LMR51430: Another burning out: Vin to SW

Part Number: LMR51430
Other Parts Discussed in Thread: TPS564201, LMR51635

Tool/software:

Warning: I'm an amateur, working on a home project. My problem seems closely related to this old thread: https://e2e.ti.com/support/power-management-group/power-management/f/power-management-forum/1241239/lmr51430-switch-regulator-keeps-burning-out-at-vin-net

I'm using the LMR51430 as a buck converter: power will eventually come from the 12V automotive battery on a fixed engine (stationary generator), and I step that down to 5V/3A for a Raspberry Pi Zero plus a cell modem. I made myself a circuit diagram (little snippet below) and a 2-layer PC board design (also below). Ordered three LMR51430 chips, and the first two have burned up on the first prototype assembly. As the board isn't otherwise populated yet, there is no load connected to the regulator, other than one 3mA LED. Right now, everything is being powered by an HP bench top lab supply at 14V. Both chips survived at most a fraction of a second!

Symptoms: First chip was fully shorted (about 1 ohm) between Vin and SW. The second chip has about 100 ohm between Vin and SW, about 200 Ohm between Gnd and Vin, and about 160 ohm between FB and Gnd. Haven't measured diode voltage drops between pins, but it probably doesn't matter.

I know there are several minor mistakes in the circuit diagram and PC board layout. First, Cin is way too large, 22uF (of type X5R) instead of the desired 4.7uF. Second, the high-frequency 100nF part of Cin is missing in the circuit diagram, but I added it to the prototype (by soldering an 0603 on top of the 22uF 1210). And looking at the layout, there should be more copper plane around the chip, and the two feedback resistors are too far from the FB pin. But is that enough to outright kill the chip, twice in a row? On the good side: All power traces are at least 0.5mm wide, and most of them a full 1mm. Ground plane both sides.

The obvious suspicion is transients on the input. The good news is that the input power is very clean: A lab supply, turned on by a PCB-mounted toggle switch, and then less than 2" of trace to Vin. The bad news is that the TVS diode on the Vin line (an SMBJ20CA-13-F) hasn't been populated yet. And before anyone even asks: No, I don't have scope traces of what happened. Since I only have one chip left, sacrificing the last one on that altar seems silly (plus, I'd have to borrow a storage scope). But I'll get more hardware ordered early next week (Happy New Year to everyone, by the way). Could the toggle switch be at fault?

I already have the following items on my to-do list for the next spin of the board: Change Cin to 4.7uF + 100nF, move the two feedback resistors to be right next to the FB pin, move the 100nF output cap to be right next to the two 22uF caps, and add a lot more copper and vias. Perhaps even add a 33pF capacitor to the feedback line. Any other good suggestions to make this work? Maybe move to a different chip? Since I hand-solder, SOT23 is pretty much the smallest I can do, and I want to stay away from chips that have pads underneath; but the TPS564201/8 and their 5A friends look like fine choices too, and give me some headroom on current. Are they more forgiving? Should I add something on the Vin line for transient management? I have ample room on the board, so even an active chip would fit, but I'd need a concrete suggestion.

And if this mishap ever happens in production, it will kill everything downstream of the LMR51430, so I'm thinking of adding a ~2A fuse (through-hole, the size of a 1/8W resistor) to its 12V input, and a crowbar circuit to its output; perhaps a few 6V 1A zener diodes. That would make repair easier and much cheaper.

Again, I apologize for being a clueless amateur, but finding information about good design is hard, so I thought asking the experts here might be useful.

  • Hello

    It sound like you have touched on all the correct points.

    I think that concentrating on the grounding is the best place to emphasize.

    Try to get C4/C5 grounds close to the regulator ground pin.

    Try to get the R4 ground close to the ground pin of the regulator.

    Don't worry too much about C6.

    I think your input cap is good; you can put in the 100nF too.

    The top of R3 should be connected closer to C4/C5 rather than the inductor.

    You want the trace to the FB pin very short.

    Try to follow the PCB example in the data sheet a little more closely, regarding the grounds.

    I think that a Zener on the input, close to the device, is a good idea to prevent transients; or some other kind of clamper, etc.

    Your switch may be contributing to the issue.  When you are ready for more testing, try to start-up the device with a different method.

    You may also want to experiment with starting the device from the EN pin after the 12Vin is applied.

    We will be happy to look at any waveforms you may capture and review you new PCB layout when it is ready.

    Thanks

  • Thank you for the guidance! In a nutshell, you're saying that a respin of the PCB is the first order of business. But since I already have a PCB, and one more chip left (before reordering), I'll try to see what I can do with little pieces of wire to improve grounding and putting the zener right next to the input, and bypass the toggle switch. I'll borrow a storage scope from my wife's lab for the tests.

    Edit: Trying to run it from a current-limited supply was obviously a bad idea, since it will oscillate on startup. Won't do that!

    I'll report back in a few days or weeks!

  • Hmm, related question: When is the LMR51635 going to be available (from distributors, in small quantities)? After I get this board done and working, the next project won't be able to use the same LMR51430 (input voltage might go over 40V, darn irrigation transformers whose nominal 24V AC can be as high as 33V), and being lazy I could share the same PCB layout. If it's going to be just a few more weeks, I'll wait for the new chip.

  • Hello

    I will need to contact the Systems engineer responsible for that device to get an answer.

    That may take a week or so due to the holidays.

    Thanks

  • Hello

    I have asked our team about LMR51635 availability.

    Thank you for your patience.

    Thanks

  • Hello Ralph,

    Suggest you follow up with distribution regarding availability of the product in question.

    Thanks.

    David.

  • Victory! Some changes to the circuit diagram: Instead of a 22uF ceramic, now using a 22uF wound aluminum (intentionally higher ESR), plus a 4.7uF ceramic on the 12V input. Added a 100nF on the input side, directly next to the input pin. Added a diode to prevent reverse input voltage, and moved the TVS to be right next to the battleground. On the EN pin, I decided to put in a resistor divider, so the chip only turns on when the input reaches 8V (as the input capacitor charges up); by that point, turnon transients might have settled some. On the output side, moved the FB pin voltage divider to be right next to the pin, and get ground from nearby. Also added a 6V zener diode as a safety crowbar: in case the LMR51430 blows out, at least the rest of the board will survive. Added a lot more copper, and a few more vias. Instead of respinning the whole board, I instead made a tiny tester board for just the LMR51430 circuit, layout below.

    Works excellently now. Tested with nearly zero load (about 3mA) and at about 1A load on the 5V. Didn't get around to borrow a storage scope yet to take a picture of transients on the input pin.

    Next battle: Using the tiny tester board as an outboard power supply for my main board, finish complete testing of the prototype, and perhaps even deploy it for a few weeks: flying wires, and double-sided foam tape to hold the tester board onto the main PC board. And then, once the LMR51635 shows up at distributors, test it (already have a tester ready for it). And only then respin the main PC board to fix the roughly half dozen issues prototyping has already found.

    Thanks for the help! In particular since I'm just an amateur, who will buy about a dozen of these chips.

  • Hello

    Glad we could help.

    I will close this post for now; you can re-open if you have more questions.

    Thanks