TI LM4051-N vs. Diodes Inc. AZ431L in current limit circuit - Page 1
Forum section: electronics / projects

author: brumbarchris
author_rank: Regular Contributor
timestamp: May 27, 2019, 07:35:43 am
content: So gentlemen, I am currently working on a circuit involving a current limit circiut (100mA) based on an 431 voltage reference variation from Diodes Inc. (AZ431L). It works fine. We wanted to replace this voltage reference with a LM4051-N from Texas instruments, as the datasheet indicates this is better in terms of accuracy. However, for some reason, it does not work. It does not work in practice and it does not work in simulation (we only looked at a simulation after we noticed it does not work in practice, expectation was that we just replace the reference and it works). Of course, we also changed the value of the R2 resistor from 12.4Ohm to 12.1Ohm (as the Vref of the two parts is slightly different). "Does not work" means the load current is about 140mA instead of 100mA and both practice and simulation indicate this value to be quite dependent on the R3 biasing resistor (which it should not, considering a sufficiently low value for that resistor is selected).As nomenclature regarding Anode-Cathode is a bit blurry in the datasheets of the two components, we had some doubts at one point that these two components might not be pin-to-pin matching, but we eventually decided they are. A second look might not hurt.I fail to understand why the TI part does not work. The block diagram of the two components indicate that in the Diodes Inc. part the REF input goes to the non-inverting input of the opamp inside the part, whereas the more detailed block diagram of the TI part indicates the same input (called FB in the TI datasheet) somehow influences both the inverting and non-inverting inputs of the opamp. Of course, these are just functional diagrams, I do not really know if in the end the parts are similar or not inside.The fact that TI has some TLVH431 makes me wonder, as it looks that this is a more exact corresponding part of the current Diodes Inc. part we are using. Unfortunatelly that is equal in performance to the existing part, so no gain changing it.so to sum up the question: why does the TI-LM4051-N not work in the same circuit?Best regards,Cristian

author: 2N3055
author_rank: Super Contributor
timestamp: May 27, 2019, 08:58:29 am
content: TL431 reference voltage is between anode and adj, with LM4051 it is between cathode and adj. In LM4051 DS it clearly shows it is connected differently in CC circuit..

author: brumbarchris
author_rank: Regular Contributor
timestamp: May 27, 2019, 09:48:08 am
content: Good catch...which means that the Anode/Cathode annotations in this picture from the first page of the LM4051-N datasheet are plainly wrong, right? I initially thought they might be just "non-standard" as these are not real diodes.The datasheet does indeed go on further down to talk about feedback being in-between the Cathode and the FB pin, and calls the Cathode to be "output" of the shunt regulator configuration. Should have made me thinking...better than I had.Anyway, the overall news is not good, means we also need to change from the current NPN configuration to a PNP transistor.Regards,Cristian

author: 2N3055
author_rank: Super Contributor
timestamp: May 27, 2019, 09:51:21 am
content: No it is right, but it basicaly works like LM317, it regulates voltage across upper resistor in divider, not bottom one

author: brumbarchris
author_rank: Regular Contributor
timestamp: May 27, 2019, 11:35:09 am
content: Are you sure it is right?They even state further down in chapter8.3 Feature descriptionthat:QuoteThey are referring to the fixed voltage variant, precisely the one on which they indicate the terminal designations on the first page. And the picture does not match this phrase.Regards,Cristian

author: 2N3055
author_rank: Super Contributor
timestamp: May 27, 2019, 11:53:07 am
content: With all integrated shunt references both the symbol and cathode/anode nomenclature is arbitrary. It is drawn that way to resemble zener diode in this use. It does not, however, behave the same as zener, it won't be diode i reversed.So only current direction it will support is from "dash" to "triangle", in which case we could argue that dash is anode , a positive electrode...TL431 retains full nomenclature like zener OTOH. But they will both only support current only in one way.Important thing is, that one regulates voltage across top resistor in a divider, and other one regulates voltage on bottom one. Which makes them look similar, but should be applied and used differently.