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Class D amplifier as high power opamp replacement

Michal Majchrak1
Prodigy 60 points
Other Parts Discussed in Thread: OPA551

Hi,

Please is there a possibility how to use class d amplifier as a voice coil driver? My problem is that I am driving a kind of voice coil (simple coill actuator) with a high power opamp (OPA551 and similar) in linear way, and , the current is around 500mA, so the power dissisipation is quite big. I was thinking about replacement of the opamp with class D amplifier, with all the analog feedback that I have, but I am not sure if the claas D could work from DC to few kHz - I think all the output protections will be the problem. There is some PWM drivers from TI (DRV2511) but I have very high sensitivity to quite small ripple currents so I hope that there is king of more efficient "audio" way how to drive these coils.

Have you any suggestions?

Thanks a lot.

  • 0
    TI__Mastermind 24010 points

    Hi Michal,

    Based on your description I think a Class-D solution might be able to work in this system. Do you know the voice coil equivalent impedance is at the frequency ranges you would be interested in? I need to know what the equivalent 

    500mA would create quite a lot of power dissipation with a opamp compared to class-D optimized for that power range! Would this be peak current or rms current? 

    One issue I foresee is the super low frequency range (and DC). The input capacitors might block frequencies below 20Hz. But if this is being used for audio then that shouldn't matter as the human ear can't hear anything below 20Hz. 

    Please let me know more of your system requirements so we can direct you to the best amplifier.

    Best Regards,

    Robert Clifton

  • 0 in reply to Robert Clifton56
    Prodigy 60 points

    Hi Robert,

    Thanks for such a fast respons.

    The problem is that this is not for audio application, and the frequency will be from 0Hz (DC) to approx. 150Hz. It is for small mechanical actuator, which has the coil made directly on PCB and coil resistance is around 10Ohm. The current is 400mA RMS,

    So there have to be DC coupling with signal source I think (DAC or MCU DAC, I2C, or just analog).

    Thanks

    Michal

  • 0 in reply to Michal Majchrak1
    TI__Mastermind 24010 points

    Hi Michal,

    The DC blocking capacitors used in analog input will block the lower frequencies. You need them unless you bias your signal to the same voltage as what's seen on the input pins (AVDD/2). 

    You could also use PWM input or I2S input to achieve the same results without having to deal with those capacitors. 

    Let me know what you think.

    Regards,

    Robert Clifton

  • 0 in reply to Robert Clifton56
    Prodigy 60 points

    Hi,

    Yes you are right, I was thinking about using also "digital" inputs, but I am most troubled about how the protections will work, for example if the DC output protection will shut down the output stage...what do you think?

    Thak you

    Michal

  • 0 in reply to Michal Majchrak1
    TI__Mastermind 24010 points

    Hi Michal,

    The device will have an automatic DC output (PVDD/2), since it will switch at a 50% duty cycle in most cases. So you could technically get the DC signal you are looking for. 

    Regards,

    Robert Clifton