Audio HP Amp TI LM4811 - am I getting crazy? - Page 1
Forum section: electronics / projects

author: Saimoun
author_rank: Frequent Contributor
timestamp: July 29, 2020, 06:17:54 pm
content: Hi all!So glad to join the community here - I have been following Dave's videos for a while now, really really great stuff!!My first post - an issue of course!I am trying this IC from TI called LM4811 - datasheet here:https://www.ti.com/lit/ds/symlink/lm4811.pdfOn the paper it's all good, it seems very simple, here is the "typical application" from the datasheet.So I made a PCB and ordered it, soldered it, no shorts and everything seems perfectly ok! But when it comes to testing, it seems that the Bias voltage (internally created by the IC) is *heavily* influenced by the CLOCK pin! yes - the CLOCK pin!Here are my schematics:To test I tried measuring the voltage on the Bias pin directly or on the Vin pin - since this should be at the same average voltage (they are identical since for now I am sending a 1.65V DC from the DAC). Here are measurements when the CLOCK pin is at 0 (GND) and when the CLOCK pin was at 1 (5V) while starting up. The yellow line is the Vin pin and the blue line is the CLOCK pin.CLOCK at GND:CLOCK at 5V:Sometimes the bias voltage changes by itself, oscillating between 0 and 1V when the CLOCK is at GND, and between 5V and 3V when the CLOCK is at 5V.I double-triple checked the PCB layout and the soldering. Nothing complicated, just the IC and a few through hole components around. Is there something I missed in the datasheet maybe?Thank you in advance!CheersSimon

author: Saimoun
author_rank: Frequent Contributor
timestamp: July 29, 2020, 06:26:32 pm
content: Ooh and I forgot to say that the CLOCK pin seems like it pulled down somehow - if I put a 1K resistor between 5V and the CLOCK pin it reads at around 0.5V. If I put 100Ohm resistor it reads at around 1V. To put it at 5V I need to connect it directly to 5V.

author: magic
author_rank: Super Contributor
timestamp: July 29, 2020, 07:36:23 pm
content: Are you sure the chip is genuine?Are you sure you didn't connect shutdown wrong?What the clock pin is for and why are you connecting it to some fixed voltage? I'm lazy and didn't download the datasheet

author: Saimoun
author_rank: Frequent Contributor
timestamp: July 29, 2020, 07:52:24 pm
content: It's all good - we're all lazyChip is genuine, bought from Digikey. I already tried two of them and both have the same behavior.The CLOCK and UP/DN pin work to change the volume. As I understood they should be at GND when "resting". Here is all the datasheet is saying about it:The Shutdown pin is connected to GND and just to check again I am looking at the datasheet that the device is active when Shutdown is low... And actually this is very confusing, I am not sure anymore :/How do you guys understand it, should it be LOW or HIGH to put the device active?Quote

author: magic
author_rank: Super Contributor
timestamp: July 29, 2020, 08:01:53 pm
content: Quote from: simonlasnier on July 29, 2020, 06:26:32 pmIt looks like there is a diode from CLOCK to GND inside the device. With 100Ω to 5V it's consuming 40mA of current if the pin is at 1VAnd the datasheet says that it should be pulled to 1.4V to register as high? Makes no sense.If it isn't fake, are you sure you didn't mount it backwards? Does it work at all, by the way? Do the VIN pins really track the voltage at BYPASS or was that just your speculation? Can you get any sound from it?Shutdown seems correct. "Shutdown active high" means that high will make it shut down.

author: Saimoun
author_rank: Frequent Contributor
timestamp: July 29, 2020, 08:29:19 pm
content: Ok about the shutdown - thanks.What makes you think there is a diode from CLOCK to GND?These values were just what I remembered, I checked again and it's more like 0V (or almost) with a 10k resistor, and 0.3V with a 100Ohm resistor. But there is clearly current flowing because the 100 Ohm resistor got really hot really quickly!!Which makes me think: what the hell happens when I set the pin to high without any resistor?!The chip seems to work, the VIN pins get to the bypass voltage (yes I measured), and if I put a sine way on VIN I have the same wave (ish - it's cut because of the wrong bias voltage) inverted on the output.

author: magic
author_rank: Super Contributor
timestamp: July 29, 2020, 09:25:20 pm
content: That's some paranormal stuffCheck if there is continuity from power supply to VDD.Do you have a ceramic capacitor between VDD and GND close to the chip?

