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TAS5711: How to calculate Audio Amplifier output power in BTL & SE Configurations?

Manmeet Dhaliwal
Prodigy 90 points
Part Number: TAS5711

Tool/software:

What is the formula to calculate power at a speaker in both BTL & SE configurations?

Let's assume an 8 ohm speaker load.  (RL = 8)

My understanding in BTL mode is that the voltage at the speaker is essentially doubled (PVDD * 2) where PVDD  = Peak output voltage on the Amplifier outputs

Then it would be:

Prms = ((PVDD x 2/)√2)^2 / RL

And for Single Ended, you wouldn't multiply PVDD by 2, so the formula would be:

Prms = (PVDD/√2)^2/ RL

However when I look at the power values shown in the TAS5711 datasheet, they don't seem to match the calculations above.

For example for BTL load, PVDD 18V, 8ohm Load:

Datasheet says ~21 Watts, but if i use the formulas above, I calculate 81 Watts.

In fact if I use my Single Ended formula above instead, but for the BTL mode, I calculate 20.25W which is matching the datasheet.
The same situation is true for all power outputs given on page 10 of the datasheet (AC Characteristics)

Am I missing something here?
Is the output voltage not = PVDD, but half of PVDD instead on each output?

How do I calculate maximum power to a load in BTL and SE cases given that I know PVDD to the amp and load impedance of the speaker?

  • 0
    TI__Genius 9175 points

    Hi Manmeet,

    For BTL, you output is a sine wave that goes both positive and negative (peak to peak of 2xPVDD). The RMS value for a sine wave with 0 DC bias is the amplitude/√(2) = PVDD/√(2). This means an RMS output power of  (PVDD/√(2))^2 / RL.

    For SE, your output is a sine wave with an amplitude of PVDD/2 and a DC offset of PVDD/2 that is filtered out with a DC block capacitor or removed by having the other end of the speaker tied to PVDD/2. This means your RMS voltage would be PVDD/(2*√(2)) so your output power would be (PVDD/(2*√(2)))^2 /RL

    Regards,

    Ramsey

  • 0 in reply to Ramsey Foote
    Prodigy 90 points

    Thank you very much, I see now my error was that I used Vpp (peak to peak) instead of Vp (peak only) in my measurements

    It makes sense now