• State Resolved
  • Locked Locked
  • Replies 3 replies
  • Subscribers 38 subscribers
  • Views 557 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

Online SwitcherPro value choices

Jeff Lindstrom
Prodigy 20 points
Other Parts Discussed in Thread: TPS54331, TPS54610, TPS54620

I have been comparing the results of SwitcherPro designs with the specs for the TPS54331 and am seeing discrepancies.

 

For instance, the resistors chosen by SwitcherPro for an output voltage of 5.2V are 10.0K and 1.78K, but the formula in the spec sheet says that those values would result in 5.3V and a better choice (using standard values) is 10.0K and 1.82K.

 

The same thing happens for the EN input, with the recommended values (for a minimum input voltage of 6.1V) being 348K and 75.0K, but changing the formula to solve for Vstart and using those values yields 6.7V, way too high for my requirements.

 

Has anyone else noticed problems like this?

 

Jeff

 

  • 0
    TI__Expert 3875 points

    Hi Jeff,

    You did not post all of your design specs. So, to answer your questions, I will use the following design which appears to be similar to the one that you describe:



    As far as the output voltage is concerned, please take a look at the "Analysis" tab, "Output1" sub-tab. You will notice that in the first row of the table, SwitcherPro gives you an actual "Output Voltage" range from 5.049 to 5.547:



    So, Vout = 5.2 is the target, but if you build a large number of these circuits, SwitcherPro is predicting that you will see a distribution of Vout between 5.05 and 5.55. This calculation is based on the tolerance of R1 and R2 (SwitcherPro is assuming you are using 1% resistors) and the allowed variation in theTPS54331 voltage reference, which according to the data sheet is, 0.8V +/- 3.5%. If your application requires a very well controlled Vout value, please consider a part with a trimmed reference, like the TPS54620 or TPS54610.

    You are correct in your second observation. R6 and R7 are calculated incorrect. SwitcherPro tries to estimate good values of UVLO start and stop. But for this device, the calculation produced a set of values(UVLO_Start = 6.74 and UVLO_Stop 5.73) that will not work (unless you operate at Vin_max). I will ask the developers to fix the calculation.

    Best Regards,
    Michael Krasnicki

    Michael Krasnicki
    Analog eLab
    Texas Instruments

  • 0 in reply to Michael Krasnicki
    Prodigy 20 points

    Hi Michael,

    Thanks for your confirmation of R6 & R7, although I think it would also apply to R1 & R2 as I was only considering the nominal output, not the range of outputs due to tolerances.

     

    Regards,

    Jeff

     

  • 0 in reply to Jeff Lindstrom
    TI__Expert 3875 points

    Hi Jeff,

    SwitcherPro uses a different equation flow than the data sheet. We consider small variations, such as 1.72k vs. 1.82k, to be acceptable given the overall tolerance variability of the generated design.

    Regards,
    Michael

    Michael Krasnicki
    Analog eLab
    Texas Instruments