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I'm about to put a LD1117 Low drop-out regulator in SOT-223 package into one of my projects and wonder if the heat sink is internally connected to anything.

Unfortunately the data-sheet doesn't mention the heat sink lug at all but states that the device is "pin to pin compatible with the other standard voltage regulators".

I'd like to solder the heat sink lug to ground for best thermal contact, but since it isn't stated in the datasheet I'm hesitating a bit.

So what to do? Connect the heat sink lug to ground or better not?

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  • \$\begingroup\$ You can follow this design procedure and make a proper design, even without a good datasheet: electronics.stackexchange.com/a/10762/411 . Physics is physics, so all silicon semiconductors have similar maximum junction temperatures, and all SOT223 have similar junction-to-ambient thermal resistance. \$\endgroup\$ – markrages Oct 6 '15 at 5:55
  • \$\begingroup\$ (but don't ground the tab, as the answers have pointed out) \$\endgroup\$ – markrages Oct 6 '15 at 5:56
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ST's LD1117 data sheet seems to be missing information about the tab's connection.

The LD1117 is electrically compatible with other voltage regulators like TI's LM1117

Looking at page 2 of the datasheet, the SOT-223 package diagram states "Tab is Vout".

So, you could solder the tab to a large area of copper on a PCB, or a heatsink, but they must be insulated from ground, and could be connected to Vout.

Edit
FYI Ultimately, I didn't use the LM1117 because I needed to get heat out on a small PCB, so I used a Micrel MIC5209, which does have the heatsink tab connected to ground. Their are a bunch of parts with that configuration of a ground heatsink tab, but IIRC they were all noticeably more expensive than an LM/LD1117

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  • \$\begingroup\$ Guess which datasheet I was reading? The ST one with the missing information. Thanks for pointing out the other one. \$\endgroup\$ – Nils Pipenbrinck Oct 6 '15 at 1:43
  • \$\begingroup\$ @NilsPipenbrinck - You're welcome. I usually go to ST for their datasheets, but TI is noticeably better in this case. I remember looking at that part a couple of years ago, and making the assumption that the tab would be ground. Then someone (on LeafLabs forum) was recommending it (because its very low-cost) so I checked before actually designing the PCB, and found the datasheets for equivalent parts was better. \$\endgroup\$ – gbulmer Oct 6 '15 at 1:45
  • \$\begingroup\$ Did you try looking on page 6? "Note: The TAB is connected to the VOUT." \$\endgroup\$ – Ignacio Vazquez-Abrams Oct 6 '15 at 1:48
  • \$\begingroup\$ Doh! I read the ST datasheet more than two years ago, and went to the TI datasheet instead :-/ Ultimately, I didn't use the LM1117 because I needed to get heat out on a small PCB, so I used a Micrel MIC5209, which does have the heatsink tab connected to ground. \$\endgroup\$ – gbulmer Oct 6 '15 at 1:54
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The tab is connected to the voltage output. Either connect it to the output supply rail or leave it floating.

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