author: tooki
author_rank: Super Contributor
timestamp: July 29, 2020, 09:53:07 pm
content: Did you build the “typical application” circuit or the demo board circuit? The big difference are the pull down resistors.Also, in your schematic, Vin1 and Vout1 are both floating. Is this actually the case? It’s possible that’s an unstable configuration.Quote from: magic on July 29, 2020, 08:01:53 pmWhy does that not make sense? It’s a device that supports a 2V supply voltage, and 1.4V is 0.7x2V.

author: magic
author_rank: Super Contributor
timestamp: July 29, 2020, 09:55:19 pm
content: Well, it makes no sense that it's impossible to pull this pin above 1V

author: Saimoun
author_rank: Frequent Contributor
timestamp: July 30, 2020, 07:44:58 am
content: Quote from: magic on July 29, 2020, 09:55:19 pmI agree with that!And yes tooki I agree the 1.4V makes sense, it's just the minimum value to be considered "high" (shutdown, up/dn and clock pins are digital inputs).Quote from: tooki on July 29, 2020, 09:53:07 pmTo be honest I tried everything at this point! Pull-down, pull-up, and directly tied to GND and to +5V.The schematics I showed was the simplified version, but my original version - the one where I started experiencing the issue - is the one I put above. Let me get the schematics and the PCB for the original version, I'll put it here.

author: magic
author_rank: Super Contributor
timestamp: July 30, 2020, 07:50:11 am
content: Again, are you sure there is no problem with VDD and the chip isn't taking power through ESD diodes from other pins?

author: Saimoun
author_rank: Frequent Contributor
timestamp: July 30, 2020, 08:13:28 am
content: Here is the original schematics and PCB.Schematics (ignore the fact that all nets have a "2-" in front of the name):PCB top:PCB bottom:

author: OM222O
author_rank: Frequent Contributor
timestamp: July 30, 2020, 08:28:59 am
content: the pcb is very messy and non standard (just look at the 10 pin SOIC, looks like a 6 pin jobby) and there was a problem with your schematic it seems.Please upload the schematics.Also this seems like easyeda. If so please share the link rather than photos.

author: magic
author_rank: Super Contributor
timestamp: July 30, 2020, 08:31:55 am
content: No ideaAny chance they sent you a wrong chip?BTW, simply rotating C12 180° would decrease the length of connection from C12 to the GND pin for free. Probably not important here.

author: Saimoun
author_rank: Frequent Contributor
timestamp: July 30, 2020, 08:42:56 am
content: Quote from: OM222O on July 30, 2020, 08:28:59 amSorry the picture upload went wrong - the post above has the schematics now.And yes the PCB is a bit messy - it's just a PCB to test the IC. And actually the strange solder pads for the IC is on purpose, it is my way to be able to solder a tiny package as I have never soldered SMD before. But as I said the soldering went really well, no shorts and I tested each individual pin to make sure it was connected.Here is a picture of the soldered PCB - the "dot" of the IC is at the top right.

author: Saimoun
author_rank: Frequent Contributor
timestamp: July 30, 2020, 08:49:06 am
content: Quote from: magic on July 30, 2020, 07:50:11 amThat's a good point - I did check on the VDD pin directly and it's getting +5V. But yes very interesting when I remove the +5V and let the VDD pin float - I get the exact same result = the voltage on the voltage pin follows the CLOCK pin.But it's only the clock pin (whether I have VDD connected or not), if I move the up/dn or the shutdown pin nothing happens.

author: magic
author_rank: Super Contributor
timestamp: July 30, 2020, 08:56:33 am
content: Can you desolder the chip and check if VDD has any internal connection? You should be able to detect something using diode test between different pins.Maybe you damaged it somehow

author: Saimoun
author_rank: Frequent Contributor
timestamp: July 30, 2020, 09:56:33 am
content: I still have two more chips so I could test with them. I tested with two already tho - and both have the same issue.But I am starting to think maybe Mouser (yes it was Mouser not digikey actually) sent the wrong chips? I would never think that would be possible but the marking says 8565 G11 for a chip named LM4811?!I have been googling for a SSOP-10 package with 8565 marking but without any luck. Quite terrible that the LM4811 datasheet does not show the marking!I also checked my order and it was the only SSOP-10 package so I could not have mixed it up with other chips. And the antistatic bag does say LM4811MM/NOPB.EDIT: my bad the datasheet *does* show the marking, it says G11 and maybe with another marking, so it kinda seems it fits?

