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TPS5405 calculations error

m b1
Prodigy 80 points
Other Parts Discussed in Thread: TPS5405, TPS54260, PCM1794A

A couple of the calculations on the TPS5405 datasheet are confusing me; either I'm going wrong somewhere (probable) or there is an error on datasheet.

Firstly, in equation (1): If I plug in 300kHz I get: 21.82 * 3e5^-1.167 = 8.85e-6. It should be around 8.55e4; where did I go wrong?

Now, Equation 3: In the example, what is the fsw meant to be!? this is not stated. I tried making fsw the subject to derive it;

fsw = (Vout / ( L / ( (Vin-Vout) / (Io * LIR) ) ) ) / Vin

using (from datasheet) Vout=5, Vin=12, Io=1, LIR=0.3 and L=5.4e-6; I get a value of 1800411.5.. or 1.8MHz

Is the 5.4uH result in the datasheet right? Or is my maths wrong?? 1.1MHz is listed as the maximum switching frequency (fsw). 180kHz would seem much more likely, which would require 54uH inductance.

  • 0
    TI__Guru**** 191445 points

    For Eq. 1, you need to input the Fsw in MHz.  So R = 21.82 * 0.3^-1.167 = 88.9 k.  The datasheet claims it is 85.5 k, which is not a standard value though.

    Fsw is whatever value you set using Eq. 1.  Back tracing, the Fsw for that example is in fact 1.8 mHz if you assume Vout = 5 V whicjh is not stated. In any case, I can tell you that the inductor equation Eq.3 is correct.  It is a standard equation I have seen many times.

    I can tell you right now that this datasheet is not very good.  the design example is not complete or consistent.  It would not be allowed to be published under our current datasheet guidelines.  I will bring this to the attention of the technical owner.

  • 0 in reply to JohnTucker
    Prodigy 80 points

    Thanks. I didn't mean to call into question the equation itself, rather the fact I was unable to get a reasonable answer made me question the workings of myself and/or the datasheet.

    The TPS5405 has a fixed 5V output, and it's maximum fsw is listed as 1.1Mhz, so the example given is not only incomplete, but invalid, for this item.

    The datasheet did not seem up to TI's usual high standards; if it is going to be revised could I make some suggestions: the axes of the graphs on p5/6 aren't labelled; is y output mA? or mW? It would be nice to see 19/24v efficiency curve too, and more complete worked examples for the calculations.

    ADDITIONAL QUESTION:: what should the values for Rc and Cc be (schematic, p2); or how should they be calculated? I can't see anything that explains it there.

  • 0 in reply to m b1
    TI__Guru**** 191445 points

    I'll pass your feed back along.

    Yes Rc and Cc must be calculated.  They are the compensation components that are used to stabilize the control loop.  Unfortunately there is no information in the datasheet on how to do it.  I usually use pspice modeling to determine the compensation, but there is no model for it either.  I could possibly give you some simple equations that would get you in the ball park.  Let me know.

  • 0 in reply to JohnTucker
    Prodigy 80 points

    ^Yes please!

  • 0 in reply to m b1
    TI__Guru**** 191445 points

    I have a bit of back log right now...it may be a day or two.

  • 0 in reply to JohnTucker
    Prodigy 80 points

    would be great if you were able to let me know today!

  • 0 in reply to m b1
    TI__Guru**** 191445 points

    I recommend you try the procedure in this document starting on page 33:

    http://www.ti.com/lit/ds/symlink/tps54260.pdf

    Use gmea = 92 umho. gmps = 9 A/V and Vref = 0.8 V.

  • 0 in reply to JohnTucker
    Prodigy 80 points

    Thanks, what should be the values of Cout and Resr be for equations 41 and 42? 

  • 0 in reply to m b1
    TI__Guru**** 191445 points

    That is your choice based on your ripple and transient requirements.

  • 0 in reply to JohnTucker
    Prodigy 80 points

    So they are not the capacitance value of Cout from the schematic on P2 of the TPS5405 datasheet, and the ESR of that cap, then? I want low ripple (it is powering PCM1794a and OP-AMPs) but don't know how to translate that into the required values..

  • 0 in reply to m b1
    TI__Guru**** 191445 points

    In that same document I referenced earlier, see the "Output Capacitor" section starting on page 30.  That can help you pick an output capacitor(s).  Generally speaking for low ripple use ceramic types with X5R or better dielectric.  Make sure to use 10V or 16V rating to minimize the effect of dc bias.  I would probably not use less than 47 uF, maybe more depending on your load step requirements.

  • 0 in reply to JohnTucker
    Prodigy 80 points

    John, thanks again but I think we have our wires crossed a little here. I am trying to ascertain what the symbols Cout and Resr from equations 41/42 refer to in the TPS5405 circuit. I assume that Cout is the value of the output cap (Cout P2 of TPS5405 datasheet) in H. But what is 'Resr'? Obviously the ESR of something, but what? The inductor? The output capacitor? Both added? Something else??

  • 0 in reply to m b1
    TI__Guru**** 191445 points

    Resr is the ESR of the output capacitor.