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TL431 linear power supply

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spec:

--- Quote from: T3sl4co1l on December 14, 2018, 09:09:05 pm ---
--- Quote from: spec on December 14, 2018, 07:32:44 pm ---Good evening to you too Tim.
Apart from the obvious vref applications in a benign environment, I have never been able to do anything with the TL431 and its family. So, I am biased and, for me it is one of those avoid chips. I wrote a critique of the 431 on ETO a couple of years ago, but can't seem to find it.

--- End quote ---

Basic scheme has already been mentioned:



(I do in fact sometimes draw schematics; most of these are from my back catalog, hence the simple style.)  Which the OP has already been set on the path too -- everything else is just an embellishment of this: adjustable voltage, current limiting, beefed up follower, better pull-up bias, etc.

It's just a three-terminal op-amp, no mystery to it. :-+

Tim

--- End quote ---
Thanks.

spec:
UPDATE #1   2018_12_18 The attached schematic has the opamp input connections reversed- see reply #77 for revised schematic.

mike_mike

Current limiting added to PSU circuit version 3 of reply #45, as per attached schematic:

David Hess:

--- Quote from: T3sl4co1l on December 14, 2018, 09:09:05 pm ---It's just a three-terminal op-amp, no mystery to it. :-+
--- End quote ---

The common emitter output makes it more like a three terminal operational transconductance amplifier which I think is where people get into trouble frequency compensating it as an error amplifier.  It would be interesting to measure the transconductance versus current.

T3sl4co1l:
It's dominant pole (voltage mode) compensated, so it's a really bad gm amp, at least.  It's true at DC, yes (but, DC isn't where we need compensation).

Obvious difference: loading the output with C is fine for a gm amp, fatal for a '431!

So it's more like an op-amp where they forgot the pull-up / follower part of the output.

Tim

David Hess:
I was not sure about how much effect the internal compensation had but of course the part has to be unity gain stable.  The TI datasheet includes a shmoo plot of stability versus capacitve load which has the typical characteristics of a unity gain compensated operational amplifier with stability the worst at low gains and low output current.

If anything, that makes things worse.  It is too bad the world could not standardize on something like the LM10; it works well as a shunt reference also.

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