5V to 3.3V LDO (ie LM1117) - Page 1
Forum section: electronics / projects

author: westfw
author_rank: Super Contributor
timestamp: December 12, 2014, 03:53:27 am
content: Realistically, can I do 3.3V@350mA with an SMT (SOT223) LDO regulator (that's about 0.6W dissipation), or should I be looking at other packages? How much copper pour area would I be looking at to keep this "warm" rather than "hot."(Yes, I know I can calculate this. I'm looking for real-world "fuzzy" experience. A lot of designs throw a 1117 on a PCB and say "look, a 3.3V 1A regulator." I'd like to do better than that, without having to work too hard :-) )

author: marshallh
author_rank: Unknown
timestamp: December 12, 2014, 03:57:07 am
content: No problem, just put a bit more space on hte tab and some vias on it.

author: sleemanj
author_rank: Super Contributor
timestamp: December 12, 2014, 04:41:21 am
content: Have a read of page 5 of the AMS1117 datasheethttp://www.advanced-monolithic.com/pdf/ds1117.pdfparticularly with regard to...Quote from: datasheetIn other words, even just having a ground plane on the other side or an internal layer, not connected to the device electrically, will have a significant improvement in dissipation.Also included is a table of guidelines giving junction to ambient for various copper/board areas, note that they appear to have the bottom side of their test board is complelety covered with copper without electrical connection (as indicated above), and that only the smaller top side area is connected (to the tab).

author: westfw
author_rank: Super Contributor
timestamp: December 12, 2014, 09:29:44 am
content: QuoteWhere they, alas, have examples that are pretty depressingly large.(80C/W for 2600mm^2 of (combined) copper area. 55C/W for 5000mm^2.)Sigh.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 12, 2014, 09:59:12 am
content: Yes, it's quite true, in a multilayer board, having solid copper on the adjacent layer is almost as good as having solid copper on the same layer.I saw the results from a UL surge test last month, test was something like 80A through a 100 mil trace. Poof! But guess where it failed: not at a pin, or at a neck, but right where the trace crossed an unusually wide gap on the adjacent inner layer.In fact, necking and thermal spokes are almost blameless, thermally and electrically. There is very little thermal or electrical resistance in such a short length of narrow material (and the added inductance of current going very slightly out of its way amounts to picohenries), so the added value to rework (easier to solder) can be valuable, especially for through-pins connected to inner layers, which can heatsink so well that the entire pin never melts!Tim

author: Howardlong
author_rank: Super Contributor
timestamp: December 12, 2014, 10:10:45 am
content: Choosing a regulator with the tab as ground helps in the thermal mitigation.Also the LT1117 draws a depressing 5mA quiescent. This migt or might not be a problem for your design, but makes it near useless for small portable battery powered devices.

author: tom66
author_rank: Super Contributor
timestamp: December 12, 2014, 10:15:18 am
content: Personally, beyond ~0.3W in a SOT223 I start to get a little nervous. It's too much power for a small device, having seen quite a few 1117 regulators fail on other equipment (mainly LCD TV main boards...) I would try a buck converter, like LMR10515. You only need a small inductor, two capacitors and two resistors to make it work. The solution is likely to be smaller too, and the no load quiescent current is ~3mA, less than the 1117. Or, for much lower Iq, something like TLV62565. Fixed voltage versions may also be available reducing BOM size further.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 12, 2014, 12:50:56 pm
content: Personally, beyond ~2W in a SOT223, SOT89 or SO-8, I get nervous.That's assuming enough copper pour to handle that. At least 1 in^2.If they're using minimal pads, in a tight enclosure that gets way too hot, sure, 0.3W is getting into the stupid territory. But that's consumer crap for 'ya...'1117 is bad for another reason: it's not an LDO. It has a minimum dropout voltage at any load current, because it's built with a Sziklai output stage (as opposed to the Darlington ~1.2V in the original '317). The 1.7V drop in this case isn't too bad, but it's kind of marginal, especially if the 5V has a pretty sizable variation (like 10% regulation).I don't have any recommendations offhand, but there are quite many out there. Be sure to inspect quiescent current versus supply voltage (if there's no graph or maximum limit, go elsewhere -- it's probably a hog under out-of-spec conditions), especially for bipolar types, and stability under capacitive load (if there's no discussion or graph of C value and ESR range, go elsewhere).Tim