TPA3255 Troubleshooting

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cod3gen

Member
Joined 2007
autobilstereo.no
#1
Hi all,

I have been working on a TPA3255 project for a while now, and already have some of my boards up and running but i am experiencing that the boards are clipping really soon. The design is more or less based upon the TPA3255 EVM with Post-Filter Feedback circuit and pcb layout is inspired by another project on diyaudio. I am only able to get 22vpp out of it on both 4R and 8R loads while running at 48.5v before it starts to clip, however by raising the voltage to ~57v i was able to push it to 25v before clip. The sound of the amp before it clips is amazing, very clear and open.

PCB have a cu thickness of 2oz, attached is schematic used and front/back pcb.

What have been done so far to eliminate where the issue is located,

- Bypass entire input section, and attaching a separated balanced buffer directly before R_in. Same result
- Switch from 4ohm to 8ohm load. Start to clip at same output voltage, so it does not seem to be current related.
- Switch OC resistor to 30k, instead of 22k. Same result
- Different PSU, Same result.

Tomorrow i will remove the heatsink and solder cable directly on to TPA3255 in order to measure its input voltage in case of defects on the PCBs.

I would like your inputs on this, maybe there is something i cannot see.. 🙂
 

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#3
What channel A&B or C&D? If the channel is C&D, your layout is the problem. TPA3255's VBG ground is ultra sensitive, ground of VBG should not reference high current ground plane. Lift C24 and air wire it to ground of C29 or pins 12&13 gnd.

Also check if your power supply has current limiter. Your amp will clip early if the power supply cannot deliver enough power.

https://e2e.ti.com/support/audio/f/6/t/723596

Regards,
Lester
 
#5
Grounding C24 to C29 gave the same results, also checked voltage on pins 37/34 and 30/34, there is a voltage drop of about 120mv from zero load and till clipping load.

I have also bypassed PFFB now, but still starts to clip at about the same output voltage level, however it can now be driven much hardering into clipping than before.

Switched from 450khz to 600khz, clipping starts at same level here to.

Could a possible wrong capitance of BST have such an impact?
 
#7
Try to put 12V to VBG with series resistor 100K to 470K ohms. (VBG = 2.5V to 3VDC) It will force AVDD to 10.5V. (Not a solution but to check if it will raise clipping voltage) Put 100K to 470K 0805 across C35 and C24

You need to change layout on the analog side of the chip, the caps are too far from the chip.
 
#9
I see, have you tried using different buffer?

When I first power up my TPA3255 the clipping point is also low, then I realized that I connected a bulb limiter lol.

Also check if it's not clipping but overheating of the chip.
 
#13
JLesterP said:
I see, have you tried using different buffer?

When I first power up my TPA3255 the clipping point is also low, then I realized that I connected a bulb limiter lol.

Also check if it's not clipping but overheating of the chip.
Click to expand...

Yeah, i have had it connected to a MiniDSP 2x8, as well as right now a custom buffer connected to Akitika 1khz osc. And both with and without onboard buffer. No difference as to where the clipping starts except when i removed the post filter feedback section, this raised a couple volts on outputs before clipping starts.

No bulb limiter connected 😛

The chip is on a 2mm thick copper plate, which is placed on an 10mm thick aluminium plate measuring about 10x30cm. Should be sufficient cooling for these tests. If cooling(or bulb limiter) was an issue, would guess switching between 8r/4r would have a great impact on there the clipping occurs, but it occurs on the same voltage with both 8r and 4r.
 
#16
JLesterP said:
Looks like reset pin doesn't have a 3.3V supply connection?
Click to expand...

MIC8114 is not supplied by 3.3v supply, however it is supplied by +12v and sensing of VDDA. When circuit detects correct voltage reset pin is released and lifted high. During regular operation at +48v i measured 3.09v on reset pin of TPA3255. Just to be sure i also now tried to force it to 3.3v, with the same result.
 
#19
In SE mode it should swing close from ground to the + rail when measuring from out A to ground. If in BTL mode you should get nearly 2x the supplyvoltage peak to peak when measuring from out A to outB. Are you sure the post filter feedback is disconnected. Enabling post filter feedback reduces the gain. I'm wondering if the clipping occurs before the chip.

On my tpa3255 I tested with 100 ohm loads first. I am using a linear power supply. I wanted to know the maximum RMS voltage output when the power supply is lightly loaded. I then increased the load till 3 ohms. This sort of gave me a crude dynamic range. Try with a high Z load ie 100 Ohms or more.

There is a TI note somewhere that says as long as the filter caps are at least 2x the power supply voltage you shouldn't fry the chip if there is no load and you drive the output to clipping.

There is a pin on the 3255 that sets the max output current, is that ok?
 
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