I had a working design of a buck switching regulator based on a TPS62125. It worked fine, with minimal (<100mVpp) noise. I made some changes to my design, seemingly unrelated to the switcher. The result is audible noise from the switcher (a high pitch whine, that gets louder as draw increases), as well as this lovely waveform:
Here's my current layout:
The only changes I made to the switcher directly is adding room for a 250uF electrolytic cap (problem persists with and without it), adding a .2mF supercap elsewhere in the design, but off this power rail (problem also persists with and without), and adding a footprint for C33, but not populating. The amplitude of the noise does not seem to be affected by load.
Where do I go from here? How do I troubleshoot this? I thought I spotted a short between the two pins of the IC that lead to the two pads for the inductor. It doesn't look like there's an actual bridge while looking with a loupe, but could this be the source of the problem (pins are too close because of excess solder, creating some sort of stray capacitance)?
The only other thing I can think of is that it's possible that I installed a wrong capacitor for the input or output caps (did this by hand). Would that produce these results? Could it be a bad inductor?