 points

    Janani,

    Thank you for your interest in our Application Report. I have reviewed the equation and found it to be correct. Please note that in Equation (1) and (2) in the report, Cmin is the minimum required ceramic input capacitance in μF.

    For the case in the report, Cmin = (10 x 0.3 x 0.7 x 1000) / (333 x 0.075) μF = (2100 / 24.975) μF = 84 μF


    I hope this answers your question. If it has, please press the "Verify Answer" button below.


    - Daniel

  • 0 in reply to Daniel Ruiz
    Intellectual 330 points
    Hi!

    Thanks for the reply. I can understand that the output value is supposed to be taken in uF

    But when I Calculate the Capacitance as per the formula the answer is in mF.

    Could you tell me how this formula was arrived at and what is the significance of 1000 in the numerator?
  • 0 in reply to Janani Baskar
    TI__Intellectual 2910 points

    I understand that some confusion can come from how the equation is presented. Note that Fsw is in kHz and Cmin is in uF. The 1000 factor in the numerator is there to make the math work so that the result is in uF. The intent was to simplify the equations.

    You can verify the equation is correct by using base units (no SI multiplier) and removing the 1000 factor (which, again, is only there so that the units in the result work out to be uF). Using the example from the document:

    Cmin = (10 [A] x 0.3 x 0.7) / ( 333000 [Hz] x 0.075 [V] ) = (2.1 [A])/(24975 [V/s]) = 8.40e-5 [F] = 84 [uF]

    I hope this helps clarify.

    Regards,


    - Daniel

  • 0 in reply to Daniel Ruiz
    Intellectual 330 points
    As you can see, only when you remove this 1000 factor in the numerator, do you get an answer in uF.

    When I multiply 1000 to the answer you have provided the result would be in mF

    Hence the 1000 factor in the numerator is unnecessary.

    Thanks!
  • 0 in reply to Janani Baskar
    TI__Intellectual 2910 points

    To clarify: I changed two things in my last post -- (1) Removed the factor of 1000, and (2) Entered the Frequency in Hz instead of kHz. Doing one of those things without the other will give an incorrect result. If you plan to use the equation for your calculations and decide not to use the factor of 1000 in the numerator, ensure that you use the frequency in Hz instead of kHz in the denominator.

    You can also see that in all my posts, my result was 84uF.

    Cmin = (10 x 0.3 x 0.7 x 1000) / (333 x 0.075) μF = (2100 / 24.975) μF = 84 μF

    and

    Cmin = (10 [A] x 0.3 x 0.7) / ( 333000 [Hz] x 0.075 [V] ) = (2.1 [A])/(24975 [V/s]) = 8.40e-5 [F] = 84 [uF]

    are equivalent.

    The factor of 1000 is used so that one can use the frequency in kHz instead of Hz and get a result in uF.