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TPA3118D2EVM - LC Filter

Patrick Frank
Prodigy 40 points

Other Parts Discussed in Thread: TPA3118D2, TPA3118D2EVM

Hello,

i have a question regarding the TPA3118D2.

In the TPA3118D2EVM are 3 different parts for the inductors L1-L4 noted.

- Must i also change C21-C24, when i change L1-L4? If yes, which value must i use?

- What makes the difference between these 3 inductor types, sound quality or only EMI?

- What is the value for the inductor type "FB1812" in µH?

Patrick

  • 0
    TI__Expert 7925 points

    Hello, Patrick. Picking a filter for the TPA3118D2 takes a little bit of planning. The schematic for the TPA3118D2 EVM (here 4478.TPA3118D2EVM Rev C Schematic - Reference Designs.pdf) will show the 10 µH/0.68 µF solution that works for most cases, and the BOM for the parts is in the EVM User's Guide.

    If you decide to use the DG6045C 2.2 µH inductors listed on the schematic, you won't necessarily need to change capacitors C21–C24, but dropping their value can help you attenuate frequencies immediately greater than 20 kHz. However, dropping their value also increases the intensity of the Q-point of the cutoff of the LC filter, which can cause large peaks at the corner frequency. For audio measurements, it's a good idea to filter out frequencies greater than the audio range. With C = 0.68 µF and L = 2.2 µF in the schematic, the cutoff frequency for the filter is 130 kHz, which could allow some of the PWM harmonics to appear at your output, but it probably won't hurt too much. We picked the 2.2 µF DG6045C inductor for design that need a small inductor with a high current rating. Please play around with filter numbers with this filter design tool and by following our guide on L-C Filter Design.

    The FB1812 is not an inductor at all, but a surface mount ferrite bead sized 1812. Ferrite beads are available from a number of manufacturers with that footprint size. A ferrite bead doesn't have a value in µH, so to say, but it has an impedance profile that increases with frequency. This choice for your magnetics would be for "filter-free" designs that don't filter the PWM out from the switching amplifier until higher frequencies, for EMI reduction purposes. A signal whose frequency gets into the 1–100 MHz range will be attenuated by a ferrite bead's rising impedance, which is good for reducing higher harmonics from switching for EMI tests. If you choose a ferrite bead for your output filter, swap out C21–C24 with 1 nF capacitors. You'll also want to make sure the bead has a high current rating to use with the TPA3118D2; 8 A with 125 Ω at 100 MHz should do the trick.

    Best Regards,
    Matt