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Take a look at the pictures below. The TO-220 with a thin tab is the LM317 (STMicroelectronics) and the TO-220 with the thicker tab is the IRF540N (International Rectifier).

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If you take a look at the thermal resistances of the two TO-220s, you will find that the junction to case thermal resistance of the LM317 is around 5 degC/W and that of the IRF540N is 1.15 degC/W (which is the value for pretty much any TO-220).

So I want to know why STMicroelectronics decided to put such a thin, high thermal resistance tab on the LM317 instead of the usual sized tab.

JRE
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Prathik Prashanth
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2 Answers2

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If you read the ST datasheet you will see that they offer both the thick and thin tabs, with the LM317T being the default "single gauge" and the LM317T-DG having the thicker ~1.3mm tab.

As of this writing, and using Digikey.com pricing (probably quite a bit higher than production quantity pricing- and more likely indicative of list price ratio), the cost of 1,000 pieces is

ST LM317T $262.50 USD/1000

ST LM317T-DG $358.75 USD/1000

As well as (potential*) part cost savings, the part is lighter so shipping by air will likely be cheaper and the end product will be slightly lighter.

* potential since some suppliers currently show the -DG variant as cheaper

P.S. When the LM317 was first introduced in the mid-1970s (~45 years ago) it was common to use linear regulators in situations where they would dissipate a lot of power. These days it is rare for a linear regulator to be used in a situation where the TO-220 is dissipating anything near what it is capable of, because that would be wasting power that could be saved with switchmode power supply. So there are quite a few TO-220 package regulators that are used dissipating well under 1W, often without a heat sink at all. The same is true of the even older LM78xx and LM79xx regulators.

Spehro Pefhany
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    Perhaps pricing will also vary by popularity --> volume buys, whatever their & the manufacturer's decision processes are for pricing, possibly whether they are low on stock of a popular part... – Technophile Jun 02 '21 at 03:21
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Cost-cutting, nothing more. Less copper (which is what that tab is made of) means less money to make it, and the LM317 is an incredibly cheap part that they've shaved off every bit of cost they can.

I suspect the different die area will also play a role, perhaps even a larger one, in determining thermal resistance. The IRF540N is a reasonably high-power device, on a relatively large die (though smaller than the original IRF540), and a larger die will always have a lower thermal resistance, all else being equal. The '317, again, is cost-cut to the extreme, and less die area is, you guessed it, less money.

Hearth
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  • Since when does copper have aluminium color? – lalala Jun 02 '21 at 14:34
  • @lalala if the lead in my pencil is graphite, maybe the copper in his tab is aluminium? :D – Cylindric Jun 02 '21 at 15:03
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    @lalala It's tin plated. Or tin/lead plated if it's an older part. If you take a TO-220 and snip the tab in half, you'll see the copper color underneath. – Hearth Jun 02 '21 at 15:25
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    @Cylindric See above; I'm quite sure it's copper. I've not seen a part yet with an aluminum tab, though it's not outside the realm of possibility. Only on through-hole parts, though; D²PAKs (surface-mount version of TO-220) must have a copper tab, because aluminum is nigh impossible to solder to. – Hearth Jun 02 '21 at 15:26