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30th December 2013, 04:02 PM  #1 
diyAudio Member
Join Date: Oct 2004
Location: Denmark

LM317+TL431, really?
Been looking at ways to improve the old LM317 and I got the idea to use a TL431 as a reference.
Looking at the TI TL431 datasheet, at page 30 you find the implementation shown in attachment 1. TL431 is used as a reference at the LM317 ADJ leg, with a 8K2 resistor from input to ADJ leg. TL431 cathode current varies with input voltage. I changed the resistor to 820R, with the TL431 set up for 17.5V OUT and that gives a current through the TL431 of 10.8mA with a 25V input voltage Attachment 2, 3 and 4 shows the rejection, impedance and noise of this implementation. So far it seems to work very well, rejection is standard for LM317, the output impedance however, looks way to good to be true, the noise looks fairly standard for a LM317 regulator. However, what if we move the 820R resistor from the Input to the Output? Well, that I did, and I now reduced it to 120R which gives the same 10.8mA through the TL431. This current would now be fixed at 10.8mA(as fixed as it can be with the 1.25V reference in LM317). Attachment 5, 6 and 7 shows rejection, impedance and noise of this implementation. The rejection improvements are substantial, looks like the Lm317 and TL431 rejection is added? Output impedance is one tenth of the datasheet implementation? Noise however, doesn't change much. Can the improvements in rejection and output impedance really be true or is this some kind of LTSpice fluke? Any other possible issues with this implementation, like stability issues? Last edited by Neutrality; 30th December 2013 at 05:31 PM. 
30th December 2013, 04:11 PM  #2 
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Where do you have the DC voltage?
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30th December 2013, 04:15 PM  #3 
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30th December 2013, 04:30 PM  #4 
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Do you get any DC voltage out also?
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30th December 2013, 04:44 PM  #5 
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Join Date: Oct 2004
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Off course I do, 17.5 Vout just as I wanted.
Been using LTSpice for a few years now and I think I know how to use it. If I simulate the LM317 in a typical datasheet implementation(R1=120R, R2=1K5//C=22uF, 17.5 Vout), it performs more or less as you would expect from the datasheet with regards to rejection, impedance and noise so the way I simulate it should be just fine. So what are you trying to say? 
30th December 2013, 05:47 PM  #6 
diyAudio Member

Does your TL431 model incorporate it's reactive components? The TI model was really only thermally sensitive.
When you bode plot the LM317 it's important to reference the analyzer inputs to output rather than ground. 
30th December 2013, 05:54 PM  #7  
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Join Date: Oct 2004
Location: Denmark

Quote:
I am using the model from here : Realistic SPICE model for TL431: stability, noise, impedance and performance simulation of TL431 shunt regulator  Audio PerfectionAudio Perfection The "Eugene" model, it is the most accurate model available, matches the datasheet very well. Models, DC, AC, Noise, Transient and Output impedance. As for your second comment, what? 

30th December 2013, 06:22 PM  #8 
diyAudio Member

The LM317 voltage reference is compared to the output not ground. See this application note from omicronlab:
http://www.omicronlab.com/fileadmin...ility_V1_2.pdf I use Christophe Basso's TL431 model  he's written an exhaustive (!) paper on the topic: http://cbasso.pagespersoorange.fr/D...%20control.pdf 
30th December 2013, 06:24 PM  #9  
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Join Date: Oct 2004
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Quote:
Interesting files you linked to. How good is his model and where can I get it? Last edited by Neutrality; 30th December 2013 at 06:33 PM. 

30th December 2013, 07:16 PM  #10 
diyAudio Member
Join Date: Oct 2004
Location: Denmark

Further simulation shows that it is C1 that gives me these exaggerated results. If you instead of having it between TL431 Vref and LM317 Out, you place C1 between TL431 Vref and TL431 Cathode, you get the normal LM317 rejection, impedance and noise. Though low frequency rejection is improved a lot as one would expect.
Interesting, the question now is, why?, and what does it mean. Last edited by Neutrality; 30th December 2013 at 07:22 PM. 
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