Take the classic LM1117 linear regulator http://cache.national.com/ds/LM/LM1117.pdf
Pin 2 is Vout. The tab is also Vout. From a component layout perspective, it would be nice if I could connect the output capacitor to the tab instead of the pin.
What is the functional difference between the tab and the pin? Does one have less current carrying capability than the other? Can I leave the pin floating and instead connect the tab to the output capacitor? Must I hook the output capacitor to the pin? Can I just short the pin to the tab?
Answer
There is no functional difference between the tab and the pin.
On a lot of SMT versions of these kind of devices the middle pin is cut off and not used - just the tab is used:
Tabbed packages are often used for SMT parts which, despite their small size, have to dissipate up to a couple of Watts. The tab will drain much more heat to the PCB's copper than a pin would.
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