Power Supply startup weirdness - Page 1
Forum section: electronics / projects

author: AndrewLoomis
author_rank: Contributor
timestamp: May 30, 2018, 12:26:20 am
content: My 5V switching buck converter design has some kind of problem with it, the startup is unreliable, and I have attached a scope capture of what a "successful" startup looks like. CH1 is output voltage, CH2 is EN pin, also input voltage. It shoots up to about 1V, dips, then finally goes up to 5V. A failure looks like the same, but instead of going up to 5V, it goes from 1V down to 0.At first I thought the problem was that the input voltage was not high enough to regulate (I have about 2000uF of bulk capacitance on input for motor control, so takes a while to charge up), so I increased the soft start capacitor value and now it seems to start with plenty of input voltage, the reliability is better, but not absolute. What would cause this weird startup behavior? I also attached my design. I have 3.3V and 5V variants that both have the same problem. Please ignore U14, the latch, I'm not using it right now.

author: David Hess
author_rank: Super Contributor
timestamp: May 30, 2018, 01:17:24 am
content: There is an awful lot of noise shown. What does the switching waveform look like and what does the input and output ripple voltage look like?

author: AndrewLoomis
author_rank: Contributor
timestamp: May 30, 2018, 01:39:48 am
content: I sent in my oscilloscope for repair, so I can't do any more probing right now, but I did measure the output ripple, and I remember it being on the order of 10s of mV peak to peak.

author: David Hess
author_rank: Super Contributor
timestamp: May 30, 2018, 02:02:00 am
content: The screenshots show a lot more than that so I wondered if the switching waveform was getting into the feedback network which can cause all kinds of weird problems.Sometimes switching regulators can latch off because of their negative input resistance characteristic but I do not think that is happening here. Some designs have fault protection or foldback current limiting which shuts the converter off but the datasheet does not show anything like that.

author: JS
author_rank: Frequent Contributor
timestamp: May 30, 2018, 02:31:26 am
content: I'm fine with the noise on the output, it the ripple is good enough for the load is good enough for me.The enable pin has a lot of noise too and that's not good, now the question is if that's really there or picked by the probe or probing technique, but looks like a bit too much for being from probing. If it's really there you should solve that and try again.JS

author: AndrewLoomis
author_rank: Contributor
timestamp: May 30, 2018, 10:54:13 am
content: Thanks for the replies David and JS, I'll take more measurements when I get my scope back... This is bad timing for a repair, but oh well. The EN pin has a threshold at 1.2V, so I don't think the noise reaches quite that far. I was probing using the ground clip of of the probe, connecting to the power input ground, so the ground loop was large and picking up all the switching noise. I just did this for convenience, but in the future I'll use the ground spring tip for better probing.I've been trying to understand switching regulators more, and there's something called "crossover frequency", which is determined by the compensation network. The datasheet just says to keep it below 1/10th the switching frequency (I'm using 500kHz and 300kHz switching freq), so I have it set to 25kHz. Could this be having some unforeseen affect on the startup? Or does this have more to do with inductor value and/or output capacitance?

author: SiliconWizard
author_rank: Super Contributor
timestamp: May 30, 2018, 02:09:07 pm
content: What's your input voltage range in all conditions?The AOZ1284 datasheet states something that could be an annoyance:QuoteSo maybe in some cases, your input voltage temporarily drops below 4.5V and prevents the regulator to operate properly? Just a thought. (You may also want to double check the EN input, but I suppose you have).

author: AndrewLoomis
author_rank: Contributor
timestamp: May 30, 2018, 02:14:10 pm
content: My input voltage is from a 4S Lipo battery, so it varies from about 14.8-16.8V. It is unlikely that it drops below the input threshold. I specifically chose this regulator because of the high max input voltage (36V). The previous regulator I was using had a 18V max input and every other time I plugged in the battery, the regulator exploded...

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 30, 2018, 02:14:17 pm
content: Show layout.Tim

author: AndrewLoomis
author_rank: Contributor
timestamp: May 30, 2018, 02:21:31 pm
content: PNG images of layout, and EAGLE files in ZIP attached.

author: SiliconWizard
author_rank: Super Contributor
timestamp: May 30, 2018, 03:27:22 pm
content: Quote from: AndrewLoomis on May 30, 2018, 02:14:10 pmOk then... so if you're certain it's not an EN signal "bug", then maybe it could be that during the soft-start period, the circuit you're powering with this regulator draws a lot of current which could prevent the regulator from getting to steady state. One possibility could be that there's something in your circuit that draws high currents when powered at low voltages (such as if it contained any step-up regulator), and then the longer the soft-start period, the worse it could get. Just a thought.If not, then, maybe the inductor doesn't have the right specs or there is some kind of routing issue (but I doubt it would prevent the regulator from starting unless you did something extremely wrong.)

