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TAS5717 with 4 ohms load

Joel Bodenmann
Prodigy 20 points
Other Parts Discussed in Thread: TAS5717

Hello,

I have two questions:

1. Figure 13 of the TAS5717 datasheet shows the output power vs. supply voltage characteristics of the chip. The note says that the dotted line represents the thermally limited region. I don't really understand this graph. Does it mean that I cannot use a 16V supply voltage to drive my 10W speakers?

2. All the graphs (including and especially Figure 13) are always shown with an 8 ohms load. However, the specs seem to allow a 4 ohms load. How will the THD behave at 4 ohms?

Kind regards,

~ Joel Bodenmann

  • 0
    TI__Genius 9175 points

    Hi Joel,

    You can use a 16V supply but you cannot try and push over 10 watts from it. This plot suggests that the higher the supply voltage the theoretical maximum output is indicated by the dotted line but it is warning that it cannot be achieved safely.

    As for THD with 4 ohms it will likely be a little bit higher. The part is rated for 8 ohms but is 4 ohm tolerant.

    Regards,

    Matt

     

  • 0 in reply to Matthew Beardsworth
    Prodigy 20 points

    Hello Matt,

    Thank you very much for your answer.
    So when I understand this correctly, I can use a supply voltage of 14V to archive a 10W output from it, right? But that graph is only at 8 ohms. Am I correct that I have to lower my supply voltage when I use 4 ohm speakers in order not to break (overheat) the chip?

    Regards,

    ~ Joel

  • 0 in reply to Joel Bodenmann
    TI__Genius 9175 points

    Hi Joel,

    That is a safe assumption. Assuming no losses in the chip 10W can be achived in a 4ohm load with 6.32V RMS.

    P = 6.32^2 / 4

    6.32RMS = 8.94V peak

    So if the output can swing all the way the rail voltage of 8.94V, then you would get 6.32V RMS or 10 Watts. However, due to losses, realistically your rail should be a bit higher since it cannot swing all the way to the supply voltage due to a 2x Vds drop on the h-bridge output stage. This may take a little testing to find what it should be but the smaller you can make the rail voltage relative to the desired output power, the more efficient it will be.

    Keep in mind that you can run 4 ohms with a 14V supply but you cannot ask the amp to swing the full 14V due to thermal limitations. From before, roughly a 9V rail will give you 10W at 4 ohms. So that is 5V of waist if you limit the amp to a 10W output power.

    Regards,

    Matt