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LM25085: Stability about constant on time regulators

Leo DENIS
Prodigy 50 points
Part Number: LM25085


Hi,  

There is no information about stability for constant on time regulators. Could you please tell me why there is no information about this ? I need fast response time but i need also that the output doesn't overshoot or undershoot too much ... 

If i have a LC, i have a phase shift of 180 ° and with the negative feedback I have 180 ° and according to the stability criterion at 360° i should have a gain relatively low  to prevent some oscillations ... I think i m missing something ... Why there is no problems of stability with this control mode ? 

I did some measurements. I injected some pertubations on my feedBack, I measured the phase of the output and the phase of the feedback by changing the frequency of the pertubations, I did the same thing with the gain. ->>>> I Did a bode diagramm :D ! 

And the phase margin and gain margin are bad ... Below 42 ° for the phase margin which is bad :( I also made some step load because it is possible that i did bad measurements previously and i saw a lot of oscillation on my output voltage ... 0A5 to 3A - there is at least 2 oscillations ... I m looking for having only one overshoot or undershoot depending on the step load type ... 

So if there is an explanation about why there is no problem of stability, i would be very happy :) 

Maybe my ripple on the output voltage is too high or too low... Is this could explain my bad stability ? And if the stability depends on the ripple it would be nice to provide some informations about this :D Like this i could do compromises between stability and ripple :D  

Thanks a lot ! :D and have a nice day ! 

  • 0
    TI__Expert 6245 points
    Hello Denis,

    COT converters have some ripple injection which introduces phase margin even with a high Q output filter. Often stability seems poor and can be improved if additional ripple is injected. I will find more information for you.

    Regards, Robert
  • 0 in reply to Robert Blattner
    TI__Expert 6245 points
    Hello Denis,

    I recommend that you look at the applications note with document number “SLVA546”. You can search for this note at the TI website. Its name is “D-CAP2 Frequency Response Model” and explains how COTs and ripple injection work.

    Regards, Robert
  • 0 in reply to Robert Blattner
    Prodigy 50 points
    Ok thank you very much Robert !

    Just one question : Why ripple introduce phase margin ? Is there an application note about this ? I m really interested to understand this affirmation !

    Have a nice day !

    Regards, Léo
  • 0 in reply to Leo DENIS
    TI__Expert 6245 points
    Hello Denis,

    Ripple injection is performed using an RC filter that emulates the inductors LR time constant. In effect, this is almost like emulated current mode. See figure 1 in SLVA546. Another way of looking at the injected ripple is that the feedback pin sees a voltage that looks as though a high ESR output capacitor is used in place of the ceramics that are really used. Since near crossover, the output capacitor now appears resistive, there is good phase margin.

    Regards, Robert
  • 0 in reply to Robert Blattner
    TI__Expert 6245 points
    Hello Denis,

    If you have no further questions, I would like to close this thread. If you would like to close it, please click resolved.

    Regards, Robert