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TPA6138A2: choosing appropriate passive component values for LPF and HPF

Aaron Knoll
Prodigy 10 points
Part Number: TPA6138A2

Hello,

I am looking for assistance in choosing appropriate passive component values for use with the TPA6138A2. I will be generating two 0-3.3V PWM signals via an MCU and wish to produce analog audio signals suitable for listening via headphones (one for each stereo channel). My MCU (an ATmega324A) is able to source up to 10mA, though I would image drawing that much current would likely be ill-advised. It is my understanding that the TPA6138A2 is ideal for PWM -> Analog. I'm aware of the fundamental necessity to ground-bias the PWM signals with AC-coupling capacitors, produce analog signals with LPFs, and amplify the signals. My questions are as follows:

  1. Can I use the TPA6138A2 for all required stages post PWM generation?
  2. Which components in Figure 10 of the data sheet correspond to R_IN and R_fb in Table 1, and which components control the LPF.
  3. What values should be used for the passive components in Figure 10 of the data sheet, assuming single-ended inputs (like those mentioned earlier), a LPF cutoff frequency of 18 kHz, and a variable gain (between 0V/V and a reasonable gain level, to be adjusted via a user controlled digital volume potentiometer)?

Thank you.

  • 0
    TI__Guru* 84915 points

    Hi, Aaron,

    Welcome to E2E and thank you for your interest in our products!

    Please take a look at my responses below and let me know if anything else is required.

    1.- Can I use the TPA6138A2 for all required stages post PWM generation?

    Yes, actually the most part of our audio amplifiers (I could say, all our analog amplifiers) can be used with a PWM input signal. Some of them support a PWM signal without any filtering stage, but preferably it is better to include a filtering stage between the PWM generator and the device. Usually, there's a out-of-band noise that must be reduced/filtered. Otherwise, the amplifier will also increase this noise. For more details, please consult this app note:

    http://www.ti.com/lit/an/slaa313/slaa313.pdf

    In this case, the TPA6138A2 can be used for the filtering stage and the amplification stage.

    2.- Which components in Figure 10 of the data sheet correspond to R_IN and R_fb in Table 1, and which components control the LPF.

    The Figure 10 is the circuit diagram for the TPA6138A2 in differential configuration with additional passive components for filtering purposes. R_IN would be R1 and R_FB would be R2 in Figure 10.

    3.- What values should be used for the passive components in Figure 10 of the data sheet, assuming single-ended inputs (like those mentioned earlier), a LPF cutoff frequency of 18 kHz, and a variable gain (between 0V/V and a reasonable gain level, to be adjusted via a user controlled digital volume potentiometer)?

    The active filter in the figure 10 is a second order filter in multiple feedback configuration. An example of the suggested components can be found in our EVM schematic. It is configured for a second order low-pass filter with cutoff frequency of 48Khz and DC-blocking capacitor. I think this configuration was selected just ro remove out-of-band noise without affecting the audible band.

    I would suggest to take a look at this website for details about the multiple feedback configuration:

    https://www.allaboutcircuits.com/technical-articles/inductor-out-op-amp-in-an-introduction-to-second-order-active-filters/

    In addition, I suggest to take a look at our filter design tool. You can design your own filter for single-ended configuration by selecting the low-pass filter and multiple feedback configuration

    https://webench.ti.com/filter-design-tool/design/3

    I hope this helps you! Please let me know if you have additional questions or comments on this!

    Have a good day!

    Best regards,
    Luis Fernando Rodríguez S.