author: AndrewLoomis
author_rank: Contributor
timestamp: May 30, 2018, 03:37:54 pm
content: QuoteThat's interesting, the 3.3V supply does draw a high current right of the bat, it it powering the Raspberry Pi Compute Module 3, which boots immediately on startup, and draws about 700mA. This is the inductor I'm using:https://www.digikey.com/product-detail/en/pulse-electronics-power/PA4341.103ANLT/553-4009-6-ND/7943333I think the specs are sound, it's current rating and saturation are much higher than the current flowing through it in my circuit. I kind of arbitrarily chose the crossover frequency (determined by the resistor and capacitor on the COMP pin) of the switcher, just keeping it below 1/10th the switching frequency. Does anyone know what effects the crossover frequency has?

author: SiliconWizard
author_rank: Super Contributor
timestamp: May 30, 2018, 04:01:15 pm
content: Quote from: AndrewLoomis on May 30, 2018, 03:37:54 pmI suspect it draws 700 mA on average but a whole lot more for a few ms while starting, and may draw even more because of the soft start of your regulator. I don't know about the PI3 module but I've used regular Pi3's and instant current can get over 2.5A easily (which is why a lot of USB PS's can make them fail). Could you try monitoring the instant current on a scope using a small shunt resistor between the 3.3V output and your module? (Unless you have already.)

author: AndrewLoomis
author_rank: Contributor
timestamp: May 30, 2018, 04:53:58 pm
content: Once my scope comes back from repair, I will measure the instant current, but the buck converter I'm using can supply up to 4A, and I have used 3A buck converters in the past powering this same module with no problems (https://www.digikey.com/product-detail/en/texas-instruments/TPS62091RGTR/296-46388-1-ND/7033562). So I don't think a lack of output current is the problem. What components would you recommend tweaking just to see if the startup waveform changes? Tweaking the soft start some more?

author: maginnovision
author_rank: Super Contributor
timestamp: May 30, 2018, 05:07:40 pm
content: Quote from: AndrewLoomis on May 30, 2018, 02:14:10 pmWas that a TI chip? tps54425?

author: AndrewLoomis
author_rank: Contributor
timestamp: May 30, 2018, 05:22:30 pm
content: QuoteThe previous regulator I was using (that kept exploding) was a TPS65261 (dual switching regulator, perfect, except for the low max input voltage).https://www.digikey.com/product-detail/en/texas-instruments/TPS65261RHBR/296-39591-1-ND/5149051

author: SiliconWizard
author_rank: Super Contributor
timestamp: May 30, 2018, 05:34:10 pm
content: I suppose VDD is your battery voltage?

author: AndrewLoomis
author_rank: Contributor
timestamp: May 30, 2018, 05:38:20 pm
content: QuoteYes, it goes through a reset-able fuse for each of the two regulators (F1 and F2).https://www.digikey.com/product-detail/en/bel-fuse-inc/0ZRB0400FF1A/507-1322-ND/1560177I've attached the full schematic if you're interested. Battery connection on page 1, power supplies on page 6.

author: AndrewLoomis
author_rank: Contributor
timestamp: May 30, 2018, 06:44:55 pm
content: During the failed startup attempts (when the voltage did not reach 3.3V), whenever I turn off the input voltage, the green LED that is connected to the 3.3V output voltage flickers on then off quickly. It looks like the regulator starts working momentarily before losing power. Why would this happen?

author: SiliconWizard
author_rank: Super Contributor
timestamp: May 30, 2018, 07:04:09 pm
content: Couldn't there be a potential issue with your power enable circuit around U14?

author: AndrewLoomis
author_rank: Contributor
timestamp: May 30, 2018, 07:12:03 pm
content: QuoteI'm actually not using that part at all right now, it's unpopulated. Just the 100k resistor to VFUSED is connected.

author: maginnovision
author_rank: Super Contributor
timestamp: May 30, 2018, 07:14:08 pm
content: Quote from: AndrewLoomis on May 30, 2018, 05:22:30 pmOk, same problem. Max input of 18v, with abs max of 20v. The fix is to add some TVS diodes to clamp it at a safe voltage. They are very sensitive.

author: AndrewLoomis
author_rank: Contributor
timestamp: May 30, 2018, 07:17:27 pm
content: QuoteThat's a good point, I considered adding input protection, but I thought that I might want 5S or even 6S batteries (up to 25V) to be compatible with my design, so I just decided to go with higher input voltage rating, just to be safe.

