Raspberry Pi RP2350A SMPS - Inductor Polarity - Page 1
Forum section: electronics / projects

author: EEVblog
author_rank: Unknown
timestamp: August 11, 2024, 11:02:35 pm
content: One of my Patrons pointed me to this Raspberry Pi design document that talk about them having issues with SMD inductor polarity on their new design.How sensitive must the controller (built into the new RPi chip) be to be impacted by this?https://datasheets.raspberrypi.com/rp2350/hardware-design-with-rp2350.pdfQuote

author: moffy
author_rank: Super Contributor
timestamp: August 11, 2024, 11:47:42 pm
content: That is a very nasty issue, maybe a case for a small shielded enclosure around this psu and its components.

author: floobydust
author_rank: Super Contributor
timestamp: August 11, 2024, 11:49:57 pm
content: You don't want any stray B-field from the inductor coupling to sensitive nodes. I haven't seen any Abracon part numbers yet, the freq., the current.Having a polarity dot, flipping that - solves nothing IMHO. I think it's another problem that exists. Try applying the Right-hand Rule for more confusion lol.Many "shielded" inductors use glue with ferrite particles in it.I would guess having that AVDD resistor R5 nearby is bad practice, and a stability problem with the Vcore reg might be manifesting.More in6.3.8.3edit:"Even with nominally fully shielded inductors, leakage magnetic field coupling into the loop formed by the output VREG_LX node through the inductor and output capacitor (COUT) seems to affect the regulator control loop and output voltage. Field orientation (and hence inductor orientation) matters - the inductor has to be the right way around to make sure the regulator operates properly especially at higher output currents and for higher load transients. This necessitates an inductor with marked polarity."

author: idpromnut
author_rank: Unknown
timestamp: August 11, 2024, 11:55:52 pm
content: From the document the RPi people released, the switcher is integrated into the RP2350 IC itself, and the problem stems from something in that circuit (or coupled near it?) going screwy if the inductor is installed "backwards". The only thing I've been able to find was that it involved inductor kickback. As was said above, I'm only "familiar" with the magnetic field collapsing inducing a current/voltage in a circuit.

author: Xena E
author_rank: Frequent Contributor
timestamp: August 12, 2024, 12:02:29 am
content: This is interesting.I have come across issues where the outer winding layers of inductors have had to be connected into the circuit in the most 'electrically quiet' orientation to stop a problem, self screening if you will, but I'd say that shows a design to be very marginal.If needs must I suppose, but it seems a bit wank.

author: EEVblog
author_rank: Unknown
timestamp: August 12, 2024, 12:10:29 am
content: Quote from: floobydust on August 11, 2024, 11:49:57 pmThat was my first reaction.Problem for them is they are stuck with the switcher being inside the RP2350A silicon.Could very well be some instability or some other issue at the die design/layout level.Can't say I've ever heard of this issue with dedicated switcher chips.

author: EEVblog
author_rank: Unknown
timestamp: August 12, 2024, 12:14:05 am
content: Quote from: floobydust on August 11, 2024, 11:49:57 pmR5?

author: PCB.Wiz
author_rank: Super Contributor
timestamp: August 12, 2024, 12:25:05 am
content: Quote from: Xena E on August 12, 2024, 12:02:29 amYes, I've recall building two identical power supplies and one popped the uA723 sometimes and the other never did.A lot of digging and un-twisting of the transformer primary 2 blue wires, uncovered the mains winding was switched-pin nearest secondary layer in one case and switched-pin further away in the OK case.Easily fixed.I can see it would make sense to connect the outside winding layer to the CAP rather than the SWITCH, but that's really acapacitiveeffect.Anymagneticeffect must be second order as a rod-core will be close to rotationally symmetric in field, unless the shielding ferrite itself is asymmetric ?Quote from: Xena E on August 12, 2024, 12:02:29 amAn easy check will be to swap one around on a board, and see what actually happens

author: ataradov
author_rank: Super Contributor
timestamp: August 12, 2024, 12:49:43 am
content: While there are theoretical concerns, if it really matters, then the designs is just broken. Or they are over cautious.Switching converters inside the MCUs are tricky in general. Atmel's SAM D51 has a noisy buck converter and the errata basically says to not use it if you also want to use PLL. No amount of strange requirements is going to fix things, so you have to errata out the broken feature.Swapping it around and seeing what happens is likely not going to show anything. If it does, then the converter is broken for sure.

author: Xena E
author_rank: Frequent Contributor
timestamp: August 12, 2024, 01:30:44 am
content: Quote from: ataradov on August 12, 2024, 12:49:43 amIf the inductors mag field is causing the problems when orientated one way, this is a parasitic degenerative/regenerative issue.The design is just broken.

