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TPA3255: Analog Input Scale for Full Range?

Christopher Müller
Prodigy 80 points
Part Number: TPA3255
Other Parts Discussed in Thread: PCM5242

I have a 0.9 Vrms differential signal wich I want to feed to the TPA3255.

And now I wanted to calculate how I need to pre-amp this signal to feed into the TPA3255.

PVDD = 55 V

Rload = 8 Ohm Speaker

I am a little confused about the datasheet of the TPA3255 and its input voltages.

The intro text states 2 vrms

The example tells me +/- 3.9 V max wich would be 2.76 Vrms.

However, when I look to the Analog Inputs electrical characteristics I find this:

The above max input voltage swing of 7 volts would mean +/- 3.5 volts. But the example above uses more input swing. This is confusing.

If I calculate my Vout to swing up to my 55 V Volt rail and using the 21.5 dB Gain from the datasheet the result would be

+/- 4.63 Volts swing -> 3.27 Vrms

Is my calculation correct and I should feed a 3.27 Vrms to each analog input?

Best regards

  • 0
    TI__Prodigy 385 points

    Hi Muller,

    1、The intro text states 2Vrms indicates when it use with PCM5242, PCM5242 can support 2Vrms in single-ended.

    2、About input voltage range, maximum input voltage swing, peak - peak value is 7V, ignore the examples(+/- 3.9 V)

    3、When you use 21.5dB Gain, the maximum input voltage is +/- 4.63 Volts swing so that can output 55V on load.

  • 0 in reply to Yang Wang
    Prodigy 80 points

    Thanks for the reply.

    But this still stands in contrast to each other and does not answer my question if I should feed +/- 3.5 Volts or +/- 4.63 volts max for full range.

    You stated "...when you use 21.5 dB Gain..."

    Should I use this value? I mean this is form the datasheet from TI... and it is not consistent with the rest.

    So my simple question is:

    Wich RMS Voltage do I need to feed each analog pin to reach 55V full swing at the output to get max. power to my speaker.

    Best regards

  • 0 in reply to Christopher Müller
    TI__Prodigy 385 points

    Hi Muller,

    When PVDD=55V, the maximum output voltage on 8ohm load is about 52V(any channel refer to GND) due to voltage dissipation. The gain is 21.5dB(11.885), so the max input voltage is +/-2.19V(refer to GND), that is 4.38V peak-peak, so the RMS value is 1.55Vrms.

  • 0 in reply to Yang Wang
    Prodigy 80 points

    Ok, thank you for the reply.

    Could you enplane that any further? 

    If have a Signal with +/-2.19 V as you say and amplify this by 21.5 dB I will get only +/- 26Volts. 

    But as we use TPA in PBTL Mode (example from first post), it should swing to +/- 52 V or am I wrong?

    Does the TPA that internally?

    Best regards

  • 0 in reply to Christopher Müller
    TI__Prodigy 385 points

    Hi Muller,

    Pls notice that I say all the amplitude is referred to GND. When the input is +/-2.19 V, that is Vpp=4.38V, and one channel's maximum output is 4.38*21.5dB=~52V(refer to GND, don't forget the DC offset is 26V, actually the minimum voltage is 0V), that is this output's Vpp=52V. So use 4.38V/52V=21.5dB. But when use BTL, will appear OUTP-OUTN, so the voltage of BTL is +/-52V.