author: SiliconWizard
author_rank: Super Contributor
timestamp: May 30, 2018, 11:50:35 pm
content: I noticed that according to the DS, the voltage on EN pin should not get higher that 5V, but you're pulling it up to VFUSED which is much higher. (And yes I think this limit is annoying.) I don't know whether that could explain the issue you're having, but you're sort of out of spec.(Likewise, the output of the U14 latch may not like that very much, although the 100k pull-up should limit the current to acceptable levels... but the internal clamp diodes are certainly going to conduct.)So you should probably pull up EN to VLATCH or something.

author: AndrewLoomis
author_rank: Contributor
timestamp: May 31, 2018, 12:09:48 am
content: Yeah, the whole "what is the max input voltage of EN pin?" was a question I was asking myself when designing this. Usually, manufacturers put this in the electrical characteristics part of the datasheet, but it is omitted in the AOZ1284 datasheet. This excerpt from the datasheet is the only mention of the EN pin sensitivity:QuoteActually, just caught this: (can't believe I missed this!!) in the Absolute Max Ratings, the datasheet states that the rating for the EN pin -0.3V to 6V. 16V is a bit more than 6V.... I'm guessing this might be the cause of the weird startup behavior? Have I fried the internal EN circuits by doing this?Btw: SiliconWizard, thanks for the suggestion to pull up to VLATCH, I think I'll definitely do that as the solution.

author: SiliconWizard
author_rank: Super Contributor
timestamp: May 31, 2018, 01:15:12 am
content: Well, the EN voltage range is not very clear if you don't read it all, although the 6V absolute max additionally suggests a 5V max operating range. This is also unusual since most regulators can have their enable pin tied to the input voltage.Actually I noticed this limit while I was looking for the input current of the EN pin (which is usually stated in most datasheets, but again is not in this particular one) to make sure the 100k pull-up was adequate or if you should lower it.I don't know if that explains your issue nor if it has harmed the regulator, but it's worth a try.

author: Gavin
author_rank: Newbie
timestamp: September 07, 2018, 02:32:54 pm
content: Have you figured out your issue? I'm having a similar issue with the same converter, outputting 5vdc for a raspberry pi and supporting circuitry. I can get the AOZ1284 to output 5v with minimal load (an LED) but when i plug in the pi and cycle power, nothing. scope shows that it is charging/discharging the SS cap instead of it charging and leveling off and the output doesn't rise more than 1v. I can post screen shots / schematics if that helps the cause. Plugging in the Pi after the converter has 5vdc steady allows for the pi to properly boot so i'm assuming its large inrush current is stopping the converter from its start up routine properly.

author: perieanuo
author_rank: Frequent Contributor
timestamp: September 09, 2018, 02:56:23 pm
content: capacitors imho are too small,put 100uF or more on input filtering and assure those are conform to datasheet.then try 7 uH inductance.after this, do the scope stuff.you're maybe seriously underestimate caps and inductanceEdit:I mean 47 uH of course or more...Envoyé de mon iPad en utilisant Tapatalk

author: Gavin
author_rank: Newbie
timestamp: September 12, 2018, 08:05:32 pm
content: Attached is my schematic. I added a 100uF cap to the input to test and had the same results with an Rpi attached to the 5V output.As you can see, originally there was 76uF on the output so I removed the 22uF caps until the circuit finally powered with the RPI load attached. I had to remove all 22uF caps and therefore had 10uF. Of course this increased the noise considerably on the 5V rail. Is there anything else I can do to make the circuit function with more output capacitance?I thought about adding in a p channel fet to the output driven by a bjt so that when the reg is stable and outputs 5 volts the mosfet turns on and powers the rest of the circuitry... I feel like that should not have to be the only solution, however.** disregard the disconnected EN pin. it is properly enabled on startup with a voltage divider.

author: thm_w
author_rank: Super Contributor
timestamp: September 12, 2018, 09:12:24 pm
content: Oddly the AOZ1280 and AOZ1282 support EN voltage up to the input voltage range (26V+). The internal block diagram is essentially identical.

author: Gavin
author_rank: Newbie
timestamp: September 13, 2018, 06:46:07 pm
content: Well I fixed my problem. Turned out to be a layout issue with ground loops. Maybe this can help other people that think their circuit is correct and should function.My output caps were not close enough to the regulators ground pin and had to take a long route on the ground path to get there and the same for the input caps ground. To solve the issue on my prototype board, i ran a bodge wire from the output caps ground -> reg ground -> input caps ground. This allowed the circuit to function properly under all loading conditions.App note AN045 from Richtek on EMI reduction in switching regulators lead me down the correct path.