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TPS40040: unable to change part value??

Other Parts Discussed in Thread: SWITCHERPRO, TPS40040

I'm using the TPS40040 in a design. SwitcherPro do not able me to change the resistor RefDes R5 in my simulation (resistor that fix the output voltage), even if the RefDes text is in red!! It's poping me a window, but only the RefDes can be change, not the value.

The name of my simulation is "TPS40040 5V to 1.8V @2.8A rev2 Modification good".



  • Hello,

    You can change one of the resistors dividers, but not both since the other one is auto-calculated. We are letting the users change the top divider (R6 in your case) and bottom divider is auto-calculated.

    If you want to change your output voltage, you can do that by clicking on the 'Edit Design Properties' button on the left side of the top bar where your design info (Design name, part number, VinMin, VinMax etc.) is displayed.

    If you want to keep your passive components, remember to uncheck the option 'Allow switcherPro to pick components'.


    Hetal Joshi

    Senior Developer and Project Manager

    Analog eLab Design Center

    HPA Design Tools

    Texas Instruments, Inc.


    TI makes no warranties and assumes no liability for applications assistance or customer product design. You are fully responsible for all design decisions and engineering with regard to your products, including decisions relating to application of TI products. By providing technical information, TI does not intend to offer or provide engineering services or advice concerning your designs.

  • Hi Hetal,


    Thanks for the hint.

    But in that case, it seems that SwitcherPro is not very accurate for the resistor values. In my case, I try to get an output voltage of 1.8V, and the simulation gives a much higher output, which is 1.816 to 1.885V!! It seems that SwitcherPro doesn't has a proper resistor list completed. I was expected a value of 4.99K . I try several voltage output around 1.75V and 1.8V (through "Edit Design Proterties"). The only value that came out is 4.87k and 5.11K. For my case, it's not very accurate. Can we expect a better accuracy of the simulation?




  • Hello,

    4.99k is a 1% standard value, but at this point switcherPro automatically picks 2% resistors as standard values, therefore you get 4.87K or 5.11K depending on larger or smaller standard values.

    We have a plan to introduce a standard value preference into SwitcherPro.  You would set this up just one time and then all designs after that would use your preference either 1,2 or 5% standard values for standard caps and resistors. 

    Sorry for any inconvenience this may have caused you.  You can typically change out the R2 resistor (or bottom) voltage divider in the feedback.  If you put a 1% value there it will work as expected and just give a closer output voltage.  If you need an exact output voltage it's probably better to use two resistors to calibrate it to an exact number.  SwitcherPro is not setup to do this kind of work.  Right now it gives you a bit higher output voltage than you need, just as you have in this case.  you designed for 1.6V out and it gives you 1.885V out.