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LM48580: LM48580 output protection and signal levels

Mark Pearson
Genius 12705 points
Part Number: LM48580
Other Parts Discussed in Thread: TPA2100P1

I have some questions on this one that I can’t find in the data sheets:

 

Are the speaker outputs current-limited or short circuit protected, either differentially or single-ended, or both?

 

Does each of the OUT+ and OUT- signals center around ground, or is there an offset?  (I’m assuming it’s a BTL-type output.)

 

What power rating should the series resistors that are connected to the speaker be?  We are driving a piezo speaker, with C=0.87uF, and are using two 5.11ohm 0603 resistors in series, and that seems much too low of power rating to me.  With piezo speakers, my understanding is most of the actual power is dissipated in the resistors (depending on the piezo ESR.)  Therefore, for this part at a 31Vpp output, that’s ~10-11w.

  • 0
    TI__Guru** 109165 points

    Hi Mark,

    This device is protected from short circuit, the device resume normal operation when the short condition is removed.
    The BTL output of LM48580 looks like the following figure, where OUT+ and OUT- are pink and yellow lines. The differential BTL output will be the subtraction of OUT+ - OUT-.

    There are some equations you can use on this document: www.ti.com/.../slos595.pdf on page 10. The piezo speaker capacitive behavior tends to create a short circuit at high frequencies, the series resistor is used to avoid this.

    Best regards,
    -Ivan Salazar
    Texas Instruments

  • 0 in reply to Ivan Salazar
    Prodigy 20 points
    Ivan,

    To clarify, are the OUT+/OUT- outputs short-circuit protected both to Gnd and to each other?

    It appears then that there is a positive dc bias at OUT+ and OUT- at 1/2 boost supply voltage at all times, correct? (~+15vdc in your plots.) However, the blue plot appears to have a bias on it despite it being the BTL output. Why is that?

    Thanks very much.
    Don Diez
  • 0 in reply to Ivan Salazar
    Prodigy 20 points
    Ivan,

    Is there a big difference in efficiency between using a series resistor vs. using a resistor and inductor? I'm referring to the verbiage in TPA2100P1 data sheet you provided.

    Thanks very much.
    Don
  • 0 in reply to Don Diez
    TI__Guru** 109165 points
    Hi Don,

    Yes the outputs are protected for both short-circuit to GND and to each other.
    Blue plot is the Vbst waveform. For the BTL output you would need to subtract the other two plots.
    The difference in efficiency is due to the current flowing through the resistors. The current demand from a resistor-inductor load is lower. Although there is not comparative documentation between the two kind of loads.

    Best regards,
    -Ivan Salazar
    Texas Instruments