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TPS2500 - Low frequency oscillations

Other Parts Discussed in Thread: TPS2500, TPS2501

I've replicated the example layout of the TSP2500 as much as possible, and I'm seeing what looks like a low frequency switching oscillation at the output, which is coupling large spikes onto the 3.3V power supply that is feeding the TPS2500.  Under no load conditions, the frequency is 20 Hz, and with a load (a USB flash drive) the frequency changes to 360 Hz.  The magnitude of the ripple is large at ~200 mV.

Reading the datasheet I'm not seeing anything these frequencies relate to, and they are orders of magnitude slower than any of the frequencies stated in the datasheet.  Can anyone tell me what could be causing these low frequencies?

Attached are screen shots of the output voltage from the TPS2500, the input with associated spikes, and a schematic of my implementation.