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Audio Amplifiers Forum
Glitching in the TPA3125D2
I am using the TPA3125D2 in exactly the BTL configuration depicted in Fig 30 of the 2008 datasheet. Supplies are perfectly quiet at 24V. I have an additional 20K ohms in series with two audio inputs that are being summed at Rin (return is common at Lin). \SD is tied to 24V; Mute is tied to Gnd. Gain0/1 are both at Gnd for now, but can be switched under firmware control.
I am getting a noise burst at the output every 276 mSec, lasting 52 mSec each. It seems to be white noise, above normal audio frequencies but rail-to-rail and thus audible (deafening, actually) at the output. The board is controlled by a PIC microcontroller with which I have extensive experience, and holding the microcontroller in reset has no impact on the noise, so any interaction with the firmware or other sections of the board seems to be eliminated.
Vclamp is sitting at about 10.6V, and droops by about 200 mV during each noise burst. Not sure if this is a cause, effect, or some other symptom.
Where do I go next?
Hi, Steve. Thanks for using the TPA3125D2 in your design.
Were you able to measure the power supply ripple at the amplifier itself? Can you please share a plot of this? I'm curious if the amplifier is having some issues with its PVCC, because VCLAMP relates to VCC, and you're seeing effects on it.
Are you able to measure these noise bursts on an oscilloscope and post the results here? Try to get the signal before and after the output filter.
I assume this is your own schematic and layout. It helps if you can share them. One thing to check in your layout is that the 0.1 µF ceramic capacitor for PVCC coupling is placed as closely to the PVCC (L and R) pins as possible. The same applies for the 0.1 µF and 10 µF capacitors coupled to the AVCC pins. We see very strange things happen when this isn't the case. Check to make sure your 470 µF bulk capacitors are rated for well above 24 V. I suggest 50 V rating in case there is power supply ripple we haven't measured yet. Make sure the bootstrap caps are placed as closely as possible to the switch nodes and bootstrap pins.
Are GAIN0 and GAIN1 asserted to ground by your microcontroller, or are they actually tied to ground? A part in this line of products called the TPA3106D1 experiences issues when SHUTDOWN goes low, and then GAIN0 and GAIN1 are driven from an external source (such as a microcontroller). Since your \SD is tied to your supply, I have to wonder if something similar is happening, because if \SD goes low with your PVCC, and GAIN0 and GAIN1 are driven by the microcontroller running off a separate power rail (I assume), things could get funny the next time you apply PVCC. See this forum post and this Application Note for more on this error. I am not sure if it applies here, but it sounds similar.
I hope this helps!Matt
Home Audio Applications
Any supply ripple is really small, order of millivolts at most.
Yes, as described earlier.
It accurately matches the sample schematic given in the datasheet, except for the previously described summation of two audio inputs via 20K resistors.
the only way to get them any closer would be to move them up onto the legs of the part. I'm well-experienced with RF, UHF, and microwave layouts where even a via is too long; my layouts tend to get into trouble with the manufacturability boys for being too tight. The caps here are 0603s, right on the pads of the DIP leads.
This is one slipup on my part. The local cap was insufficiently rated, and has been lifted. The bulk capacitance is at the power supply, perhaps an inch of trace away.
Gain0/1 are indeed driven by microcontroller outputs. However, the microcontroller supply is itself derived from the 24V supply that feeds the TPA3125.
Thanks for taking the time for a detailed response. Any other ideas?
My Service Request # 1-821703757 has received no response other than to direct me back here. Any chance of any further support for this issue?
Hi, Steve. I'm sorry for not writing back earlier. I am looking further into what might be the root cause of this. I intend to reply back with more information today.
Hi, Steve. I'm going to need some more information to help you further. Can you please provide the following?
I hope I can help you further on getting this information!
I have prepared images of hte schematic and layout of the relevant section and captured them in a Word doc, but I can't find any way to attach them here. The "Paste from Word" button doesn't work for the pictures, and I don't see anything to attach a file. How?
I'm getting pressure from my client to get this resolved, and still can't find any way to paste anything in here. The "Paste from Word" button doesn't work, nor do any of the normal Windows keyboard shortcuts to cut and paste.
Hi, Steve. I'm sorry I didn't reply when you first wrote. I'm also sorry your client is pressuring you at this point. I'll do my best to help you.
Attaching a file is done with the small paperclip icon in the text entry menu. If you hover over it, it will display "Insert File". It is immdiately left of the "Past from Word" button. A menu will pop up giving you the option to insert a file. It may take a few seconds to load. Please let me know if this gives you any trouble.
