Board review/sanity check, lm2576 .2v output, icsp issues? - Page 1
Forum section: electronics / beginners

author: XOIIO
author_rank: Super Contributor
timestamp: June 26, 2025, 11:07:53 pm
content: Hi all, so this is my first attempt at a buck supply with a switching regulator, I went for an LM2576 as it was fairly simple layout wise, however when applying power in, I hear coil while but only get .2v output.I was working on this iteration of the board middle of the night a couple nights in a row and only realized a while after ordering that I probably undersized the inductor just compared to other adjustable supplies I checked that use the same regulator, and I'm wondering if that's potentially the cause, or if the one I have would be good enough. Looking for SMT inductors on digikey wasn't bringing up loads and this was about the best I could find.On a similar tangent I did use a tantalum poly capacitor as well, however I do have some larger electrolytics I could bodge in, though I feel like that wouldn't cause it to have such a low output.It's also my first crack and slapping an mcu onto a board directly as well instead of another board with pin headers and and so far it seems to not be getting picked up properly either with my usbasp, but this is a very old one, so needs a firmware update and may have other issues.Unfortunately tomorrow I'm having my wisdom teeth removed and they're pretty f**ked, so I'll likely have a slow recovery, but I won't be able to reply to this super quickly. Just hoping to have my design glanced over in case I missed anything super obvious. LM2576 section is just based off the datasheet, and the atmega328pb connections off the minicore pinout on github.(I did also forget to order one size diode and popped an LED in for D2 and that could very well be affecting it since that's not the spec at all lol).Probably should have posted before ordering the boards, but a month of working on personal projects until 3/4 am after work and being impatient doesn't work well with waiting on shiny new parts. It's also my most complicated board layout so far and I'm pretty rusty so it's definitely not ideal layout wise. I'd appreciate some gentle pointers to improve things as well. This went through some other revisions but to save time designing parts and such, connector positions and overall size had to stay the same which wasn't great for how I would have rather done the psu section.https://www.digikey.ca/short/rz57v8n8Several spare cap options in there for C5 but I used 718-2180-1-ND for the time being. Starting over I probably would have tried mouser to find larger smt inductors, and go for electrolytic as well, though I'd need a couple in parallel as one large one wouldn't fit the case designed for this.edit: oh man, so I just realized I intended to stack the tantalum caps to get much closer to the spec. I've also seen different capacitor and inductor spec for different datasheet versions so wasn't quite sure if 330uH was what I needed. Another one I just checked shows 100uH. I didn't keep track of which one I had used.

author: thm_w
author_rank: Super Contributor
timestamp: June 26, 2025, 11:25:41 pm
content: D1 is backwards in your schematicThere is no input capacitance on the LM2576 in your schematic, 100uF is recommended in the datasheetNot sure what the purpose of D2 is but don't think its going to have any negative effect being an LED.In the future, if you are not 100% sure of your design, start by just building the PSU section, power that on see if it works. Then solder on the other IC's one at a time.

author: XOIIO
author_rank: Super Contributor
timestamp: June 26, 2025, 11:35:36 pm
content: Quote from: thm_w on June 26, 2025, 11:25:41 pmOh frig thought I fixed that this time around. D1 has been like that for a while and I just correct it when soldering. It's just for reverse polarity protection.I did miss the input capacitor though.. damn. Not sure how I did that, maybe based off an example I spotted somewhere or something. That's easy to bodge for the test boards at least. Would that cause it to be totally non functional though? Not sure I have one around currently. Probably don't have time to check through my spares.D2 is for the reset pin if I want to use the FTDI header, wasn't sure how well icsp would work so I added both for now, thinking worse case I can pull a chip off a pro mini clone, or program it off board. (D1 in this schematic that I forgot to link)https://github.com/MCUdude/MiniCore/blob/master/avr/images/minimal_setup_TQFP-32_PB.png

author: XOIIO
author_rank: Super Contributor
timestamp: June 28, 2025, 07:42:52 pm
content: Ok, surprisingly feeling better-ish much faster than I figured, dug around and found an adjustable buck psu with an LM2596s to compare. The inductor physically is far larger, but also 330uh. build up another board with just the voltage regulation stuff and perhaps bodge this one in with wires perhaps as a test.Still on a final dose of oxy so not super clear headed, also not able to eat much but f me, there's a designer for power supplies on their website that I think will spit out part numbers and such, uhg.https://www.ti.com/tool/WEBENCH-CIRCUIT-DESIGNERStill though with no load on the output, not sure why the 450ma saturation current of the inductor would be an issue if one at all, surely that would start to flake out once I had a higher load.