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PSRR of TPS717xx

xiangkai zhu
Prodigy 160 points

My question is about this app note  linked below .

http://www.ti.com/lit/an/slyt280/slyt280.pdf

My question is how does this formula come from?

  • 0
    TI__Mastermind 23361 points

    Hi Xiangkai,

    Normally the f should be included in the Z of any capacitor. For example, Zcpar1 = 1/ jwCpar1; w = 2pi*f. Usually, the term PSR is used when using a shunt-feedback model. gm, f, beta are the terms from the reciprocal of loop-gain transconductance. There are different ways to represent this equation, since this application has not shown the details of how this equation has been obtained. It's very hard for me to identify if this "f" is an extra. What do you think?

    Regards,
    Jason Song

  • 0 in reply to Jason Song
    TI__Mastermind 23361 points

    Hi Xiangkai,

    If you are interested to get the PSR equations, I would suggest you to read this IEEE paper. 

    https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=1362447

    As you can notice, the application lists this paper as the first reference, and this paper explains in details about the derivation of the PSR equation used in the app notes. I am closing this post for now, feel free to post any comments if you have any questions in the paper.

    Regards,
    Jason Song

  • 0 in reply to Jason Song
    TI__Prodigy 160 points

    Hi Jianan,

    I still have some questions on this ieee paper.

    The main questions are derivation of the Equ.2 and  modeling of Fig. 2 .

    Can you help me with your answer?

  • 0 in reply to xiangkai zhu
    TI__Mastermind 23361 points

    Hi Xiangkai, 

    After I reviewed the equation on paper and application note, I found neither explains the equation clearly. I have included my own derivation using the shunt feedback model. 

    Let me know if you have any questions. 

    Regards, 
    Jason Song

  • 0 in reply to Jason Song
    Prodigy 30 points

    Hello Jason,

    Thanks for your explain, and I still have some questions:

    1. How to transfer the model in page 1 to page 2? I don't quite understand.

    2. For the final result, Vout/Vin seems to be not directly related to frequency. Then how can we use it to explain the behavior of PSRR verus frequrncy?

    BR//

    Ling Qin

  • 0 in reply to Ling Qin2
    TI__Mastermind 23361 points

    Hi Ling, 

    1. How to transfer the model in page 1 to page 2? I don't quite understand.

    Page two is the small-signal analysis using the shut feedback model shown on the right part of Page1. 

    2. For the final result, Vout/Vin seems to be not directly related to frequency. Then how can we use it to explain the behavior of PSRR verus frequrncy?

    As you see, the equation I did here is only for shut feedback impedance, you will need to follow the paper for the rest of the discussion on how PSR equation can be rewritten using this shut feedback impedance. 

    Regards, 
    Jason