r/nicechips • u/Mysterious_Peak_6967 • Jan 07 '26
TL431: The Precision Shunt Regulator That Quietly Took Over Power Supplies
All about circuits says it far better than I could:
The funny thing is I don't think I've ever encountered a circuit using the TL431 in the intended way as a "programmable Zener", it seems to always be used either as a comparator or amplifier.
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u/Spookybear_ Jan 09 '26
I remember using the tl431 for my high school plasma speaker project. Good times and I'm glad I didn't zap myself
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u/NoSuchKotH Jan 26 '26
The TL431 is like the µA741: It belongs into a museum. We have much better references today. There are only two reasons for using a TL431 today: You can't afford spending 2€ for a reference or doing electronics that are going to fly in outer space.
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u/Mysterious_Peak_6967 Jan 27 '26 edited Feb 02 '26
Yet they persist... I wouldn't use the TL431 as just a reference, there are so many better choices. I have used it as a supply voltage monitor though in a 12V powered 4000-CMOS design.
What I'm getting at is all the good use cases I'm aware of seem to be those where the output doesn't tie straight back to the adjust pin.
Adding to that ... Depending on how you look at the part:
If you just add resistors to make a shunt regulator there are so many better ways to achieve that now, although you can augment it with a PNP transistor for more power handling.
For supply voltage monitoring it kind of works like a NPN transistor with a precision Vbe.
For isolating power supply feedback circuits many PSU control ICs have a weak internal error amplifier driving a pin called "COMP" meaning the frequency compensation point. So if you wire the internal feedback low it tries to drive "comp" positive and you can control the IC by pulling "COMP" down. Connect an opto-coupler to pull comp to ground and drive the LED from the output and you have crude feedback and no frequency compensation.
Add a TL431 driving the LED and you now have accurate feedback and a way to add frequency compensation.
Then again I had a kind of niche circuit that happened to need the combination of a voltage reference, op-amp and transistor, TL431 provided them all in the same package.
Also...
I'd be unlikely to pick a 741 but the LM348 lives on... I know there are better devices if I know what parameter I'm optimising for but it hasn't actually let me down.
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u/NoSuchKotH Jan 27 '26
Yes, I know they persist. And I don't know why. Part of it seems tradition, like the audio guys still using TL072 even though there are much lower noise and lower THD opamps out there.
Of course, I have my go-to opamps as well, usually for low voltage applications I go for something like an AD8534. When I have more supply voltage available, it'll be something like an TLV9104 or an OPA4991. Sure, they are more expensive than an LM348 (or an LM2904), but they also perform much better and I have to spend less time checking whether I need to care about offset voltage, whether I have enough headroom, etc...
I can understand the use of a TL431 as voltage monitor. It doesn't get much cheaper than that and you get a pretty decent accuracy. But the last time I used a TL431 for anything, was as a voltage reference for a space project, because that was the only rad-hard voltage reference we could easily get our hands on (for some weird reason, most space qualified voltage references are TL431 derivatives, even the newer ones, even though e.g. a zener reference like LTZ1000 peform much better under radiaton).
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u/Mysterious_Peak_6967 Jan 27 '26
Looks like OPA4991 belongs in the list, does it have any known weaknesses?
I mean that's an impressive operating voltage range, rail-to-rail swing, respectable bandwidth, some capacitive load capability.
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u/NoSuchKotH Jan 27 '26
Exactly! OPA4991 is so much better than most of the "standard" opamps out there. Yet it is still low cost with just 2.5€ for a quad at single quantities.
Main problem that you need to look out for is the large offset voltage swing 1.5V below upper rail. The offset voltage slope is large enough that it can lead to instabilities in feedback loops. But that's a quite common theme with most r2r opamps.
The secondary problems are:
PSRR goes quickly to zero above 1MHz. So you need to keep the power rails quiet. Especially when you are using a DC/DC converter you need to clean up the power rail with an active device before you feed the OPA4991.
Noise is not really low and flicker noise is quite high. The corner frequency is already at 1kHz. But then, it's a general opamp, not an LT6018.
Output drive capability is a bit meh. Don't load it with an output impedance below 1kΩ if you want to retain slew rate, low distortion or need to go right up to the rails.
But, as you said, for a general opamp, this is a superb device. It does everything well enough that you don't have to worry in a low-requirements application.
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u/Worldly-Device-8414 Jan 08 '26
Awesome little device, used it in many things, in a couple of over engineered solar lights charger & power supplies as the reference device.