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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?