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LM48560TLEVAL: External power supply for ultrasonic speakers

Mark Vermeulen
Prodigy 10 points
Part Number: LM48560TLEVAL
Other Parts Discussed in Thread: LM48560, DRV2700, DRV8871

Tool/software:

I am testing the LM48560 to drive up to 5 ultrasonic speakers, 150nF each, 30Vpp max, at a frequency varying between 38kHz and 42kHz.

It seems the internal boost boost converter will be stressed too much, can I use an external power supply to 15VDC, to feed the IC at TP2. As the output is full bridge I believe this will give me the desired 30Vpp across the speakers.

Would it be possible to simply feed 15VDC into TP2 to achieve this?

  • 0
    TI__Guru** 102555 points

    Hi Mark,

    I think this should work, make sure to disable the internal boost before powering up the VBST pin externally. Please test this thoroughly as it is not the conventional use-case, the performance may not follow the data sheet.

    Best regards,
    -Ivan Salazar
    Applications Engineer

  • 0 in reply to Ivan Salazar
    Prodigy 10 points

    Dear Ivan
    It seems the boost voltage from the speaker is strongly dependent on the input voltage as well as the capacitive load of the connected speakers. Is there a recommended way to boost the speaker voltage without resorting to an external boost converter?
    Mark

  • 0 in reply to Mark Vermeulen
    TI__Guru** 102555 points

    Hi Mark,

    The output of a piezo speaker depends considerably on its capacitance. Generally, the frequency response is more limited as the capacitance increases such as the example below from DRV2700 documentation:

    Considering such high frequencies, you're trying to drive even higher than 20k, the voltage drive will be limited.
    LM48560 and in general all the audio related drivers are rated for up to 20k.

    Best regards,
    -Ivan Salazar
    Applications Engineer

  • 0 in reply to Ivan Salazar
    Prodigy 10 points

    thank you, I am aware of this.

    However, ultrasonic speakers are piezo speakers and require a driver han can handle capaciive loads, The LM48560 can handle the bandwidth but indeed at ultrasonic frequencies the impedance of such speakers drops. I have not yet found a driver IC more suitable than the LM48560. To increase power output and reduce load issues I've put an inductor in parallel to the speaker so that the LC resonance is just below 38kHz This seems to reduce the current draw from the LM48560 around 38..42kHz so it seems I can still use the LM48560 as suitable driver.

    I've also tested the DRV8871 H-bridge motor driver as this can handle higher currents and handle the switch speed, but so far the performance of the LM48560 seems better. Would there be another recommendation you have for this application?

  • 0 in reply to Mark Vermeulen
    TI__Guru** 102555 points

    Hi Mark,

    I'll double check if there's other devices more suitable for such application. Please allow a couple of days to get back to you on this.

    Best regards,
    -Ivan Salazar
    Applications Engineer

  • 0 in reply to Ivan Salazar
    TI__Guru** 102555 points

    HI Mark,

    Just wanted to come and close this one. I think your workaround is the best solution as of now.
    Other devices would still require external power supply, and you'd still have to compensate for the loading effect at higher frequencies with a piezo load.

    Best regards,
    -Ivan Salazar
    Applications Engineer