author: langwadt
author_rank: Super Contributor
timestamp: August 12, 2024, 01:34:56 am
content: Quote from: ataradov on August 12, 2024, 12:49:43 amfrom the quoted section of the datasheet: "We found that the regulator performance could be massively improved if the inductor is 'the right way round'."that leaves broken ...

author: radiolistener
author_rank: Super Contributor
timestamp: August 12, 2024, 01:47:09 am
content: I think this is their big mistake to put switching converter inside MCU. Someone wrote that they also had ADC nonlinearity issue. Most of all, this issue with converter is also the root of cause for ADC issue...And even if they fix this issue, it would still be a problem. Because such MCU cannot be used in solutions which is critical to noise interference and power supply purity and since they embedded that regulator inside MCU, there is no way to replace it with linear one...

author: ataradov
author_rank: Super Contributor
timestamp: August 12, 2024, 01:57:24 am
content: Oh, I did not see that it was basically mandatory to use it in a switching mode. Usually switching mode is included as an option if you really want to shave some power consumption.In that case the whole device is junk. I don't see that being too popular if it requires components from a single supplier that need to be mounted the right way.

author: ArdWar
author_rank: Frequent Contributor
timestamp: August 12, 2024, 02:18:50 am
content: I'm still not exactly sure what the problem is. If it's B field coupling then surely it's theorientationthat is critical instead of thepolarity. Like if you assemble the inductor with the right polarity but laid the PCB out the other way then the B field problem is exactly the same??Edit: now reading a bit more :duh: it's indeed the orientation that matters. Now even with the correct polarity and orientation, some indicator can be placed sideways, that will be interesting huh.Oh welp... at least you can bypass it right?

author: ejeffrey
author_rank: Super Contributor
timestamp: August 12, 2024, 02:19:06 am
content: Yeah. It's hard enough to make sure that diodes and LEDs get assembled in the right orientation and people know to look for that. Making sure inductors get assembled the right way is just too much trouble. Not to mention if the manufacturer starts winding inductors the other direction, or some manufacturing engineer tries to substitute an alternate part.Of course all those problems are something you can deal with with assembly notes and proper manufacturing checks and supply chain management. But for what should be a simple low power regulator it's not worth the trouble. And it means the whole design or at least the board layout is marginal, so I wouldn't really trust it long term anyway even if made "correctly".

author: radiolistener
author_rank: Super Contributor
timestamp: August 12, 2024, 02:30:46 am
content: Quote from: ejeffrey on August 12, 2024, 02:19:06 amThe main problem is not the wrong inductor orientation in space, but the fact that changing inductor orientation affects MCU functions... That is an indication about serious issue in MCU design. And even if you rotate inductor in such way that MCU start to work, it don't fix the issue. It's just hide it...Quote from: ejeffrey on August 12, 2024, 02:19:06 amYes, exactly

author: floobydust
author_rank: Super Contributor
timestamp: August 12, 2024, 02:57:05 am
content: Raspberry Pi Foundation isn't doing IC design, IMHO. It would be high-level architecture only.I have seen semiconductor IP licensed, the Lego blocks such as FLASH memory are a chunk you buy (license) from a specialty design firm. Developing that in-house is too costly and time consuming.This Vcore buck-converter, I'm assuming it's a Broadcom piece of IP. 1.1V at several 100's mA for the two cores?Inductor is a custom Abracon 3.3uH 2016  guessing like theASMPM-0806 seriesbut with a polarity dot.Wonder what happens if you place it on its side, and what is picking up the B-field in the MCU. Very strange and bad news."The regulator’s output voltage can be varied between 0.55 V and 3.3 V, but RP2350 may not operate reliably with its digital core supply (DVDD) at a voltage other than 1.1 V"Quote from: EEVblog on August 12, 2024, 12:14:05 amIt's the AVDD RFILTor R5 on the Pico schematic that I thought was originally picking up noise - not Vcore reg. I'm an optimist lol.

author: SiliconWizard
author_rank: Super Contributor
timestamp: August 12, 2024, 03:22:02 am
content: In this case, regarding Flash, I doubt it's on the same die. It's probably a second die in the same package.Regarding the buck converter, who knows if they have designed it or bought an IP. (Similarly for the ADC.)Even if they bought some IP, integrating it properly on the die requires skills. Even just the pinout may influence the results.I haven't followed closely the problem here, but I've already seen this kind of problems: switching converters with EMI coupling to the feedback network and throwing regulation completely out of whack.

author: profdc9
author_rank: Frequent Contributor
timestamp: August 12, 2024, 03:32:42 am
content: I would guess there's probably a synchronous buck converter consisting of a P-channel high-side and a N-channel low side on the chip.Think that perhaps adding a Schottky diode (SS14) or a small snubber capacitor (1000 pF) at the VREG_LX terminal might help? Maybe this would smooth any transients when the high-side switch turns off.

author: EEVblog
author_rank: Unknown
timestamp: August 12, 2024, 05:14:11 am
content: Quote from: ataradov on August 12, 2024, 01:57:24 amThat's a huge red flag. Potential show-stopper. If I was the design engineer deciding whether or not to implement this micro in a design, and I knew this, I'd do anything to avoid using it.

author: oPossum
author_rank: Super Contributor
timestamp: August 12, 2024, 05:29:47 am
content: An external linear regulator can be used.An example:https://x.com/dangerousproto/status/1821903736446550197https://x.com/dangerousproto/status/1822232583104495992

author: ataradov
author_rank: Super Contributor
timestamp: August 12, 2024, 05:52:28 am
content: Quote from: oPossum on August 12, 2024, 05:29:47 amI'm surprised that this is not an official solution. You don't even need to think hard to figure out that an LDO is cheaper than the inductor and all the supporting components.This makes me think there may be issues with this. May be sequencing issues or something like that. Things may appear to work until they start to break.Hopefully Dangerous prototypes cleared that use with the RP guys.

author: gamalot
author_rank: Super Contributor
timestamp: August 12, 2024, 06:12:57 am
content: In the schematic on page 5 it is R3, while in the PCB silkscreen on page 6 it is R5.

author: Geoff-AU
author_rank: Frequent Contributor
timestamp: August 12, 2024, 06:14:31 am
content: With this kind of hocus-pocus the RP2350A would beperfectfor audiophile projects.Having a manufacturer custom-design an indexable inductor is an end-run around understanding and fixing the real cause. It also ignores all the lessons hardware design companies learned about supply chains during the pandemic.Meh.

author: Hydron
author_rank: Super Contributor
timestamp: August 12, 2024, 06:17:47 am
content: Datasheet states "Alternatively, DVDD can be supplied directly from an off-chip power source."I've got a few RP2350s to play with but none of the special inductors, happy to hear a LDO has been used successfully, and datasheet seems to back it up (though I haven't read enough to know if there are sequencing considerations). Shame I didn't see this thread earlier, I would have asked the RPi guys directly.

author: Xena E
author_rank: Frequent Contributor
timestamp: August 12, 2024, 06:37:08 am
content: Quote from: PCB.Wiz on August 12, 2024, 12:25:05 amIt would make a good Dave VideoI'm sure the research has been thoroughly done.From the PDF:QuoteSo, what is the axis of the magnetic field on these inductors?In the conjunction that the magnetic axis of the inductor is horizontal, the only thing that will effect its magnetic field polarity is the manufacturer's direction of 'winding' reversing the component on the board won't change that.If OTOH the component creates a vertical field axis, it can be changed simply by reversal.Quote from: Geoff-AU on August 12, 2024, 06:14:31 amPiss poor.Bad luck on the guys who had to sort it.Quote from: Hydron on August 12, 2024, 06:17:47 amI'm sure the membership will be interested in your experiences...

author: tszaboo
author_rank: Super Contributor
timestamp: August 12, 2024, 10:06:45 am
content: Quote from: ataradov on August 12, 2024, 05:52:28 amWell, they are using a crappy 1 cent inductor to do the switcher.Which is not a critique, I use the same type of inductor if it does the job, and trying to keep the BOM low.Probably this type:https://abracon.com/Magnetics/power/ASMPL.pdfThe shielding on these inductors is non existent.I bet you, that a Würth MAPI like 74438343022 (likely a 3.3uH part would be needed) would do the job and wouldn't cause any orientation issues.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: August 12, 2024, 10:46:21 pm
content: Quote from: tszaboo on August 12, 2024, 10:06:45 amUnlikely to be that one as they state thisLX must be fully shielded, 3.3μH ±20% and with a maximum DC resistance of 250mΩ. Saturation current should be at least 1.5A.Based on the datasheet info, I'd say it is an interpolation of these two existing part numbers that Abracon already make from Digikey infoAOTA-B201610S4R7MT IND 4.7UH ±20% 1.3A iSat 1.6A Shielded 235mOhm MaxAOTA-B201610S2R2MT IND 2.2 µH ±20% 2 A iSat 2.6A Shielded 74mOhm MaxAOTA-B201608S2R2MT IND 2.2UH 1.8A iSat 2.3A Shielded 148mOhm Max

author: idpromnut
author_rank: Unknown
timestamp: August 13, 2024, 11:31:38 pm
content: So after re-reading the design doc a few more times, it looks like the placement of the switching inductor and output capacitor are such that they need to be close to the RP2350. That close proximity of the inductor to output capacitor is causing some of the magnetic field to couple into the output capacitor which in turn is upsetting the switcher control circuitry in the RP2350 (Page 6 of the Hardware Design with RP2350). They note that "shielded" inductor is not what they expected. I'm still not clear on exactly how the magnetic field is coupling into the capacitor but I'm not a great circuit designer.

author: dobsonr741
author_rank: Frequent Contributor
timestamp: August 23, 2024, 04:29:30 am
content: I think it refers to RP2040's ADC hiccup, is that right?Code:Someone wrote that they also had ADC nonlinearity issue. Most of all, this issue with converter is also the root of cause for ADC issue...The RP2040 issue is well explained inhttps://github.com/raspberrypi/pico-feedback/issues/91

author: iMo
author_rank: Super Contributor
timestamp: August 23, 2024, 06:22:03 am
content: Quote from: dobsonr741 on August 23, 2024, 04:29:30 amThat had been explained by a guy from rpi company in past - a bug in the sizing of the capacitors in the SAR ADC (they use capacitors instead of resistors in there), not fixable (as it would require complete new set of photo-lithography masks, imho).Anyhow, as has been discussed in a different thread on the 2350 the only way is to use LDOs for the 3.3V and 1.1V voltages (when you want somehow clean noise background).PS: it does not matter what kind of inductor you are using, or what "polarity" it has - the switchers operate such they produce rather energetic current flows, with any resistances in the circuit (like the onchip tracks) it will always create ripples.BTW., big players - like ADI, TI - recommend to use LDOs for filtering the switcher's ripples - a) they will sell one chip more, b) the modern LDOs have got a pretty good PSRR (like ~90+dB) at those freqs, thus they filter the ripples better than a, say, second order low pass LC filter (2xLC).

author: ArdWar
author_rank: Frequent Contributor
timestamp: August 23, 2024, 08:16:56 am
content: Oh come on, there are a lot other MCU having embedded DCDC. STM32WB, LPC55, nRF52 etc. All are robust enough not to trip on itself, all are designed reasonable enough by post regulating the DCDC with internal LDO for the actual VCore etc, all provide easy (and documented) means to bypass or disable it.

author: iMo
author_rank: Super Contributor
timestamp: August 23, 2024, 10:00:25 am
content: Quote from: ArdWar on August 23, 2024, 08:16:56 amI would not compare rpi company with the players like STM or NXP etc.Btw you may disable the 1.1V switcher in sw..https://www.eevblog.com/forum/microcontrollers/possible-click-bait-title-the-raspberry-pi-pico-2-now-has-extra-risc-v-cores/msg5604377/#msg5604377.."click bait title" .. a new eevblog feature ??

author: MK14
author_rank: Super Contributor
timestamp: August 23, 2024, 10:56:09 pm
content: Quote from: iMo on August 23, 2024, 10:00:25 amNo, not an EEVBLOG Forum feature, that was me!Originally, before news of the RP2350/PICO2 release became public, Eben Upton (the main man, at least in terms of public perception, as regards the Raspberry PI organizations), seemed adamant, that there would be NO RISC-V's, in Raspberry Pi's, anytime soon.So, I thought it would make a good joke, to start the title, with something on the lines of "possible-click-bait-title".But, almost immediately after creating the thread. Remembered, that this forum, does NOT seem to appreciate, my sense of humor. So, I changed the title, before anyone had time to make posts in that thread.Unfortunately, as good as the forum software is, in many respects. It DOES NOT, physically change the web url addresses, to reflect the new (edited) thread title.Hence the 'funny' url to that thread, you quoted.

author: deanclaxton
author_rank: Regular Contributor
timestamp: October 02, 2024, 11:09:00 pm
content: I'm working on a board right now with RP2350 on it, and tempted to go straight down the 1.1V LDO route instead of trying to source the recommended inductor and following the layout guides around this.I imagine I'll just leave VREG_FB and VREG_LX as not connected. VREG_VIN still requires power though for the on-chip power-on-reset circuit so I'll still provide 3.3V there with a 4.7uF cap to VREG_PGND which would be tied to the GND plane. I imagine I could get away with a much smaller cap for the VREG_VIN if thats all its being used for (POR). Then again maybe not (RC?). Probably just go with 4.7uF and be done with it.Has anyone tested this LDO option as yet?

author: phil from seattle
author_rank: Super Contributor
timestamp: October 19, 2024, 04:19:00 pm
content: Quote from: deanclaxton on October 02, 2024, 11:09:00 pmRead the dangerous prototypes blog about their experience. They use a TI TLV74311 + 2 caps. I'm incorporating this into an RP2350B design. Haven't yet spun the PCB but getting close.