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Original question:

TAS2781EVM: Standard config for amplifier evaluation with resistive load and APx

TAS2781EVM: Achieving 10W into 4Ω with Class-H enabled

Adam Lock
Prodigy 245 points
Part Number: TAS2781EVM
Other Parts Discussed in Thread: TPS61089, TAS2781, LM5155EVM-BST, , LM5155

Hello,

I have been having issues getting the EVM to output a portion of the rated power into 4Ω at 1%. Figure 6-4 and 6-5 of the datasheet lead me to believe that I should have no issue hitting 10W into 4Ω.

With the TAS2781 and integrated TPS61089 I am only able to get around 6W before significant clipping occurs. The PVDDH rail does reach up to ~12V at this time, but I start to see PVDDH fluctuate and the FB pin drops below the FB voltage.

Because of this I switched over to a LM5155EVM-BST that is more than capable of the 10W required. After struggling to get the Class H configuration to work properly between the TAS2781EVM and LM5155EVM I resorted to fixing the LM5155EVM-BST output voltage to ~12V and then ~13V. At 13V I am still seeing ~2.3% THD at 10W and -4dBFS. I am using 12AWG jumpers and only see the rail drop to 12.998V at C8/C9. The distortion is also asymmetric with clipping only happening on the negative side of the waveform with the current voltage rail.

I have added in a 15uH inductor in series with the positive side of the resistive load. I believe this should be inline with the datasheet.

I will continue to boost the voltage rail via a bench supply and see if I can hit the target distortion at a high rail voltage.

Do you have any possible ideas why the EVM would not be hitting 10W with a 12V input on PVDDH?

-Adam

  • 0
    TI__Mastermind 18245 points

    Hi Adam, 

    The datasheet plots are taken using a fixed voltage power supply, i.e. not from a boost. in that case the 1% THD point will be dominated by the output beginning to clip against the supply. for the 12V fixed supply we would expect ~7.5-8W into 8Ohms and ~15-16W into 4 Ohms. (depending on test setup)

    For the boost case the peak output power will be limited by the power rating from the boost, and the size of the inductor. It is not expected that the EVM can deliver 12V peak continuously across 4 ohms, the voltage will drop. 

    Based on your statement when using the LM5155 I would expect that if the voltage is not dropping you should be getting something ~15W at 1%. 

    Can you check using a fixed benchtop power supply of 12V into the PVDDH jumpers on the EVM and check if you can get some higher output power? Clipping only on the negative cycle of the output sounds like there may be some setup issue. if we remove the variable of the boost it could give some clarity, 

    Also, Are you using the Class-H when testing with the on board boost?

    Regards,
    Arthur