Thanks, Steve. This helps. Are you able to provide the other data I'm asking for below? I have a suspicion this problem has to do with the BYPASS cap, and the waveforms I'm asking for will help me determine if this is correct.
I'm having trouble getting those scope plots, because connecting the scope to the PC also ties the system Gnd to PC Gnd which is line Gnd (standard desktop PC). I'll have to set up to do the scope capture and freeze it, then transfer to the PC. My scope is fully floating, as is my policy. Yes, I'm well aware of the possible hazard. Not to start any religious wars, but my position is that if you don't understand electronics enough to safely float a scope, you don't have any business using one.
I have done a bit of further exploration. With the inputs completely disconnected, the TPA3125 has a continuous rail-to-rail 290 KHz square wave output. Lifting R661A and R662A so there's no path to earth or line (the design includes 5KV of isolation to line and earth, other than these two earth connections) does not change the noise. Connecting either audio input to the speaker output of my laptop (with no audio coming out) makes the noise come in bursts as described above, still a 290 KHz square wave during the "on" time. Disconnecting the laptop's wall-wart, so the laptop is itself fully floating, resumes continuous noise.
There's 10K across the TPA3125 inputs. Does that need to be significantly lower? Do I need to add low-impedance terminations to the inputs? Not shown in the datasheet; and to what would I terminate them?
Bypass is absolutely quiet. The various Vcc pins show a small amount of noise with the noise bursts, but that seems to be an effect rather than a cause. The final output stage slapping the rails would surely inject some noise back onto the supply rails. The only other signal that moves is Vclamp, which rests at about 10.6V, and drops sharply by about 200 mV during the noise bursts.
I'm suspecting rather strongly that I need some termination on the inputs, but I'm not sure just what's called for here.
Thank you for the additional details, Steve. I have to think about how these affect the noise problem you're having.
Have you observed this on more than one TPA3125 IC? If not, are you able to use another one and see that this phenomenon still occurs?
I have a set of 8 boards made up with identical configurations. All that I have checked (probably at least 5 or 6 if not all 8) show identical problems. However, I have characterized it a bit further.
I did indeed have a power supply problem (purchased power supply). On four of the eight units, the power supply was alternately going into thermal shutdown, even though total system current (even under worst case inrush conditions) should be well under half the supply's rating. So we're looking at changing that supply in the design, of course. Meanwhile, I'm continuing checkout using a robust and trusted bench supply to provide 24V. Now I get a continuous hiss from the TPA3125, with the inputs open except for the 10K shorting resistor shown on the schematic.
Looking at it on the scope, I see a full rail-to-rail swing square wave at about 295 KHz. I presume this to be the class-D modulation waveform. Greatly attenuated though still visible after the output filter.
Looking past the output filter, there are bursts of noise repeating at a 300 Hz interval. The 300 Hz repetition rate is what I hear audibly. Each burst is a rapidly decaying sinusoid at 54 KHz. There is nothing else in the system that should be running at either of those rates.
Does that provide any more clues?
Hi, Steve. Thanks for the clarification.
You are correct that the rail-to-rail swing square wave at 295 kHz is the switching output of the class-D. (Page 4 of the data sheet confirms this.) Again, if you can capture this scope plot and post it here (using the "Insert image" button, immediately to the left of that paperclip button), it will offer so many insights into what you're seeing. That way I can see just how visible the switching still is after the output filter.
I still need to understand and comment on a few things about the design.
Sorry for the barrage of questions, but these are things that really stuck out at me upon review of your schematic and layout.
I like your idea of bypass caps right on the pins, on the opposite side of the board. I'm used to working up in the MHz and GHz realms, and have done lots of designs like that. Even heard of (but never actually tried) sticking an 0603 cap into an unplated hole in a 62 mil thick board going right to the ground plane on the other side, with a part pin sitting right on top of the cap - can't get much tighter bypassing than that!
That said, I wasn't anticipating anything quite so fussy at audio frequencies. However, the sharp edges of the class-D switching could certainly stir things up. I'll try some tighter bypassing when I get back to the lab midmorning tomorrow (EDT).
The inputs are a pair of mini-phono jacks, which have two connections for the ring; hence the double pin shown on the schematic. The two inputs are additively mixed, except that one has a mute clamp which can be operated by firmware. The inputs are not ground-referenced except via the 1 meg ground resistors. The wiring is exactly that shown in the datasheet for the BTL configuration, using the two channels in push-pull to produce a combined 20W into a single speaker, which is also connected via a mini-phono plug. Does that help explain things a bit better?
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