TPA3251D2EVM: TPA3251D2EVM clips at 50% Power

Matthew Grainger34

Part Number: TPA3251D2EVM

I seem to be getting exactly half of the power output I should be seeing for a given output impedance for Mode 00 BTL 2.0. I'm getting about 35W and 70W into 8r and 4r respectively @ 36Vin. I've also measured across the A and B inputs to the amp IC and I'm seeing 2Vrms rather than 4Vrms before clipping?

At first I thought I had misunderstood the configuration but see that full power is obtainable due to the inverting setup from the first Op-amp in the schematic, effectively creating a differential input from one single ended one.

I think I've set all the jumpers correctly so not sure what I'm doing wrong, presumably it's something obvious.

Thanks

over 8 years ago

0 Robert Clifton56 over 8 years ago

TI__Mastermind 27500 points

Hello Matthew,

Can you please tell me how you have J4 and J19? If you're driving it single ended (RCA input) you need to set both of them to SE input.

Robert

0 Matthew Grainger34 over 8 years ago in reply to Robert Clifton56

Prodigy 100 points

Hi Robert, yes they are both set to SE, I am driving it from A and C inputs.

0 Robert Clifton56 over 8 years ago in reply to Matthew Grainger34

TI__Mastermind 27500 points

Hi Matthew,

Do you know the input signal's frequency you have been using?

Robert

0 Matthew Grainger34 over 8 years ago in reply to Robert Clifton56

Prodigy 100 points

Around 100Hz but it's the same at any frequency, 1Vrms is is the max I can have at the RCA input before clipping.

0 Robert Clifton56 over 8 years ago in reply to Matthew Grainger34

TI__Mastermind 27500 points

Hi Matthew,

I am assuming you are using the Clip_OTW LED as the indicator for clipping? The Clip_OTW LED comes on before THD+N=1%. If possible I would recommend checking the output through an oscilloscope to see when exactly the signal clips.

Robert

0 Matthew Grainger34 over 8 years ago in reply to Robert Clifton56

Prodigy 100 points

With a speaker connected I get very little additional output by further increasing the volume once the clip light is illuminated and it becomes obviously distorted. There is no way a further 50% power is available with the clip LED on. There is something fundamental not right here since it clips from 50% Vin at 1Vrms, it should be 2Vrms.. ?

0 Robert Clifton56 over 8 years ago in reply to Matthew Grainger34

TI__Mastermind 27500 points

What kind of power supply are you using? One thing you can try is make sure there isn't a drop in the voltage being supplied to the system.

0 Matthew Grainger34 over 8 years ago in reply to Robert Clifton56

Prodigy 100 points

It's a bench PSU at 36V with 20A current limit and decent cables. Is there a setting on the amp IC that causes it to only accept half the input signal before clipping early?

0 Matthew Grainger34 over 8 years ago in reply to Matthew Grainger34

Prodigy 100 points

I'm seeing Vin/2 at the PWM A and B output relative to Gnd so I think the whole amp output has a DC offset of half the Input voltage.This would make sense as I can only put half the signal I should be able to put in at the RCA before clipping which when added to the offset would hit the rail. I also measured 3.8V DC at the InA and InB pins relative to Gnd, is this right? Seems like the 10uF caps aren't blocking all the DC offset from the single supply 6V biased op-amps.

0 Robert Clifton56 over 8 years ago in reply to Matthew Grainger34

TI__Mastermind 27500 points

Having 3.8V DC coming out of InA and InB is correct, which suggests the input side of the amplifier is behaving as expected.

How are you measuring the PWM outputs? If you are using an oscilloscope could you please send screenshots?

0 Matthew Grainger34 over 8 years ago in reply to Robert Clifton56

Prodigy 100 points

I have done some measurements at 36V, 1kHz input and the output power of each channel in BTL mode before the illumination of the clip LED is as follows:

51W into 8 Ohm, 95W into 4 ohm, 123.5W into 2 ohm. This is seems rather low compared to the datasheet.

I get 72W and 138W into 8 ohm and 4 ohm respectively before flat top clipping which is in line with the 1% THD+N figures in the datasheet. The waveforms however look far worse than 1% THD+N, see image.

There is also 1.2V pk-pk at around 570kHz imposed on the output signal at all times, irrespective of power output.

0 Matthew Grainger34 over 8 years ago in reply to Matthew Grainger34

Prodigy 100 points

Anyone ??

0 Matthew Grainger34 over 8 years ago in reply to Matthew Grainger34

Prodigy 100 points

I have built the RC filter as advised in the TI white paper and the output looks okay.
I would still like to know if it is normal to have 1.2V pk-pk @ 600kHz switching frequency on the output at all times, I thought the filtering would be better?

0 Justin Bohr over 8 years ago in reply to Matthew Grainger34

TI__Mastermind 27990 points

Hi Matt,

What are you using as your input source? And what are you using to measure the THD+N? Could you show me a capture of the 600 kHz peaks? Is this a measurement after the LC filter or before at the PWM outputs, and is it with or without a load?

Justin

0 Matthew Grainger34 over 8 years ago in reply to Justin Bohr

Prodigy 100 points

I'm using a clean signal generator, the 600kHz sine wave is present irrespective of load, I'm not able to take a picture at the moment but it's just a 600Khz sine wave with a bit of crossover distortion at around 1.2V pk-pk at the output terminals (after all filtering).

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TPA3251D2EVM: TPA3251D2EVM clips at 50% Power