TL431 model with temperature dependance? (fatal error) - Page 1
Forum section: electronics / projects

author: incf
author_rank: Frequent Contributor
timestamp: April 23, 2025, 12:09:03 pm
content: I'm looking to model using a TL431 type device as a "power good" comparator from -40C to 105C. Particularly, I'm interested in how much the "soft knee" at around 400uA moves around.Nexperia has a TL431 model that supposedly models temperature dependent behavior. (existing TL431 models break in unexpected ways when I change the temperature of the simulation)But I ran into a problem, QSPICE (the LTSpice successor) reports an error "Fatal error: BEF•X2: Current sensing from a behavioral current source is only supported for Thévenin equivalents(V=... sources)." with this model.I emailed the author of QSPICE, but he is very busy.When I look at the contents of the file I see a line that it appears to be complaining aboutCode:BEF ef1 ef2 I=((V(ef1,ef2))/25.25)**3But I am not sure that I fully understand the issue.Might it be possible to work around this limitation by modifying the model? (I'd guess something like adding an intermediary behavior voltage source that the behavior current source can read)https://assets.nexperia.com/documents/spice-model/TL431.txtAlternatively: Do any "good" TL431 type models exist that model input and output behavior vs temperature? I think the output voltage/current probably varies with temperature. I can 'sort of' imagine what it will do based off of the typical specs. But I would feel better with a model.I think the "TL431ED" model is quite good for that, except for that I'm not quite convinced that it behaves if the temperature is not 20CEdit: I think the error must be saying "use a voltage source with a series resistor to emulate a current source"

author: Benta
author_rank: Super Contributor
timestamp: April 23, 2025, 08:58:55 pm
content: Why not just use one of the MAX803/09/10 parts?Much easier, with good precision.

author: incf
author_rank: Frequent Contributor
timestamp: April 23, 2025, 09:26:51 pm
content: Cost and availability.I ended up making some (hopefully sensible) assumptions about worst case behavior and ignoring the TL431ED's input reference current being invalid at low temperatures.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: April 23, 2025, 10:27:15 pm
content: Quote from: incf on April 23, 2025, 12:09:03 pmWhy ?You are going into the least specified area of the data, on a real Hail-Mary expedition.If you must use a 'TL431' at 500uA, why not just buy one rated properly at that point?My notes findTI tla431 typ 150uAand these at lcscShenzhen-Fuman-Elec-431S < 100uAPJSEMI-TL431LA < 85uAALJ-TL431D < 80uAThere are also many 1.25V ref parts with sub 100uA knee points.I see the new Runic RS421V (1.24V 50uA) does graph Knee I vs Temp, but I've not seen price/stock on that yet.

author: incf
author_rank: Frequent Contributor
timestamp: April 23, 2025, 11:12:34 pm
content: Quote from: PCB.Wiz on April 23, 2025, 10:27:15 pmYou are right about the 500uA operating point. Operating it as a comparator had me thinking about it wrong initially.I eventually realized is that the "Ika(min) Minimum Cathode Current for Regulation" is the only thing that is guaranteed, so I modified everything to accommodate the standard 1mA max "cutoff" current and 2.5V off state voltage.I didn't give those parts you mentioned enough consideration - particularly the 1.25V ones. They have very nice specs. The pricing is mostly acceptable. Stocking levels of the "dirt cheap" versions (under 2 cents) is where it falls short in my opinion.I'd rather design for the cheap and well-stocked commodity part - I don't desire the performance so much as I desire having a guarantee of what its going to do. What I failed to understand initially was the connection between "regulation current" spec and the inferred "cutoff current" from the graphs.Admittedly, the learning curve was a bit steeper than I would have liked (looking at the internal structure, models, and specs to try to figure out if the specs what the in linear mode actually mean when operating in comparator mode).