How does the pinout work for this version of the LM317?

How does the pinout work for this version of the LM317?
Posted by WarFreak131 on 06 Dec, 2020 17:57
I am looking to use this LM317 in a project I am going to build: https://datasheet.lcsc.com/szlcsc/Unisonic-Tech-LM317AG-TN3-R_C75510.pdf

The LM317 is a three terminal device, but the TO-252-2 package only has 2 pins plus a tab. The tab is usually ground, but not in this case. The sample drawings only say what pin #1 is. What would be pin #2 is cut off of the package. Is the tab considered to be pin 2, or pin 3?

Thanks!

#1 Reply
Posted by radar_macgyver on 06 Dec, 2020 18:21
Tab would be pin 2 (VOUT). The tab is always the center pin (not ground) on 3-terminal packages (unless specified otherwise).

#2 Reply
Posted by Siwastaja on 06 Dec, 2020 18:46
Proper datasheets specify the pinout unambiguously, mentioning "tab" directly, or specifying "tab = pin #x".

But, when they fail at doing that, you just need to know that on such classical tab + 2 legs packages, the tab is always pin #2, and the legs are #1 and #3. Many such packages even have a short stub leg #2 available, you can see with a multimeter how it connects to the tab. I sometimes (ab)use this stub leg, for example solder a bypass capacitor there.

No, the tab or thermal pad isn't always ground. Often it is, and when it is, it's quite helpful.

#3 Reply
Posted by Zero999 on 06 Dec, 2020 20:24
Yes, the middle pin is normally connected to the tab, but in some components it's isolated, in which case it will be stated on the data sheet.

In the case of the common bipolar voltage regulator ICs, 78xx, 79xx, LM317, LM337, the centre pin is normally the most negative voltage on the regulator, which is the adjust pin, on the LM317, but is the input on negative regulators such as the LM337.