author: Saimoun
author_rank: Frequent Contributor
timestamp: July 30, 2020, 10:04:35 am
content: But actually on LCSC the picture shows a 9235 G11 marking... Not sure how much we can trust this anyways :/https://lcsc.com/product-detail/Audio-Power-OpAmps_Texas-Instruments-Texas-Instruments-LM4811MM-NOPB_C529566.html

author: magic
author_rank: Super Contributor
timestamp: July 30, 2020, 10:28:12 am
content: It is possible and it rarely happens but the table in the datasheet says that marking is G11.

author: Saimoun
author_rank: Frequent Contributor
timestamp: July 30, 2020, 12:18:35 pm
content: Yeps I saw as well - thanksI made a few more measurements - now with a completely new chip, which I was super careful with not to touch in case I could have fried the previous ones. There are no shorts and all pins are soldered as they should. There is nothing else on the PCB, just the chip. Measuring resistance between the digital pins and ground, and between the digital pins and VDD, with my multimeter.UP/DN and SHUTDOWN pins always give something above 1M(to both GND and VDD pins) while the CLOCK gives around 10kto GND (and around 100k to VDD)?!Then I tested the following: 10kpull-down resistors on Shutdown, CLOCK and UP/DN, and give 5V on VDD (and 0V on GND). All other pins floating.I get the following, yellow is the Bypass pin and blue is VDD.(at around 7s I removed the pulldown resistor on CLOCK, making it float, and again this affected the Bypass pin).I am getting out of ideas of what to test, and out of chips...

author: OM222O
author_rank: Frequent Contributor
timestamp: July 30, 2020, 12:41:49 pm
content: those pins are meant to be driven actively. I don't understand why you're "testing" the IC with EVERYTHING FLOATING. connect it to an MCU, drive the volume control and clock pins, add your audio source and speakers, see if it works as expected or not. many ICs may not function well under random circumstances. I remember having a seven segment driver which kept burning out under "testing" with floating pins, and I thought it was a bad chip, but when actually connected it to everything, worked as expected.

author: Saimoun
author_rank: Frequent Contributor
timestamp: July 30, 2020, 01:42:08 pm
content: Fair point OM2020 - but at the end the only floating pins were the inputs since the outputs are internally tied to GND and I pulled down the digital ones. And the inputs are meant to be analog anyways so it should not make much difference?And tbh I would rather not drive the pins from a MCU since the amount of current the clock pin seems to sink.But ok I will try to follow their Demo board schematics then - it has buttons and jumpers to control the digital pins.

author: tooki
author_rank: Super Contributor
timestamp: July 30, 2020, 03:09:34 pm
content: Let’s review some assumptions and facts:1. It’s extraordinarily unlikely that the chips are fake since they were bought from a reputable vendor2. It’s extraordinarily unlikely that multiple chips in a row would leave the factory faulty3. The datasheet lists a maximum current draw of 3mA for the chip4. Given that the IC is specified as being a low-power part, it wouldn’t make sense for its support components to draw large amounts of current.So, what’s left?5. Does the schematic actually make sense and work in theory?6. Does the PCB match the schematic?7. Are all the support components good, and of the correct values?8. Was the PCB assembled properly?9. Was care taken to ensure the chip isn’t damaged during soldering, due to excessive heat or ESD damage?10. Is the device being tested under sensible conditions?You probably have more than one PCB. Try building up another one.Try leaving out the low pass filters on the inputs (omit C1/9 and bridge R17/21).Make sure you have a load on the outputs.What are you powering it from?Remember that if it’s not the IC that’s bad, then it’s something else, i.e. the rest of the circuit.Take the original populated PCB and remove just the IC. What happens to the voltage at the pads for the clock, up/down, shutdown, bypass, and VDD pins as you feed in high and low signals on the clock, up/down, and shutdown inputs? Somehow, I suspect that there is a short elsewhere on the board acting as a near short to ground.

author: OM222O
author_rank: Frequent Contributor
timestamp: July 30, 2020, 03:11:49 pm
content: An MCU won't "burn" or "source" amperes of current! They're internally limited by the small fets that activate them. They have a "max current rating" that is usually less than 40mA and even then, the voltage will drop to close to 0. You can literally short out an MCU pin to GND forever and nothing happens. These pins are not meant to be analog. CLOCK definetly means digital, there are timing diagrams for them in the datasheet! Floating pins cause unreliable and random operation modes. Even if they were analog (op amp inputs for example) "best practice" is to tie them to GND or VCC if they're not used. Floating pins are always bad.

