Inverting Buck LMZ12001 failure - Page 1
Forum section: electronics / projects

author: jacobfeder
author_rank: Contributor
timestamp: June 08, 2022, 02:12:43 am
content: Hi,I have a design that requires a negative power supply. I'm using the LMZ12001 (https://www.ti.com/lit/ds/symlink/lmz12001.pdf) in an inverting configuration as outlined in this TI application note (https://www.ti.com/lit/an/snva425a/snva425a.pdf?ts=1654616902995). The schematic and board layout are attached.The problem is that the converter is failing to start properly. Counterintuitively, the -5.5V rail rises to +0.5V. I have tried several things, none of which have solved the problem:- Replacing the LMZ12001 chip- Adding a 10uF ceramic capacitor between VIN and GND near the IC- Removing C6- Adding a jumper wire from the anode of LED3 to the EN pin of U4 and a second jumper from EN to VIN through a 1k pullup resistor. The intention here is to ensure that the EN pin stays above the 1.2V threshold but not above 6.5V.- Removing U4 and powering the circuity on the -5.5V rail through a separate power supply. There are no shorts and the consumption is ~80mA @ -5.5V.- Checked the surrounding passives for correct values / no shortsI'm at a bit of a loss as to what could be going wrong with the IC, so any ideas on what to try would be greatly appreciated!Thanks!Jacob

author: Someone
author_rank: Super Contributor
timestamp: June 08, 2022, 02:21:59 am
content: The negative rail coming up above 0 is likely from the load between +5.5 and -5.5, try disconnecting the - output from any loads or use just a dummy load -5.5 to 0V?Also the appnote is for a specific switcher, most buck to inverting implementations include protection diodes.

author: jacobfeder
author_rank: Contributor
timestamp: June 08, 2022, 02:51:13 am
content: Thanks for your reply. Could you explain in more detail what you mean by protection diodes? Also the app note does specify that it includes LMZ1200x models.Thanks,Jacob

author: Someone
author_rank: Super Contributor
timestamp: June 08, 2022, 07:21:35 am
content: Quote from: jacobfeder on June 08, 2022, 02:51:13 amThere is discussion of the problems here:"Inverting Buck Regulator Saves Space and Power"https://www.power-mag.com/pdf/feature_pdf/1327592178_Austria_Feature_Layout_1.pdfYou have more input capacitance than the app note, the initial +ve excursion of the -ve rail might be the problem. Also the app not has some rather detailed design for the enable input, integrated switchers can do all sorts of weird things in startup.

author: Geek459
author_rank: Contributor
timestamp: June 08, 2022, 09:14:13 pm
content: Hi jacobfeder, where have you found a LMZ12001 application for negative output ? Probably i mistake but for me U4 can not output negative voltage especially if you have only a positive input. you have connected the output to ground and at pin4 (GND) you expect -5,5v , in this condition internal low mosfet can not drive the inductor.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 08, 2022, 09:48:53 pm
content: Have heard reports that those things might be, either counterfeit sometimes (particularly from less reputable sources), or maybe vulnerable in some way (which isn't clear; the example I know of was reasonably well protected, so it seems unlikely to me).Tim

author: jacobfeder
author_rank: Contributor
timestamp: June 08, 2022, 10:43:04 pm
content: Thanks all for your feedback.Quote from: T3sl4co1l on June 08, 2022, 09:48:53 pmThese parts are from digikey so I would hope they aren't counterfeit...Quote from: Geek459 on June 08, 2022, 09:14:13 pmThe application note is linked in the original post. From page 2: "This configuration can be applied to the whole family of LMZ1420x and LMZ1200x modules so long as the input voltage and output current limits are observed".Quote from: Someone on June 08, 2022, 07:21:35 amI liked your theory about the positive excursion due to the load. I did two things:1) Remove all of the load ICs2) At some point I noticed that the copper connecting the feedback resistors had broken (likely due to soldering and desoldering new ICs so many times). I manually bridged the connection.Now the plot thickens: the converter still does not start up properly. When power is applied, it creates an internal short. However, I am able to get it to start up reliably by doing the following:- applying VIN (I am powering it with a benchtop supply which I put in current limit mode to prevent the IC from heating up too much while it is internally shorted during this initial period)- use a jumper wire to manually short the -5.5V to GND- remove the shortThen the converter regulates to -5.5V and operates normally. I am starting to suspect it is a defect in the IC design - here is my theory: when the IC is first powered (but before regulation happens) there is some protection diode or internal leakage that causes the -5.5V rail to increase to ~ +0.3V. Once the EN pin threshold is tripped, the IC can't actually turn on the low side FET properly because the source voltage is too high (0.3V).I'd be curious to hear what people think about this.Thanks,Jacob

author: Someone
author_rank: Super Contributor
timestamp: June 08, 2022, 11:39:06 pm
content: Quote from: jacobfeder on June 08, 2022, 10:43:04 pmThere may also be some damage to the device carried forward between experiments, always wanting to burn another fresh chip for each test gets expensive. Even without a sensitive configuration like this, for dual/split supplies I usually put a low forward voltage reverse blocking diode to avoid any of the powered devices (or regulators/supplies) clamping to 0V through their protection paths.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 09, 2022, 12:05:00 am
content: Quote from: jacobfeder on June 08, 2022, 10:43:04 pmAlright, that should be fine then.Ah, I have an idea. Try a reverse bias diode in parallel with the output. Must be schottky, B340 or bigger say.Consider what happens on startup: a step input charges the input capacitors, C6 in series with C31 (and the ceramics in parallel with them). This puts the output up to VIN/2 if not for internal diodes. Likely you're forward-biasing substrate diodes or the like, forcing it into CMOS latchup. Current continues to flow until reduced below threshold (100mA or so??) and then normal operation resumes. (If it isn't destroyed already, from the ~amperes surge?)Consider additionally, what a bench supply does in current limit: with current set nonzero and no load connected, its output charges to VOUT (whatever that's set at). When a load is connected, surge current flows from its output capacitor (bench supplies almost ALWAYS have an output bypass cap; they don't need one, but they almost always have one) into the load. This surge current can be many times the current limit, indeed over 100A if the leads are short and the cap's ESR is modest to low.The proper way to use a bench supply is set the output to zero, either by hard-shorting it (thus forcing the current limit condition at VOUT ~ 0), or by setting it to zero (current and/or voltage, whichever works) then ramping up. This isn't often documented so don't feel bad for missing it -- but it is one of the many gotchas that AC circuits (dynamic responses) exhibit, that simple DC explanations ("it's constant current, how can I go wrong!") miss.I would remove C6 entirely, and move the ceramic caps on that side to GND. With everything on common GND plane anyway, the input caps can all share -- no need to have them dedicated to each converter. Also, I guess the screenshot isn't the whole board, but just a small section of a larger board? Don't be shy with grounding, pour that shit all over, get it covered real good, oh yeah. The VINs can then all be supplied from a common pour, with a couple bypasses, and that'll pretty much do it.A TVS from GND to VIN may also be a good idea to limit inrush peak voltage -- or electrolytics typically handle this as well (by having enough ESR to dampen any overshoot) -- which you already have, I guess (unless those are polymers), so that shouldn't be a problem here.Or like maybe a series or antiparallel diode (the latter is also provided by a unidirectional TVS) just to prevent reverse connected supplies, since -- center-negative barrel jacks are uncommon, but definitely show up from time to time. YMMV.Tim