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TPS544B25: Target Ripple and Undershoot not mating

Rajat Singh
Intellectual 290 points
Part Number: TPS544B25

Hi Team,

I was designing power rail for FPGA.I have used excel sheet for designing and then I tried to simulate it on PSPICE.I am not able to mate the target Ripple value and Undershoot.Please have a look and let me know what i am missing.

Designed Rail details are-

Vout -0.88V

Iout- 10A

Ripple - 12mv (peak to peak)

Undershoot - 10mV

I have attached excel sheet and PSPICE SIMULATION File.TPS544B25_Calculator Tool_0.88_10A_MGTAVCC.xlsx0.88_10A.zip

  • 0
    TI__Mastermind 30780 points

    Hello Rajat, 

    The undershoot, do you know what is the the current step (?A to ?A lad step), I see 7A in the spreadsheet.  and the slew rate (??A/us). 

    we we do the PSICPE modeling, the model does not take in consideration the derating, we need to make sure that is taken in consideration by derating the values properly and also have the right ESR of the COUT capacitor, that is important.  

    Thanks, 

    Tahar

  • 0 in reply to Tahar Allag
    Intellectual 290 points

    Hi Tahar,

    The current step is 0A to 7A.

    Slew Rate you can take 1A/us

    We have used same capacitor in schematic which is used in calaculation sheet. Plese let us know how can i achieve the above Targeted voltage rail value.

    Many Thanks

    Rajat

  • 0 in reply to Rajat Singh
    TI__Genius 11315 points

    Hey Rajat,

    Looking at both your startup and transient schematic, it seems that you are missing Rbias as seen below.

     

    Try these edits on your schematic and let us know the results. On my simulation I have seen a 20mV undershoot. I tried to match your DCR and ESR. The ESR on your capacitors seems to be the issue. The spreadsheet recommends 0.3mOhms.

    Thanks,
    Caleb

  • 0 in reply to Caleb Perez
    Intellectual 290 points

    Hi Caleb,

    I am not sure why you have used RBIAS for 0.88V.Can you please let me know

    I have set VOUT SCALE LOOP=1 so as per below image for 0.88V there is no need of RBIAS.

     

  • 0 in reply to Rajat Singh
    TI__Genius 11315 points

    Hey Rajat,

    You are correct, my mistake. I've fixed your calculator sheet. Your capacitance and bandwidth were too low to support your undershoot requirements. The excel sheet attached has component values that give a 6mV undershoot. You can double check by running these new values in your simulation. Let me know if you have any questions.

    TPS544B25_Calculator Tool_0.88_10A_MGTAVCC (1).xlsx

    Thanks,
    Caleb