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How I leared what a decoupling capacitor is for, the hard way

28 points by actinium226 3 days ago | 6 comments

oakwhiz 2 hours ago

Seems like a missed opportunity to try adding a capacitor dead-bug style onto the board to see if it cleans it up.

If it's really 20MHz++ noise that's screwing him, you need something faster than a through hole capacitor IMO to deal with it.

That being said, I'm not 100% convinced this is a 20MHz++ noise issue.

hadrietta 42 minutes ago

Having 1.5V Vpp ripple on a 3.3V supply rail seems more like an issue with the regulator / bulk capacitance than a decoupling capacitor, I would think?

actinium226 25 minutes ago

Yea since writing this I think it has more to do with the regulator circuit. I plan to do a small rewrite and change the title to something like "When 3.3V isn't actually 3.3V" to more accurately reflect the situation. A decoupling cap would probably still help, but there were some mistakes made on the regulator circuit.

dragontamer 19 minutes ago

Switching regulators (and even linear regulators!!) have maximum capacitance ratings.

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).

dragontamer 38 minutes ago

Yeah. Decoupling capacitors are for smaller ripples than that.

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.