LM317 symbol creation


Thanks Jim for that clarification. My most post was a direct response to the post I quoted which was a response to the question:

Maybe I should have quoted them both, but thanks anyway. :wink:

However, you might want to be careful. Your distinction between pins and tabs is becoming a bit blurred. When it comes to components in TO-220 packages I’m sure we can all agree as to what a “tab” is, and for the most part they can be ignored. They are seldom included on the schematic and don’t need a corresponding pad on the PCB. But the TO-263 (aka. DDPAK/D2PAK), the SMT cousin of the TO-220, is a 4 pin device. Pin 4 which you seem to refer to as a “tab”, requires a corresponding pad on the PCB for mechanical strength as well as thermal considerations and often pin 2 is omitted making pin 4 the only electrical connection to that terminal. The same is true for the SOT223 package.

I do agree however, many devices that use these packages can usually be represented on the schematic with 3 pin symbols.

Edit: This started out as a reply to @Sprig but once I quoted @DiBosco it became a reply to him/her.

Edit2: The above mentioned packages have multiple variants some of which have more than 4 terminations (pins), given the context of the current topic, the above refers to the 4 pin variants.


Bah, saying they’re tabs not pins is sheer pedantry! :yum:

In the end it’s up to the individual/company and how they want their libraries, but I’ll always put the tab on my symbol (and indeed call it tab).

@1.21Gigawatts SOT223 is a very good example, four pins, one of which is a tab all which could have different functions.

For a TO252 where the tab connects to board and the stubbby pin doesn’t, personally, I don;t have the stubby pin on the schematic and I do have the tab.

I’m a him BTW :grin:


Real world, and I’m working with/training someone, what do you think the reply would be if I asked them to, “Check the voltage on pin 4 of U1?” and it is this device?


Because I’d never ask a new tech to probe the voltage on “pin-2”. There is to great a chance of getting pins 1 or 3 involved in the connection and letting the smoke out.

ON EDIT: @1.21Gigawatts I’m not trying to be snarky here. This is what I know from what I have done at where I have worked; and it’s probably not that many that have the experiences that I have had.


I do not know, but I suspect, that some grounds on some chips are not internally connected; as indicated by their block diagrams. Or, if even if they are connected, have other issues that make a connection to the outside world make a good idea.


In my prior workplaces, pedantry was the norm; sorry if it seems I carried it to far here.


Well since you are the one training them … :wink: But surely they could figure out which pin was pin 4! If not it would be a good opportunity for you to teach them something.

I’m sure we have all seen examples of manufacturers datasheets showing other than standard nomenclature but in this case I would not blame them for indicating similarities between a TO-220 and TO-263 with regards to a specific component. You are of course free to call it anything you like, but to base an entire argument on your preference is a bit of a stretch.

I’m sure we are all grateful to have such an experienced authority on the subject here to set us straight. Thank you! :roll_eyes:


You took that the opposite way it was intended.

I have a more restricted knowledge base to draw from.


If that’s the case then I apologize, I only took it as it was written.


What if “pin-4” was not on the qualified schematic?

In this thread, I included an upload of a SOT-223 package where the TAB was given the assignment of pin-4; yet still the schematic equivalent of pin-2.

You are not arguing with me; these are the symbols, footprints, and datasheets from at least 5 different major device manufactures with currently purchasable parts and their documentation.

I’m going with their documentation until it changes.


You will notice that in a previous post I did agree that:

many devices that use these packages can usually be represented on the schematic with 3 pin symbols.

I will also admit that in the case of the TO-263 it is often referred to as a ‘tab’ due to it’s similarity with the TO-220. IPC documentation refers to the pad on the bottom of SOT and SOIC packages (often referred to as an “exposed pad”) as a “thermal pad”. When this pad extends beyond the edge of the package it is referred to as an “extended thermal pad”. And I have found two occurrences of these extended thermal pads being referred to as “thermal tabs”. The TO-263 fits this description, however the SOT-223 does not. So perhaps I was a bit pedantic when it comes to the TO-263 as it is a bit of a borderline case, meaning that the pin/tab requires a pad on the PCB. Even manufactures can’t agree on whether it is a TO-263-3 of TO-263-4.


The diagrams in manufacturers in data sheets are created by or for the marketing department, they should not be regarded as normative. The data sheet leaves out details such as pin numbers because they are illustrating the use, they are not designing a product. It makes no difference to the performance of the component how the customer draws their schematic.

As I said before, datasheets are serving suggestions, one should not get hung up on slavishly following the suggestions presented.


Nah, it’s all fine, we’re just having a friendly conversation far as I’m concerned. :slight_smile: