High pitch noise from switching regulator and OLED display - Page 1
Forum section: electronics / projects

author: dingari
author_rank: Contributor
timestamp: June 22, 2016, 10:40:20 am
content: I'm having some trouble with a design for a project I'm working on. It includes an SSD1351 OLED display with a 13V DC-DC FAN5331 boost regulator.Whenever the OLED display is on, a very audible high pitched noise is coming from the circuit. I can't exactly pinpoint from where it is coming from.My schematic looks like this:A couple of notes:C40 and C38 were added because we initially thought it was noise on the 13V rail. ATM C40 is populated with a 47uF capacitor.Only one inductor is populated, I wanted to have the option of using either inductor so the other footprint is unpopulated.C20 is populated with a 100pF capacitor, 180pF is an ideal value according to the FAN5331 datasheet, and this was the closest value we had. Initially we used a 100nF capacitor like in the Adafruit schematic. Changing the value had no noticeable effect.The schematic was drawn with reference to theAdafruit OLED breakout boardand the FAN5331 and SSD1351 datasheets.I've tried disconnecting the switching regulator and feeding 13V to the display from a bench power supply. I could hear a faint whine, but not as much as it is with the switching regulator.SSD1351 datasheet:https://cdn-shop.adafruit.com/datasheets/SSD1351-Revision+1.3.pdfFAN5331 datasheet:https://www.fairchildsemi.com/datasheets/FA/FAN5331.pdfThe Any ideas what might be causing this and how to avoid it?

author: Buriedcode
author_rank: Super Contributor
timestamp: June 22, 2016, 11:40:07 am
content: First I would disconnect everything the boost converter is powering, then apply a known load and power it up, and check the waveforms with a scope. Increase the load to the maximum you think your circuit will draw and check again. Whilst it could be an error in the layout of the OLED, first port of call would be that boost converter.Do you have an image of the PCB layout for the regulator? As you probably know, they can be extraordinarily sensitive with regard to layout, especially the feedback trace.

author: dingari
author_rank: Contributor
timestamp: June 22, 2016, 01:06:40 pm
content: Hi, thanks for the reply!Here is a screenshot of the layout with the relevant components highlighted:The 5V come in on the left and 13V go out of R17 on the left. I'd like to think it's okay and following all the layout guidelines in the datasheet.Quote from: Buriedcode on June 22, 2016, 11:40:07 amWhich waveforms would be of most interest here? The 13V rail, or the inductor terminals? Or the feedback pin?

author: Buriedcode
author_rank: Super Contributor
timestamp: June 22, 2016, 01:41:44 pm
content: Even with such low power, the traces look rather thin, but what caught my eye was the GND side of R18. Whilst I'm sure it does connect to GND it may have to travel a fair way and can be a long way from where the device has its GND connection. Even with low power switchers, I tend to use polygons for all the power traces, or at least traces the width of the pads. Also, via's are your friend. A solid ground plane on the underside gives a low impedance path to ground, with via's very close to every component that needs a GND - specifically the device and input/output caps. With most PCB manufacturers not caring about the number of via's its best practice to use multiple ones to stitch the top and bottom GND pours. If its a DIY PCB it can my life difficult, but I still use double sided boards for power supplies for this very reason.The datasheet has a layout that is pretty much ideal.With all that said, I have seen some replies to problems with switchers that imply it 'wont' work at all unless you have thick traces, lots of GND vias and a solid ground plane'. I don't believe that is true, and many devices work (as in regulate) with less than optimal layout, just perhaps not as efficient or as quiet with regards to EMI.As for debugging, i would cut that thin trace going from R17 off to whatever its powering - you can cut it with a razorblade or craft knife, scrape the solder mask off either side of the cut and place a 0-ohm 0805 resistor there later. That would isolate the switcher and allow you to provide a known load.Another thing, whilst the rest of the circuit is disconnected, you can use a lab power supply and measure its current draw. In the switchers datasheet I see references to '50mA' as being the maximum output current. I'm sure adafruit know their stuff, but it would also check yoru layout/design for errors if it draws > 50mA.Sorry for the disjointed reply, I try to strike a balance between speculation and 'things to try', because just providing things to check is practical - it doesn't always give one a basis for future debugging. And just talking about theory is all well and good but you've designed and built a circuit that doesn't work as expected so talking about redesigning it doesn't help much.

author: dingari
author_rank: Contributor
timestamp: June 22, 2016, 03:17:41 pm
content: Hi again!Quote from: Buriedcode on June 22, 2016, 01:41:44 pmI can see that R18 has a short path to the FAN5331 GND pin underneath R17 and C20. I'll really consider the vias on the next run. It's a manufactured PCB so that shouldn't be much hassle.Quote from: Buriedcode on June 22, 2016, 01:41:44 pmI'll try that out. But one thing: Where exactly in the datasheet do you see 50mA maximum output current? Judging by the second graph on page 4 I should be able to draw ~140mA @5V input voltage?Quote from: Buriedcode on June 22, 2016, 01:41:44 pmNo problem, I appreciate the helpQuote from: Buriedcode on June 22, 2016, 01:41:44 pmI unplugged the OLED display and connected a 50k variable resistive load instead. At around 350-400 ohm the circuit started to whine, at 13V supply voltage that comes out to 35-ish mA.I then connected the OLED display and the resistive load in parallel. The circuit whined and I saw the interference in the waveform, probing the 13V rail. The whining stopped when I got down to 130 ohms.So, is it the low current draw of the OLED display that is causing the regulator to be unstable? How can I design around this?

author: suicidaleggroll
author_rank: Super Contributor
timestamp: June 22, 2016, 03:32:42 pm
content: I think grounding is the main problem here. While R18's ground isn't ideal, C19's ground is much worse. Just try to trace the path from IC5's ground to C19's ground, you can't do it on the image you posted...how far out do you need to go? That's a huge loop. Try running a short jumper wire between the IC5 ground and C19 ground, see if that makes a difference. In a future rev you should flip C19 180 degrees and run the power line straight up instead of horizontal, that'll give the ground a straight shot from the input cap to the IC - that's how it's done in the datasheet.

author: Powermax
author_rank: Regular Contributor
timestamp: June 22, 2016, 11:22:57 pm
content: I think the problem is that inductors and MLCC capacitors can be noisy. MLCC capacitors, while nice because of their size, low ESR/ESL/tan delta (due to the inherently parallel construction), due suffer from piezoelectric effects. Voltages applied across the capacitor will cause it to flex in response, and similarly vibration and stress can cause a voltage to develop across the capacitor. So when operating MLCC capacitors at audio frequencies, you can sometimes faintly hear them. (but be careful, MLCC's are fragile too, and are easily fractured with shock and vibration.) Similarly, the windings on inductors can also be noisy. Adding epoxy resin to the windings can help stiffen up the windings and silence the inductors.You may also want to change the switching frequency the switch mode converter operates at, if it is operating at audio frequencies.

author: dingari
author_rank: Contributor
timestamp: June 23, 2016, 09:23:59 am
content: Quote from: suicidaleggroll on June 22, 2016, 03:32:42 pmWhile your observations on R18's and C19's ground path are correct, and vill be fixed in the next revision, I don't think that's what's causing this. I've used the same configuration on another board which had much more ground plane to work from, and the same noise was present there.I've run a jumper wire to shorten C19's ground path and it had no effect on the noise. And again, it is only present when a small amount of current is being drawn from the regulator.Quote from: Powermax on June 22, 2016, 11:22:57 pmThe regulator is operating at a fixed switching frequency of 1.6MHz, so it's well beyond the audible spectrum.Edit: It seems to be the piezoelectric effect in the MLCCs. After removing the ceramic capacitors from the 13V rail and temporarily replacing them with electrolytic capacitors I only hear the faint whine I heard when powering it with a lab power supply, and the waveform looks much nicer.

author: Powermax
author_rank: Regular Contributor
timestamp: November 02, 2020, 10:59:01 pm
content: Quote from: dingari on June 23, 2016, 09:23:59 amI think you can end up with sub-harmonic oscillations and audible noise when the load is highly dynamic. While it is set to run at 1.6MHz the frequency and amplitude is modulated in such a way you end up with constructive/destructive interference and it gets mixed down to audio frequencies.

author: Benta
author_rank: Super Contributor
timestamp: November 02, 2020, 11:13:18 pm
content: After more than 4 years, I'm pretty sure that the OP has found a solution...