author: Saimoun
author_rank: Frequent Contributor
timestamp: July 30, 2020, 05:15:49 pm
content: Thanks tooki for the summary.I tested with my last chip (number 4) and the 4th PCB. I can now get one of the two channels to work. The other channel is fried I think - I get a 42Ohm resistance between input and output.I think I basically fried all 3 other chips, which explains the very strange behavior on all of them.My guess about what happened: the chip must be really sensitive to changes on the CLOCK input, since each rising edge changes the volume, so I think leaving it floating just for a second makes too many changes and the chip fries. And as for the second channel of the 4th chip - I had the input tied to GND while I was testing the other channel, I guess that killed it?I also noticed how sensitive the voltage on the bypass pin is - just probing it with my oscilloscope is enough to make it drop by a few 100mV!

author: Saimoun
author_rank: Frequent Contributor
timestamp: July 30, 2020, 05:29:58 pm
content: Anyways - thanks all for the help, I guess the issue is sorted. I'll buy a few more and be even more careful this time.

author: OM222O
author_rank: Frequent Contributor
timestamp: July 30, 2020, 06:00:33 pm
content: the main problem that I suspect is the very odd shaped pad which you use to solder. Adding large amounts of solder, overheats the chip for a long period of time, burning the circuitry. Most reliable method for hand soldering SSOP packages:1)Add solder using a normal iron to all the pads excessively2)Add flux and go over the pads with an iron, removing excess solder, leaving just enough on the pads3)Heat the pads at the same time using hot air, hot plate etc, enough to melt to solder on the pads4)Grab the part with the tweezers, place it on the pads, keep pressing down and remove the heat sourceThis way there is minimal heating of the chip, practically no chance of shorts (if you don't mess up the placing step and move the chips around) and reliable joints on the first try.

author: Saimoun
author_rank: Frequent Contributor
timestamp: July 30, 2020, 06:49:52 pm
content: Thanks for the tip about amount of solder - but then I guess I can still use my technique just be very careful with the amount of solder? I am also really quick with the iron, leaving it less than a second on the pad usually.I do not have a hot plate or hot air gun so for now this might be my only option. And the large pads *really* help.

author: tooki
author_rank: Super Contributor
timestamp: July 30, 2020, 06:58:21 pm
content: Honestly, I doubt heat is the problem. It could be, but I don't think it's likely. (And honestly, you don't need those giant pads. SMD is about technique, and one of the things to unlearn is that the soldering iron needs to be as small as the pad. It absolutely doesn't.)Anyhow, I listed a whole bunch of things for you to look at, but you didn't respond to any of them. In particular, we still haven't isolated whether the issue is in the IC or in the PCB...

author: Saimoun
author_rank: Frequent Contributor
timestamp: July 30, 2020, 07:12:14 pm
content: Quote from: tooki on July 30, 2020, 06:58:21 pmThanks for the info, I clearly have a lot to learn about soldering - this was my first SMD soldering though.As I said earlier - IMO - I think the issue was in the IC - the first 3 were fried because of bad usage (floating the CLOCK pin), and one channel of the 4th one also (because setting the input to GND - my guess at least). On the other channel of the 4th one - everything works perfectly fine, including the volume change.To answer your list:5. Does the schematic actually make sense and work in theory? -> I think so, can you guys confirm?6. Does the PCB match the schematic? -> yes - checked with software also checked the physical one a bit with the multimeter7. Are all the support components good, and of the correct values? -> yes, they match the schematics8. Was the PCB assembled properly? -> yes, everything is where it is meant to be9. Was care taken to ensure the chip isn’t damaged during soldering, due to excessive heat or ESD damage?Yes that's the question - I was careful about ESD and as quick as I can during soldering, but how can I tell if it has been damaged? So far the only direct tests I found were to measure the resistance between the pins - noticing how on the fried chips some of the resistances were completely off.10. Is the device being tested under sensible conditions? -> as written above clearly the first 3 were not. I also found out that putting the oscilloscope probes on the bypass pin is a bad idea - now I only monitor input and output.

author: magic
author_rank: Super Contributor
timestamp: July 30, 2020, 08:15:40 pm
content: Is there any chance that you applied some of the control voltages (shutdown, clock, up/down) before VDD? Perhaps that's what harmed them? I don't know, the whole thing looks very weird.

author: Saimoun
author_rank: Frequent Contributor
timestamp: July 31, 2020, 08:22:38 am
content: I dont think so but I'll keep it in mind in the future

author: exe
author_rank: Unknown
timestamp: July 31, 2020, 06:18:49 pm
content: Can it be a ESD issue?