Trying to make my own LMR14050 regulator and it is falsely going into OCP? - Page 1
Forum section: electronics / beginners

author: ITman496
author_rank: Regular Contributor
timestamp: June 10, 2025, 05:36:25 am
content: Hello,I wanted to make my own resilient dc/dc converter to use in my designs and because I've always wanted to try..When I power it up, the output flashes and flickers and the waveform in the scope suggests its doing the burst, wait, burst, wait that I believe means it's in over current protection.If I inject 5v after its output into the output cap, the circuit powers up just fine, so its not shorted. I've not yet tried injecting 5v, then starting the dc/dc converter, then removing my 5v.. That will be for tomorrow.I've also tried putting a 0.1uf cap on soft start to get it to extra slowly soft start, to no avail. I also tried removing the soft start cap all-together.The Soft Start pin goes up to 1.9v.. which seems weird, isn't it supposed to go to 0.8v?What am I doing wrong? I used values from TI's own designer and simulator so I figured it would be pretty straight forward?Here is my schematic:Here is my board layout:

author: BadeBhaiya
author_rank: Regular Contributor
timestamp: June 10, 2025, 06:16:29 am
content: Not really sure about the soft start bit (I haven't used an IC with that feature before) but I have a very strong suspicion that the OCP is triggering because of C8 being 470uF and quite low ESR (10mΩ, as per the datasheet). Try replacing it with a smaller cap, say 22uF or 47uF.The OCP is likely being triggered because the inrush current on the output side is just too large in the beginning. The switching IC would have to take the large 470uF cap all the way from 0V to 5V, and with the low ESR, that looks very similar to a short to the chip.

author: ITman496
author_rank: Regular Contributor
timestamp: June 10, 2025, 07:02:41 pm
content: So I'm at a bit of a loss..I probed several points and I'm getting complete nonsense, but I've read and re-read the datasheet and nothing makes sense. I'm getting huge noise spikes all over the place and.. I'm just plain not sure why, but I'm pretty sure its why its not working!Here is a picture of my scope. Note the voltage scales are all WAY up on parts that should be normal. I'm genuinely not sure what's happening. I tried boosting the capacitor to running voltage and then powering the regulator on and then releasing it, and it just drops to nil and blinking again. It can't even power itself, let alone a 30ma load, let alone a 5a load.I am pretty sure I did the design correctly so I'm just.. completely uncertain where all this noise is coming from. I'm also fairly certain I got the bottom pad soldered? Is there something grossly wrong with my layout? I tried to get it close to the datasheet. I LCR metered my capacitors and they are still good. I'm not sure where all this noise is coming from!Blue is probed onto the output capacitor pinMagenta is probed onto the feedback pinCyan is probed on the top of the freewheel diode, so SW pin into inductorYellow is the input capacitorThe circuit draws 18mA at 12v.. only 16 inch wires connecting from my power supply to the input.

author: floobydust
author_rank: Super Contributor
timestamp: June 10, 2025, 07:41:34 pm
content: A few possibilities:C8 ground is 10 miles from the IC. This is trouble. Where are the vias? The ground-pour is sliced up and you have to consider the high currents flowing in that.I don't like all the line resistance you have - the diode and PTC yet C9 is only 39uF?Trace under L1 is going to be very noisy.

author: ITman496
author_rank: Regular Contributor
timestamp: June 10, 2025, 09:51:43 pm
content: Thank you flooby, those are all things I will fix on the board respin.Also, I did actually fix it! The issue was the diode was too big. My electronics teacher suggested I try a smaller diode with less capacitance and sure enough, as soon as I bodged a 3A thru-hole it fired right up and is stable.But as for improvements for the next version:More via stitching of the ground plane to shorten the pathsBigger input capacitance (I just used what the TI simulator website told me to use, oops)Move that trace out from under the inductor. Its shielded but still, avoid that. Also its completely unnecessary as it could have connected to the one slightly to the left.. This is what I get for doing pcb design sleep deprived on an overseas flight home!Get C8 closer to the IC, a smaller diode should help with that and also some re-jiggering of the bootstrap capacitor to allow that ground plane on top to link back up with it.Thank you for the feedback!

author: floobydust
author_rank: Super Contributor
timestamp: June 11, 2025, 03:07:20 am
content: Why do you need a 5A buck converter? A problem with too big is the high input current peaks are always there, which is bad if you have a weak power source.LMR14050can take 40V inputs so why TVS clamp down low at 15V? 30A Schottky diode? Bigger is slower. I think some things are out of balance, a bit too much brute force IMHO.I don't like the inductor right next to the ESP32/antenna because the radiated high harmonics actually lowered S/N in the GHz, it can swamp the RF RX and you get poor sensitivity. Even though it's a few MHz.Best to label terminal block pins for what they are, not colour codes.To protect 4-20mA I/O from accidental connection to power, I have seen small PTC's used, I'd have to dig for the p/n. So the resistor does not get burnt up."The output capacitor, COUT must be placed close to the junction of L and the diode D. The L, D, and COUT trace must be as short as possible to reduce conducted and radiated noise and increase overall efficiency."

author: ITman496
author_rank: Regular Contributor
timestamp: June 11, 2025, 03:20:04 am
content: Yes, you are absolutely correct that this power supply is complete overkill for what this thing is. However, I have plans to use this power supply in more things that have higher power draws, so I wanted to use this chance to try making it and learning from it for the future projects.The other reason I have the big clamps is because this device goes inside a generator and eventually my own vehicles so I wanted to attempt to harden it against spikes. The big diodes have not been treating me well, so I will likely replace them with smaller ones.. Bigger isn't always better as it turns outIn this case this doesn't matter as for this application I'm not using the esp32's radio at all, I just am using it for its ability to run arduino code and not cost too much and be quite fast.Good point, I should protect the 4-20ma input. If you can find that PTC, I'd appreciate that.I labeled the blocks the way I did because the propane tank receiver uses an ethernet style cable that is completely unlabeled so as with the last revision of this I wanted to make it foolproof for my coworkers to just smack it together and go without thinking too hard.Yup. Totally ballsed up the output capacitor. I'll be sure to fix that in the next spin of the board.. Which I will probably post here before I order

author: BennoG
author_rank: Frequent Contributor
timestamp: June 11, 2025, 06:41:23 am
content: C12 needs to be between +5V and GND not to the FB input.Also as other have mentioned move the inductor far away from the ESP.And make your ground connections solid so use more vias to get the ground paths as short as possible.Put a extra MLC between ground and VIN (100n or so) as close as possible to the LMR14050Benno