LDO regulators.

[nigelwright7557]

I recently built up a circuit with a PIC32MX220 and a few other analogue components.
The first pcb worked fine.
So  I built up a second one and it didn't work.
The problem turned out to be lots of noise on the 3v3 line.
I looked up the 3v3 ldo regulator spec and I had a 100nf instead of a 2u2 capacitor on the output.
So I swapped it for 10uf.
So  I built up a third one wit h10uf in it and that was noisy too !
It needed a 47uf to get rid of the noise.
These ldo regulators are great with low drop out voltages but can be a bit unstable even if care is taken with pcb layout.

[ANTALIFE]

So what LDO did you end up using, also post your schematic/pcb too ;^)

[mzzj]

"ESR, Stability, and the LDO Regulator"
http://www.ti.com/lit/an/slva115/slva115.pdf

http://www.ti.com/lit/an/snva167a/snva167a.pdf

[CJay]

Yeah, I found that out too after making the (stupid, I know better) assumption that pin compatible 3 terminal regulators are an easy swap out, it's definitely a good idea to read the datasheets and not assume anything with them so there's a few tubes and a box full of LDO regulators in my W/shop with very clear labels advising me how to use them.

Layout wasn't so critical for me (either that or I did it right without trying) but the bypass and filter caps, most important or the 'regulated' rail was awful.

[james_s]

I ran into a similar problem recently. I was repairing a keyboard for an old serial terminal, didn't take long to figure out that the 7805 was dead. I didn't have any 7805's in my stash but I had some other 5V 3 terminal regulator salvaged from something or other so I installed. that. Keyboard powered up but didn't work properly so I spent hours troubleshooting and testing ICs before I had the sense to scope the 5V line and sure enough, crazy oscillation.