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TPS54561 - Swinging - compensation

Christoph Klees
Prodigy 220 points
Other Parts Discussed in Thread: TPS54561

Hello,

I have a Problem with the DcDc-converter, when I do a load jump from 0A-2A@Vout=24V [Vin = 44V]

 

I use the compensation Network which is calculated by webench, but the result you find attached.

I describe the different colours :

green : Load jump

Violet : Vout = 24V

yellow : comp Pin

The first Picture Shows the unstable state and the second the stable state.

Third Picture is the Regulator with all components.

What can I do to stabalise the Regulator ?

Kind regards

 

blue : Switch

 

  • 0
    TI__Guru**** 191445 points
    What is the difference between the first and second waveform? Just zoomed in to a different time? Bottom trace is the SW node correct? It appears to definitely be under damped. If you have large transients with fast slew rate you may also need more output capacitance.
  • 0 in reply to JohnTucker
    TI__Guru**** 191445 points
    I see there is a pspice transient model for TPS54561 in the product page. The first thing I would probably do is to model your circuit to see if you get the same result of under damped response.
  • 0 in reply to JohnTucker
    Prodigy 220 points
    I do different measurements, maybe the risetime of the load current is a little bit different.
    The first one didn't stop swinging, althoug I Change the load.
    The second one stabalise the Output after a few µs.
    Bottom (blue) is the Switch node, yes.
    One question regarding the Switch node. I thought, that I have a fixed frequency, only the on time of the Switch varies.
    In the datasheet I didn't find this state, that the mosfet in "on" for such a Long time. Can you explain me what happens inside the TPS54561 ?

    What means "under damped" ? Can you give me a hint what I should do ?
    A few weeks ago I asked in this Forum which values I should take for the compensation Network, the one from webench or the calculated by myself with the formulas of the datasheet. The values had great differences.
    The answer was webench and the Output is Swinging.



    Christoph
  • 0 in reply to Christoph Klees
    TI__Mastermind 35185 points

    Hi Cristoph,

    Do the output capacitors on your schematic match those recommended by WEBENCH? I put your requirements in and the recommended output capacitance is 2x 47 uF but your schematic has 2x 22uF. The compensation network is also about the same as yours. Using different output capacitance than recommended by WEBENCH could cause the circuit to be unstable.

    Best Regards,
    Anthony

  • 0 in reply to Anthony F
    Prodigy 220 points

    Hy Anthony,

    thanks for the hint !.

    First I changed the output capacitor in webench and yes, the system is instable.

    Second, increase the capacitor in reality and the Regulator is stable.

     

    Kind regards,

    Christoph

     

     

     

  • 0 in reply to JohnTucker
    Prodigy 220 points
    We havn't pspice. I tried to use the model in LTspice but the model is coded and LTspice couldn't use it.
    Maybe in future you support LTspice......

    Kind regards,
    Christoph
  • 0 in reply to Christoph Klees
    TI__Guru**** 191445 points
    I doubt we would ever support LT spice. But I am glad that your problem is resolved.