Hello everyone,
I put some effort into designing an output filter for a class-D IC. Here I am using Coil Craft's and Murata's SPICE models to calculate the filter with an 8 Ohm load.
Lines:
Filter:
The IC that I am using TAS2780 has a post filter feedback (PFFB) feature and I thought that would be good to use to keep the THD as low as possible. So I started reading about the PFFB requirements.
It seems crazy to me to specify the filter cut-off frequency to be greater than 10MHz when this device is switching is 380kHz.
What could they mean?
Questions
I put some effort into designing an output filter for a class-D IC. Here I am using Coil Craft's and Murata's SPICE models to calculate the filter with an 8 Ohm load.
Lines:
- Red line is the capacitor impedance
- Green line is the inductor impedance
- Purple line is the filter response
Filter:
- -3dB cutoff is around 25kHz, just outside the audio band
- The filter works until about 10MHz where the parasitics of the components have a resonance.
- Sure the values can be tweaked, but this is not a bad starting point for now.
The IC that I am using TAS2780 has a post filter feedback (PFFB) feature and I thought that would be good to use to keep the THD as low as possible. So I started reading about the PFFB requirements.
It seems crazy to me to specify the filter cut-off frequency to be greater than 10MHz when this device is switching is 380kHz.
What could they mean?
- The rise/fall time of 1/10MHz = 100ns. So maybe if the rise/fall time of the IC is around 10ns or less, the filter could filter that out?
- The cut-off frequency they mean where the parasitics break the filter? This is around 10MHz for the L and C parts I am using.
Questions
- Can anyone experienced with Class-D and filter let me know if a -3dB cut-off just outside the audio band is sensible?
- Has anyone used TAS ICs from Texas Instruments before, or the TAS2780 specifically? Can you suggest a sensible filter with or without the PFFB feature.
- What the hell are they talking about in the data sheet and application report when they specify the cut-off must be greater than 10MHz?
It took some years for TI to deliver a chip with PFFB that deserves that name instead of that bad jokes around TPA3255 etc.
Oh I’m not in the loop regarding the TPA3255, is it a pain to work with?
Any comments on advice on filtering with TAS?
Any comments on advice on filtering with TAS?
you will find more recent details here: https://www.ti.com/lit/an/sloa339/s...vlXOSQ6pYkthcBU1d49r-4vyH2XXicqG55tW5Upq6ZGiA
I think this relates to EMI-filters with ferrite beads, not the common LC-filtersIt seems crazy to me to specify the filter cut-off frequency to be greater than 10MHz when this device is switching is 380kHz.
moving the filter into the feedback loop affect the stability of the amplifier.
My understanding is that the filter needs to have certain properties for the amplifier to be stable. What I don’t understand is what those properties are when using an inductor and a capacitor all of the documentation they publish is not very helpful in that respect.
My understanding is that the filter needs to have certain properties for the amplifier to be stable. What I don’t understand is what those properties are when using an inductor and a capacitor all of the documentation they publish is not very helpful in that respect.
Inserting the filter into the feedback loop gives an additional 2nd order low-pass inside the feedback loop that will degrade stability.
You could say as well that the feedback loop needs some properties to be stable with the inserted filter.
This is a common issue with feedback loops.
The implications are documented in basics of feedback loops.
You could say as well that the feedback loop needs some properties to be stable with the inserted filter.
This is a common issue with feedback loops.
The implications are documented in basics of feedback loops.
The plot shows -3dB point at about 60kHz, not 25kHz.Hello everyone,
I put some effort into designing an output filter for a class-D IC. Here I am using Coil Craft's and Murata's SPICE models to calculate the filter with an 8 Ohm load.
Lines:
View attachment 1347671
- Red line is the capacitor impedance
- Green line is the inductor impedance
- Purple line is the filter response
Filter:
- -3dB cutoff is around 25kHz, just outside the audio band
- The filter works until about 10MHz where the parasitics of the components have a resonance.
- Sure the values can be tweaked, but this is not a bad starting point for now.
Probably you know this already (regarding the EMI) and maybe you don't. The Infineon document is quite explaining.
https://www.ti.com/lit/an/slyt198/slyt198.pdf?ts=1724444222088&ref_url=https%3A%2F%2Fwww.google.pl%2F
https://www.infineon.com/dgdl/Infin...N.pdf?fileId=5546d46264a8de7e0164b359d78016f6
https://www.ti.com/lit/an/slyt198/slyt198.pdf?ts=1724444222088&ref_url=https%3A%2F%2Fwww.google.pl%2F
https://www.infineon.com/dgdl/Infin...N.pdf?fileId=5546d46264a8de7e0164b359d78016f6
Last edited:
Thanks for all the comments and useful docs. folks.
I think I have got to the bottom of it now.
It seems the post-filter feedback feature of this IC is designed for ferrite beads only. I was not expecting this.
The filter cut-off should be placed at around 10MHz to filter the harmonics of the switching frequency.
I think I have got to the bottom of it now.
It seems the post-filter feedback feature of this IC is designed for ferrite beads only. I was not expecting this.
The filter cut-off should be placed at around 10MHz to filter the harmonics of the switching frequency.
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