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DC-Coupling TPA3132D2 Inputs

Steve Hinshaw
Intellectual 515 points

Other Parts Discussed in Thread: MSP430F5638, TPA3132D2

I have an application where I need to drive the linear-compressor of a small cryocooler. Each 'motor' of the compressor requires a 15W, 50Hz sine wave drive signal and I'm looking at using 2 of the TPA3132D2 chips in the PBTL mode to individually drive each compressor motor. The sine wave will be created using a MSP430F5638 with an internally stored sine table that is DMA loaded into one of the DAC's and the DAC output is filtered with a 3rd order Butterworth filter.

Because of mechanical balance issues, we'd like to be able to offset the DC voltage across the load (compressor) by +/-1V and thus the need to DC-couple the TPA3132D2 inputs.


Because the processor will run on 3.3V, I'd like to also run the opamps of the reconstruction filter from 3.3V.

The TPA3132D2 datasheet states that the inputs are biased at 3V but gives no spec for the common-mode input range. What is the common-mode range of the TPA3132D2 inputs? Can the inputs be externally biased at 1.65V?

  • 0
    TI__Mastermind 37045 points

    Hi Steve,

    DC blocking caps are needed are both inputs. A DC offset voltage at the inputs will trigger DC protection and shutdown the amplifier. The inputs voltage Abs Max rating is -0.3V to 6.3V

    reg,

    Paul Chen

    Apps Engineer

  • 0 in reply to user5276421
    Intellectual 515 points

    Hi Paul,

    I'll probably be using a PVcc of around 12V and table 5 implies that the DC protection will not kick in until the output offset reaches 2.6V, so I'd think that I could impose up to a +/- 1V output offset without triggering the protection. I'll be using the chip at a fixed gain, so I would set the DC input voltage to produce the desired DC offset at the output.


    Is it possible to obtain the DC common mode range of the input stage?


    Thanks,

    Steve

  • 0 in reply to Steve Hinshaw
    TI__Mastermind 37045 points

    Hi Steve,

    The input will work up to the abs max range. It will start clipping close to the limit and manifest it on the output THD number. For your application THD is probably not a critical performance spec. However, depending on the gain setting, a few hundred millivolts of input offset voltage is sufficient to trigger DC protect. This amp is not intended to work without the DC blocking cap, unfortunately, we do not have much data to share.

    reg,

    Paul.

  • 0 in reply to user5276421
    Intellectual 515 points

    I thought that I’d follow-up on the question of input common-mode range of the TPA3132D2...

    I realized this morning that I can easily test the input common-mode range using the eval board. A 4-pin plug fits nicely across JP5 and JP6 and this allowed me to put 2 series connected 10k resistors between the INNL and INPL pins of JP5 and JP6 and then drive the junction of the resistors with another DC power supply to set the input common-mode voltage.

    I configured the eval board for 2 channel BTL operation and driving 20Vp-p across a 4 ohm load. I used the scope math function to give me the composite output and then did a FFT of the composite waveform to look at the distortion.

    On the low end the output signal starts to show signs of distortion below a CMV of 350mV and at 4.4V on the high end, so it looks pretty safe to drive the inputs with a 1.65V common-mode voltage. Over the range of 0.35 to 4.4V common-mode voltage there was virtually no change in the output waveform or the FFT.


    Thanks,

    Steve