Author Topic: Why with LDO Vreg TAB on Vout and non-LDO TAB on GND ?  (Read 1667 times)

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Offline Kjelt

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Why with LDO Vreg TAB on Vout and non-LDO TAB on GND ?
« on: July 11, 2017, 01:44:55 pm »
Just popped up, why is it that with a lot if not most non-LDO Voltage regulators the GND is the TAB of the TO220/DPAK... package and with LDO regulators this is the Vout.
Any particular reason advantage? The part that becomes hot has to be connected to the TAB but why does this differ between a LDO and non-LDO ?
 

Offline Benta

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Re: Why with LDO Vreg TAB on Vout and non-LDO TAB on GND ?
« Reply #1 on: July 11, 2017, 04:09:13 pm »
Because LDOs use PNP pass transistors, and the collector = substrate.
 

Offline Zero999

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Re: Why with LDO Vreg TAB on Vout and non-LDO TAB on GND ?
« Reply #2 on: July 11, 2017, 05:24:27 pm »
Because LDOs use PNP pass transistors, and the collector = substrate.
Then why is the substrate the emitter, in the case of the LM317 (output) and LM337 (input)?
http://www.ti.com/lit/ds/symlink/lm317.pdf
http://www.ti.com/lit/ds/symlink/lm137.pdf

I agree with you about it being due to the process used to manufacture the device but there's a huge variation in manufacturing process and circuit design, across different parts.

Just popped up, why is it that with a lot if not most non-LDO Voltage regulators the GND is the TAB of the TO220/DPAK... package and with LDO regulators this is the Vout.
Any particular reason advantage? The part that becomes hot has to be connected to the TAB but why does this differ between a LDO and non-LDO ?

Which parts are you talking about? I've had a quick look at some TI parts and they're tab = 0V.
http://www-3.unipv.it/lde/strumentazione_componentistica_extra/datasheet/LM2940-C.pdf
http://www.ti.com/lit/ds/symlink/lm2941.pdf
 

Offline T3sl4co1l

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Re: Why with LDO Vreg TAB on Vout and non-LDO TAB on GND ?
« Reply #3 on: July 11, 2017, 11:24:27 pm »
Collector = substrate isn't always used, because that can be very tricky for loose charge carriers diffusing into the control circuitry.

Substrate is usually the lowest or highest voltage node, depending on if it's N or P type.

Wells are diffused into the substrate, then transistors into that.  Presumably, the LM317 connects output to substrate for convenience, and it would have to be P substrate for an N-well (output transistor collector), with P and N diffusion/epitaxy on top of that (making the rest of the transistor).  Which, as it happens, means there's a huge friggin' reverse-protection diode there: the N-well (input) to P-substrate (output).  It might not be desirable to use, though (poor current distribution, no guard rings..?).

This makes PNPs very difficult, because they either have to be substrate = collector (pretty useless), or they have to be diffused on a P-well in an N-well, which is a triple liability: the doping on the emitter has to be terribly high, the collector voltage rating will only be the NPN emitter rating, i.e., ~7V, and the PNPN layering of the wells allows catastrophic SCR latchup to occur (the PNP on top may not be usable at all because of charge diffusion away from it).  This is why they invented "lateral PNP": shit hFE, but symmetrical C/E ratings (30V!), good enough for diff pairs, current mirrors and such.

I suppose LM337 could be a layer-for-layer swap (substitute P for N), though I doubt you can do that strictly so easily, because the dopants have different diffusion rates so you need to tune all the process steps again...  That's probably part of the reason why they used a different design (it's actually an LDO, just not as useful because it's negative!).

LM1117 is perhaps where things get interesting: it's more like a positive '337.  I forget if that was one fuckoff-massive lateral PNP, or a single NPN follower (not Darlington) and a slightly-less-massive lateral PNP to drive it.  I think it's still made on the classic process, not a complementary one?

Oh, so, anyway, note that tab = substrate isn't necessarily any internal connection, because nodes can be isolated through various means, allowing voltages above and below substrate to be used inside.  That's why the LM317 substrate is neither the collector (N substrate for a power device would actually be rather inconvenient, for the above reasons) nor the emitter (the diagram shows emitter resistance, for current sharing -- it's actually dozens of emitter 'fingers', wired in parallel with a little added resistance), but "ground" of a sort. :)

Tim
Seven Transistor Labs, LLC
Electronic design, from concept to prototype.
Bringing a project to life?  Send me a message!
 
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Offline Kjelt

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Re: Why with LDO Vreg TAB on Vout and non-LDO TAB on GND ?
« Reply #4 on: July 12, 2017, 07:15:41 am »
Which parts are you talking about? I've had a quick look at some TI parts and they're tab = 0V.
The ST LDO's like the LD1117
But a colleague also mentioned that the Microchip parts like the MCP170x have the Vin on the TAB.

So is the conclusion that it is up to the manufacturer ? Wouldn't it be better if the TAB was isolated but thermally coupled with the entire die ? Just trying to understand why and how this works.
 

Offline Kjelt

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Re: Why with LDO Vreg TAB on Vout and non-LDO TAB on GND ?
« Reply #5 on: July 12, 2017, 07:19:39 am »
Collector = substrate isn't always used, because that can be very tricky for loose charge carriers diffusing into the control circuitry.
Substrate is usually the lowest or highest voltage node, depending on if it's N or P type.
...........
Thanks, but I think I am a guy that needs pictures with the lecture  ;D
 

Offline T3sl4co1l

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Re: Why with LDO Vreg TAB on Vout and non-LDO TAB on GND ?
« Reply #6 on: July 12, 2017, 08:21:59 pm »
Collector = substrate isn't always used, because that can be very tricky for loose charge carriers diffusing into the control circuitry.
Substrate is usually the lowest or highest voltage node, depending on if it's N or P type.
...........
Thanks, but I think I am a guy that needs pictures with the lecture  ;D

Talk is cheap, pictures are extra :P

Tim
Seven Transistor Labs, LLC
Electronic design, from concept to prototype.
Bringing a project to life?  Send me a message!
 


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