Adding an adjustable constant current option to a LM5175 circuit - Page 1
Forum section: electronics / general

author: Noeljunior
author_rank: Newbie
timestamp: February 05, 2021, 05:04:17 pm
content: Hi! This is my first post here. This looks to be the right place but if not, please move the thread to the right place.Disclaimer: I'm no electronics engineer and I have no experience in this kind of circuit! I do occasionally design simple analog circuits and PCBs, barely messing with opamps.Context:I'm about to build an open source general purpose modular DC UPS. Meaning, it will switch between a DC input and a battery accordingly, charge the battery and regulate the battery's voltage. It will be MCU monitored and it will be able to talk to a host, if wanted. It's modular because the input voltage and current, the output voltage and current and the type of battery are meant to be arbitrary and the user may re-design, populate or configure it in whatever way he needs. This circuit is meant to be as robust and efficient as possible (limited by me design skills), as it will be on for long periods.The question is regarding the battery charging circuitry.I'll be using the LM5175 (datasheet:https://www.ti.com/lit/ds/symlink/lm5175.pdf?8ts=1612366793620). My design is highly based on its typical application design (p. 20, fig. 24) and I'm using the webench to adjust it to my needs.Here is the schematic so far. This is incomplete and not all values are yet chosen.I want to implemente a smart, but not so smart, charger, using CC/CV method for lead-acid or lithium batteries. All battery's safety features should be handled at battery side.The CV is easily configurable my adjusting R2 and R3 but now I need the CC.What I know I can do:1. use the internal current limit circuit (ISNS+ and ISNS- pins);2. messing the feedback (FB pin) so it can act as both CV and CC, whichever happens first;3. anything else?My main concern here is the stability of the system. I mean, I don't really care if it charges the battery at 0.9A or 1.1A. But whatever value the user got it adjusted, it should stay there. Also, I assume the user may use a dummy load, short the output or measure some specific TP to set the CC limit.Option 1.I believe this would be the correct way to go but I have no idea how to make this work. Usually the pins ISNS+ and ISNS- are connected across R1 (current shunt). Obviously this shut cannot be a trimmer. These pins are connect inside to an amplifier and to a comparator such that when voltage across the current shunt resistor is higher than 45 mV the IC limits the output current, by lowering the output voltage. The functional block diagram is on p.13.Question:is there a way I could add some circuit here so I could tune current with a trimmer between around 0.5A and, say 2A? How?I came with this solution based on GreatScott's idea and adapted to my problem. However I have no idea if it makes sense to use the internal comparator like this:Option 2.I thought I could implement something like some Chinese designed switched mode regulator boards do. I have some idea that, by visual inspection, it looks like they mix the voltage feedback with a current shunt resistor and a trimmer. An amplifier is used to amplify the voltage across the shunt and a comparator to compare it to the trimmer.GreatScott added this CC circuit to a board which only had a CV trimmer. Here is the whole project (https://www.instructables.com/Adding-a-Current-Limit-Feature-to-a-BuckBoost-Conv/) and the schematic regarding this addition:Question:if this a good option to my problem, will this work here? Can you please give some help adjusting it to here?Option 3.Anything else I could do you remember? I found what looks to be two other approaches to this problem.I found that the TI's reference design around LM5175 (https://www.ti.com/tool/PMP20660) has a CC setting to 11A but it's using something more around ISNS+ and ISNS- pins which I don't understand. The schematic is here (https://www.ti.com/lit/df/tidrtw7/tidrtw7.pdf?ts=1612367138154&ref_url=https%253A%252F%252Fwww.ti.com%252Ftool%252FPMP20660).There is this thread on e2e (https://e2e.ti.com/support/power-management/f/196/t/955203) where it looks someone is trying to do the same using the COMP pin. I also don't understand the idea.Thank you for your help!