Electronic – Buck converter ringing on rising and falling edges before failure

There is a lot of ripple current in the input caps in a buck converter. However, if you have a good solid ground plane you don't need to worry about "frequencies of several hundred MHz being loaded on ground of input." Further, your large output caps will have a self-resonant frequency below 100s of MHz and will essentially be inductive at those frequencies.

In all the many hundreds of buck converters I've laid out or looked at I've never seen a problem where input ground noise couples through the ouput cap's ground to the output. Many of these had a close low-inductance connection between input and output caps. Where are they suggesting the output reference is anyway? Wouldn't it be the negative terminal of the output cap?

This is not to say that ground noise can't be a problem. Poorly designed grounding schemes can absolutely lead to a noisy output. It's just that tightly coupling the grounds of the input and output caps is not a problem in my experience.

Basically this is a non-problem. You don't need to do suppress the free-wheeling ringing due to the inductor and drain-source capacitance of the open-circuit MOSFET (more likely than the diode) in a buck converter because nothing bad happens if you leave it alone. The voltage in both polarities is never bigger than the voltage due to switching so the MOSFET cannot harmed by it.

It's totally different on a flyback converter of course but this is a non-synchronous buck converter.

Just regard it is a little bit of energy that could not make it's way to the output.

After-thought - if you were really clever you might be able to find a way of harnessing these oscillations and feed that energy back to the input capacitor. That would certainly be a step in the ecologicial right direction.

The device data sheet has this circuit for reducing output ripple by up to 10 times: -

The LC network will rely on the ESR of the capacitor added being as low as possible and quite probably if you are using "any old" capacitor in the Cout position (ref diagram above) then its ESR and ESL will be poor. TI are not recommending a snubber to reduce ripple!