There might be a resonnance point on that regulator, or maybe a maximum capacitance that was violated on the feedback.
There are a TON of ways to screw up your PDN on a PCB. It's nominally a master's degree level subject.
Adding more capacitance could, in theory, further destabilize your regulator.
The overall tank circuit (the inductor + capacitor forming the bulk of the switching circuit) is incredibly fragile.
It's legend that some old switching designs stopped working as newer tantalum capacitors had less resistance, screwing with the stability of older switching designs. You kind of need to choose exactly the "expected" kind of capacitor (aluminum caps have more resistance, which increases stability of the feedback but slows down the feedback).
oakwhiz•1h ago
dragontamer•1h ago
That being said, I'm not 100% convinced this is a 20MHz++ noise issue.