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TPS57140-Q1: Dimensioning in spreadsheet gives different numbers than Webench

Franz Schellhase
Expert 3605 points
Part Number: TPS57140-Q1

Hi Team,

a customer is dimensioning the TPS57140-Q1. They are using a TI spread sheet for the TPS54x family called 'TPS54x40 & TPS54x60 Current-Mode Step-Down Converter Design Calculator', which they already had and Webench.

System requirements are the following:

  • Vin: 8-32V
  • Vout: 6V
  • Iout: 0.565A

The spreadsheet puts out a max frequency of 1480 kHz, Webench on the other hand only 652 kHz. Of course the higher one would be better because smaller inductors could be used. So a frequency of around 935 kHz would be the goal for that. Which tool should be trusted? Is it possible to use TPS57140-Q1 safely at 940 kHz?

Thank you and best regards,

Franz

  • 0
    TI__Mastermind 49647 points
    Hi Franz,

    I forwarded your enquiry to the product expert.

    Regards,
    Tim
  • 0
    TI__Mastermind 49647 points
    Hi Franz,

    The switching frequency of the TPS57140-Q1 is adjustable over a range from approximately 100kHz to 2.5MHz by placing a resistor on the RT/CLK pin. See equation (7) in section 7.3.10 of the datasheet and Figures 31 and 32 for guidance on resistor selection.

    The switching frequency choice is essentially a tradeoff of efficiency vs. size. See this blog for more detail:
    e2e.ti.com/.../the-buck-regulator-efficiency-size-tradeoff-dilemma

    Regards,
    Tim
  • 0 in reply to Timothy Hegarty
    Prodigy 80 points
    Hi Timothy,

    I am the customer you asked Franz about the differences from the Webench tool to the Excel Product Designsheet we have. In cooperation with Franz, we decided that I will correspond directly to you.

    If I understand it right, I need to simply fulfill or have to be lower than the result of the equations (8) in the datasheet for the minimum controllable on time and the frequency-shift protection. This formulas are checked in the Excel product design sheet. So I think it should be safe, if we design with a switching freqeuncy, which is lower than the lowest of the calculated value.
    Do you agree?

    But why reports the Webench tool an essential lower switching frequency as the worst case maximum switching frequency?
    Maybe because it takes more component tolerances into account?
    Or is that simply a safety margin added?

    Regards
    Jürgen
  • 0 in reply to Juergen Mayr
    TI__Expert 6860 points

    Juergen ,

    You should be able to switch at 900KHz without any issue . You have enough margin her e.

    Ambreesh