LM317+TL431, really? - Page 2 - diyAudio
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Old 1st January 2014, 11:45 PM   #11
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Inclusion of the TL431 in the feedback path increases the open loop gain by the TL431's 55dB small signal gain. Like any amplifier output error is therefore reduced by the additional excess loop gain. You can assess stability by checking the phase margin same as any other composite amplifier. The basic stability criteria is the output device needs to be faster than the control loop (see Linear's AN-47; if memory serves you want appendix C). So, given the 317 has about a 10kHz regulation bandwidth and the TL431 a 100kHz regulation bandwidth, there'll likely be tears unless compensation is applied.

Shunting the TL431 with a decent size cap like C1 effectively removes it from the feedback loop. Hence the reversion to regular 317 performance.

Essentially what you're doing is implementing a super regulator. There's an abundant literature on those which you can consult.
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Old 2nd January 2014, 10:50 AM   #12
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Great answer, just what I was looking for.

Compensation something which has an output device that is only 1/10 the speed of the control loop would be a real pita. I think I will just go ahead and use the TL431 as a standard low impedance reference for the LM317 and keep it out of the feedback loop. It still gives me great low frequency ripple rejection and that was what I was after to begin with.
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Old 2nd January 2014, 02:03 PM   #13
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You could increase line/load rejection by splitting the TL431's cathode bias resistor into two series pieces, then bypassing the midpoint to ground with a BFC. The "+3dB benefit point" is approx BFC>10uF for Fig.1, BFC>68uF for Fig.5. Cost=low, benefit=high, decision=easy.

By the way, as Fred Dieckman never tired of pointing out, the LM317 functions as a closed loop, negative feedback, voltage regulator even when its VADJ pin has no connection to the LM317 output. The LM317 will happily regulate its output when there's no Rupper, Rlower voltage divider from output to VADJ to ground.

So, for example, if you connect a TL431 shunt regulator to the raw unregulated input voltage, and then use its +13.75 volt output (cathode) to drive the VADJ pin of an LM317, the LM317's regulated output will be +15.0 volts.
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Old 2nd January 2014, 02:57 PM   #14
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Interesting ideas.

Thanks.
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Old 2nd January 2014, 03:50 PM   #15
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Yes, essentially the LM317 is a fixed 1.25v regulator. The difference in performance is just the result of 'sitting it on top' of something else giving a local voltage reference applied to Vadj pin.

With the usual resistive divider, adding a bypass cap from Vadj to 0v improves performance not by doing 'something' to the LM317 or adding gain or anything like that. It is simply making this external voltage reference quieter - by reduced AC impedance. As an alternative a zener works very well there instead if you pour a bit of current through it (try Rupper 100ohms). So, as the OP found, does using a second voltage reg. this TL431 - not really a surprise. You can do just the same with a 7805 to make any voltage you require...
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Old 2nd January 2014, 05:27 PM   #16
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Quote:
Originally Posted by martin clark View Post
Yes, essentially the LM317 is a fixed 1.25v regulator. The difference in performance is just the result of 'sitting it on top' of something else giving a local voltage reference applied to Vadj pin.

With the usual resistive divider, adding a bypass cap from Vadj to 0v improves performance not by doing 'something' to the LM317 or adding gain or anything like that. It is simply making this external voltage reference quieter - by reduced AC impedance. As an alternative a zener works very well there instead if you pour a bit of current through it (try Rupper 100ohms). So, as the OP found, does using a second voltage reg. this TL431 - not really a surprise. You can do just the same with a 7805 to make any voltage you require...
I am just not to sure about how good a zener will be with regards to noise and the datasheet are useless for noise specifications.

Simulated performance with a zener is great, but the potential noise issue could be a deal breaker.

On the other hand, the TL431 has decent noise if you get it from like Onsemi or from Exar which have several low noise variants of the TL431.
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Old 2nd January 2014, 05:33 PM   #17
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Zeners are only noisy when run starved of current (uA)

Try it with a couple of mA through the zener and it'll have a dynamic reistance c.100ohms max and easily prove 30-40dB quieter than the 317 itself... quieter than the TL431, too.
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Old 2nd January 2014, 05:56 PM   #18
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Quote:
Originally Posted by martin clark View Post
Zeners are only noisy when run starved of current (uA)

Try it with a couple of mA through the zener and it'll have a dynamic reistance c.100ohms max and easily prove 30-40dB quieter than the 317 itself... quieter than the TL431, too.
You have any measurements? I know user Christer here on DiyA did some Led and Zener measurements showing low levels of noise, however I have also subsequently read that his measurements were flawed in some way and that in reality Zeners are much worse wrt noise. Thougn I cant really remeber where I read that, somewhere on DiyA

Last edited by Neutrality; 2nd January 2014 at 06:08 PM.
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Old 2nd January 2014, 06:12 PM   #19
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Not to hand, but Christer's figures looked pretty good to me then and, checking, now too. Even if the figures given were a factor of 10x too small, the zener's noise will be utterly swamped by the internal noise of the 317 ... ~150uV output noise even in 'lowest' in 1.25v mode.

Easy to built up a reg and test it - have a go. You might be surprised.

Last edited by martin clark; 2nd January 2014 at 06:18 PM.
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Old 2nd January 2014, 06:14 PM   #20
tomchr is offline tomchr  United States
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The LM317 is a nice regulator, however, considerable improvements have been made since its introduction in the late 1970ies. If you don't mind spending a bit more, I suggest looking at the LT3080. I'm using it in my 21st Century Maida Regulator and have found LT's claims of 120 dB ripple rejection to hold up in real life! In my setup, I ran 50 Vpp of ripple into the regulator and measure 20 uV RMS of residual ripple + noise on the regulator output. I suspect the ripple component is actually buried below the noise floor.

If your local component pusher can't help you with the LT3080, I'm sure Aarhus Radio Lager (ph. 86246422 if memory serves) or RS, Farnell can help you out.

~Tom
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