This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

TPA3255: LC Filter in PBTL mode

Part Number: TPA3255
Other Parts Discussed in Thread: TPA3245, LCFILTER-CALC-TOOL

Hello!
I need to use TPA3255 in PBTL mode.
How do I make an output filter?
I have an inductor of 10 μH, 6.1 A, MA5172-AE Coilcraft
and film capacitors Film, 1 μF, 250 V,

In the document SLASEA8A, the PBTL scheme is drawn
OUTA, OUTC for one inductor and then a capacitor of 1μF
OUTB, OUTD to another inductor and then a 1μF capacitor

In the SLOU441 user manual, the output filter circuits for the PBTL mode are different.
Each output to its inductor and then a capacitor of 1μF

How to do it right?

  • Hello,

    Thanks for looking at the TPA3255! The TPA3255 supports both 2-inductor (pre-filter) or 4-inductor (post-filter) PBTL output configuration.

    - Pre-filter PBTL (2 inductor) - OUTA/OUTC and OUTB/OUTD are connected together before the inductor.
    - Post-filter PBTL (4 inductor) - Channels A/C and B/D are connected after the LC filter.

    Either one is acceptable and here are a few trade-offs that you may want to consider:

    - Inductor saturation - for the same output power each inductor in pre-filter PBTL will need to handle twice the current of post-filter PBTL, requiring larger inductors. This may either be a size or cost trade-off in your system depending on the inductors available.
    - Pre-filter has slightly better THD+N performance than post-filter, but for many applications it's negligible.

    We are updating the TPA3255 datasheet to show both pre-filter and post-filter. The TPA3245 (device in the same family) shows both pre-filter and post-filter PBTL already if you want to use it as a reference for now.

    Let me know if you have any questions.

    Thanks,
    Brian
  • Thank you, Brian!

    Next question)
    With a wide range of loads, is it enough to calculate the filter for the most powerful load?

    If I connect the load to the TPA3255 via a transformer, how will the LC filter's calculation change?

  • Hi,

    The LC filter design may need to change due to a change in load impedance.  The load impedance affects the Q of the LC filter.   For most filters driving 2-8 Ω loads, the filter doesn't change much; however, if the load is much higher than the L and C should be adjusted.

    • See the LCFILTER-CALC-TOOL on TI.com to calculate the appropriate LC filter
    • You can measure the impedance of a transformer by connecting the load to one side and measuring from the other side of the transformer. 

    Thanks,
    Brian