LM317T pinouts on TO-220 packages the same across manufacturers? - Page 1
Forum section: electronics / beginners

author: nzo
author_rank: Regular Contributor
timestamp: November 07, 2011, 09:56:51 pm
content: Hi all,I have a number of LM317T's stamped with the letters 'SGS'.Looking at the front of the chip, there is a dot above the mid lead of the 3 leads. What the dot signifies I don't know. I've had no luck finding a datasheet for the SGS chip.On National Semicon's datasheet the pinouts for an LM317T are, from left to right:L=adjust, Mid=Vout, R=Vin.How likely is it that my SGS LM317T pinouts will be in the same order?Thanks.

author: tekfan
author_rank: Frequent Contributor
timestamp: November 07, 2011, 10:03:40 pm
content: All LM317 regulators have the same pinout. I think the dot is just a leftover from the molding process.

author: ivan747
author_rank: Super Contributor
timestamp: November 07, 2011, 10:08:00 pm
content: They are generally the same pinout unless it is a special version, which would be marked differently from the standard part.Here's the datasheet for the SGS brand:http://www.datasheetcatalog.com/datasheets_pdf/L/M/3/1/LM317T.shtmlDatasheet catalog is a good site for generic IC's because it shows datasheets for multiple manufacturers on the same page.UPDATE: it seems like SGS Thompson was just ST's name back in late 80's, so just get ST's datasheet.

author: nzo
author_rank: Regular Contributor
timestamp: November 08, 2011, 12:50:53 am
content: Thanks tekfan and ivan747. Good find!

author: grenert
author_rank: Frequent Contributor
timestamp: November 08, 2011, 02:19:10 am
content: Note that the negative regulator counterpart, LM337, has a different pinout.

author: ivan747
author_rank: Super Contributor
timestamp: November 08, 2011, 02:40:57 am
content: Quote from: grenert on November 08, 2011, 02:19:10 amThe same goes for the 78xx series and the 79xx series. It once ruined one of my projects.

author: FenderBender
author_rank: Super Contributor
timestamp: November 08, 2011, 03:48:29 am
content: All LM317s should have the same pinout, but a corresponding negative regulator will typically have two pins switched around. LM337 for example. A while back I destroyed one by hooking it up wrong assuming it had the same pinout as a 317.

author: grenert
author_rank: Frequent Contributor
timestamp: November 08, 2011, 04:22:07 am
content: Ha ha, yeah, probably everyone has ruined something by making that assumption! You can include me in that club, too

author: ciccio
author_rank: Frequent Contributor
timestamp: November 08, 2011, 08:51:48 am
content: I think that everybody was baffled the first time he looked at the different pin-out of 78 and 79 series regulators,but in fact it is the same:pin 1 is the most positive (input for 78, ground for 79)pin 2 is the most negative (ground for 78, input for 79)pin 3 is the intermediate (output for both).In effect, sometime if you solder the wrong one, it will not be destroyed...Cannot find the same logic in LM317 / 337, but I believe it is due to the pass transistor being connected to the metal tab for better heat spread.

author: jimmc
author_rank: Frequent Contributor
timestamp: November 08, 2011, 12:40:31 pm
content: The various components in an IC are isolated from the substrate by p-n junctions, for correct operation all of these junctions must be reverse biased, hence the substrate must be connected to the most negative voltage seen by the IC. Complete isolation can be achieved but this requires more area on the chip.For the TO220 package the substrate is mounted on the tab (centre pin) which must therefore be connected to the most negative voltage. (The adjust pin uses the more complex isolation.)Of course, as has been mentioned, heat is extracted via the substrate which must be in thermal as well as electrical contact with the tab.There is no constraint on the other two pins, for the LM317T / LM337T pin 1 was chosen as the adjust pin.Jim