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TPS50601-SP: AC simulations show ample stability, but transient simulations show motor-boating

Part Number: TPS50601-SP

I am designing a +1.8V supply using the TPS50601 PSPICE models.  My AC simulations indicate ample phase margin (more than 73 degrees) and gain margin (more than 61dB), and yet the transient simulation shows what looks like instability when I set my load resistor to values higher than 10 ohms.

In another design where I'm using the same PSPICE models to design a +1.9V supply, and I'm also seeing the same issue where there is more than 75 degrees of phase margin and more than 50dB in gain margin.  However, I did stumble on a simulation accuracy setting that made the unstable behavior go away.  I cannot apply this same setting for the +1.8V design though, because with that setting it fails to converge after about 1.6ms into the simulation.


Is the TPS50601 transient PSPICE model known to display false instability behavior?  How can a stable design in the AC domain be unstable in transient simulations?

  • Hi Peter,

    This is not an issue we have encountered before. As you mention, the simulation settings used for transient simulation can impact the accuracy of the results. What step size are you using? And what simulation settings?

    Can you share your files or schematic so we can try to replicate your issue and provide more detailed feedback? Without more information it is hard to tell what the root cause could be.



  • Hi Sarah.  I'd be happy to send my PSPICE testbench and simulation settings.  Please provide an email and I'll send it over.


  • Thanks Peter, I received your files and follow up here with any additional comments, questions, or guidance.

    - Sarah

  • Hello Sarah.  After setting the max step size to 20ns, the motorboating issue disappeared!  The simulation also ran faster ironically, in contrast to the original max step size of 1us.

    Thank you very much for your help!