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TPS57114-EP: Switching Freq formula wrong

Ray Vincent Aranzaso
Mastermind 18180 points
Part Number: TPS57114-EP

Hi,

Good Day. I have a customer who is working with TPS57114-EP. Please see below his query for your reference. Thank you very much.

The Switching frequency formula (fsw = 171,032/RT^0.974) in page 17 does not match the RT/Freq plot on page 9.  The plot on page 9 does however match the values I get on the Webench designer.

Could you please confirm that the formulas in page 17 are not correct?

Best Regards,

Ray Vincent

  • 0
    TI__Genius 14876 points

    Hello Ray,

    The equation for the frequency is obviously an empirically-derived equation and not from any theory or calculations (since the numbers are not even/round values).  I believe the equation comes from the Excel curve fit of Figure 6 , which is probably characterized on one or a few devices.  Any of the equations should give very similar results--for sure the results should be well within the tolerance of the frequency itself (which is specified only at 500 kHz).  You will have additional variation outside of the equation from device to device, over temperature, etc.

    How is this an issue for the customer?

    Please, let us know.

    Thanks a lot!

    Regards,
    Dorian

  • 0 in reply to Dorian Brillet de Cande
    TI__Mastermind 18180 points

    Hi Dorian,

    Good Day. Please see below the response of our customer to your reply. Thank you very much.

    Well, the difference between the equation result and the plot is not insignificant. For instance, using the equations, I get a Fsw of 1.026 Mhz with an RT of 191K. But, the plot (and Webbench) give me a fsw of 0.902 Mhz with the same resistor value of 191K. That is a 10%+ difference,
    Also, the datasheet says that to get the min freq 0.3Mhz I have to use an RT of 700Kohm, while the plot shows a 604K, again, quite a significant difference, 

    Best Regards,

    Ray Vincent

  • 0 in reply to Ray Vincent Aranzaso
    TI__Genius 14876 points

    Hello Ray,

    Thank you for the valuable feedback! I will bring this topic back to our Systems team and see if we can improve this empirically-derived Equation in the datasheet.

    For the time being, I would suggest that customer refer to Figure 6 which refer to characterized data.

    Thanks again and let me know if I can help any further.

    Regards,
    Dorian

  • 0 in reply to Dorian Brillet de Cande
    TI__Genius 14876 points

    Hello Ray,

    Do you have any further questions here?

    Let me know if I can help.

    Regards,
    Dorian