I'm buying a whack of parts to start experimenting with chip amps. The 8406 I want to try both in class D and class AB. The others are class D only.
However for Class D, i'm having trouble determining specs for the filter.
Data sheets claim full LC filters are not required, and you can make a ferrite bead filter, then goes on to say you must take care selecting a proper ferrite.
How do I pick a proper ferrite? Is the lack of a suggestion just a cop-out so that if your design fails emi testing they arent to blame?
I was looking at using some 150ish ohm @ 100mhz ferrites and the 220pf caps suggested on the data sheet. Is that good enough?
I've read so much at this point I feel like im on the precipice of understanding the requirements of the filter, but i'm not quite there.
Also I dont understand spice so I cant simulate.
However for Class D, i'm having trouble determining specs for the filter.
Data sheets claim full LC filters are not required, and you can make a ferrite bead filter, then goes on to say you must take care selecting a proper ferrite.
How do I pick a proper ferrite? Is the lack of a suggestion just a cop-out so that if your design fails emi testing they arent to blame?
I was looking at using some 150ish ohm @ 100mhz ferrites and the 220pf caps suggested on the data sheet. Is that good enough?
I've read so much at this point I feel like im on the precipice of understanding the requirements of the filter, but i'm not quite there.
Also I dont understand spice so I cant simulate.
Magnetics are the most complicated passive components to select alas. They all fall short of the ideal in various ways. I'll summarize what I think about this, but take with a pinch of salt...
For a filter you want low loss(*), enough DC current handling (the audio signal counts as DC for the purposes of a class D output filter!), and importantly good linearity of the audio signal.
There are many grades of ferrite which each have their own frequency-dependent properties.
The properties at the switching frequency are pretty critical, but for suppressing EMI the behaviour up into much higher frequencies matters too. I would choose a ferrite with a frequency range including the switching frequency at its lower range, so there's a good behaviour at lots of harmonics of the switching freq.
150 ohms is great with two of them in series with an 8 ohm load - spurious currents are kept well down. If the inductance of the beads provides similar impedance at the switching frequency I'd try those beads.
Don't scrimp on the sizing, try find ones with a DC limit significantly larger than the max audio current - this should help keep things linear.
The acid test is performance - distortion at the speaker terminals and self-heating of the ferrite - both should be low.
(*) Losses pull more switching current from the output, and heat it up and the inductor. Losses at the higher harmonic frequencies aren't too problematical, there's much less power there. Ferrite tends to transition from inductive to resistive lossy characteristics at higher frequencies
For a filter you want low loss(*), enough DC current handling (the audio signal counts as DC for the purposes of a class D output filter!), and importantly good linearity of the audio signal.
There are many grades of ferrite which each have their own frequency-dependent properties.
The properties at the switching frequency are pretty critical, but for suppressing EMI the behaviour up into much higher frequencies matters too. I would choose a ferrite with a frequency range including the switching frequency at its lower range, so there's a good behaviour at lots of harmonics of the switching freq.
150 ohms is great with two of them in series with an 8 ohm load - spurious currents are kept well down. If the inductance of the beads provides similar impedance at the switching frequency I'd try those beads.
Don't scrimp on the sizing, try find ones with a DC limit significantly larger than the max audio current - this should help keep things linear.
The acid test is performance - distortion at the speaker terminals and self-heating of the ferrite - both should be low.
(*) Losses pull more switching current from the output, and heat it up and the inductor. Losses at the higher harmonic frequencies aren't too problematical, there's much less power there. Ferrite tends to transition from inductive to resistive lossy characteristics at higher frequencies
Two in series?
I also dont have a scope. its been on my list for 2 years, but unfortunately its on the "higher dollar" list, and I've been able to do lots without.
These PAM chips are "filterless", in the data sheet its mentioned that if the traces to the speakers are very very short, you can likely get away with no filter, but that most applications will require a ferrite.
I've linked a data sheet. Might help the question be answered properly.
https://www.diodes.com/assets/Datasheets/PAM8303D.pdf
My understanding is the only reason for the ferrite filter is to deal with EMI, and that the switching frequency is filtered out by a circuit within the IC.
Its just spec'ing the ferrite, I dont know where to start. Honestly the 150ohm @ 100mhz choice was/is completely arbitrary.
I also dont have a scope. its been on my list for 2 years, but unfortunately its on the "higher dollar" list, and I've been able to do lots without.
These PAM chips are "filterless", in the data sheet its mentioned that if the traces to the speakers are very very short, you can likely get away with no filter, but that most applications will require a ferrite.
I've linked a data sheet. Might help the question be answered properly.
https://www.diodes.com/assets/Datasheets/PAM8303D.pdf
My understanding is the only reason for the ferrite filter is to deal with EMI, and that the switching frequency is filtered out by a circuit within the IC.
Its just spec'ing the ferrite, I dont know where to start. Honestly the 150ohm @ 100mhz choice was/is completely arbitrary.
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.