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soundNERD 16th January 2004 11:50 AM

Lm3886 Pcb
 
2 Attachment(s)
Hi, I finished designing some pcb patterns for the LM3886. This amp will be used to power a small 8-inch subwoofer speaker.


Please tell me if there is anything wrong with it.

Thank you!

-Mike

soundNERD 17th January 2004 01:35 AM

Anybody?

Please tell me how this would work.

I based it off of the schematic on the first page of the 3886 datasheet

thanks, Mike

peranders 17th January 2004 07:23 AM

I have just taken a very short look, but where are the deoupling caps? The trace between the LM3886 pins... not good. The trace width, consider this is a >10 A device! I think the pcb will act as fuses. Otherwise I think it's not so hard to succeed.

You can get some inspiration if you take a look at my LM3886 project. Download the pdf's for the pcb layout. Note also that I'm going to have 95 um copper (2.7 oz).

soundNERD 17th January 2004 12:34 PM

The trace between the chips is the ground, so it doesn't matter, right? And the caps won't do anything, since the ps board is very close to the amp board.

I'll look at your project, though.

-Mike

soundNERD 17th January 2004 12:49 PM

Actually, I changed my mind. I think I will just put the ps on the amp board. That will eliminate the need for the decoupling caps.

If I don't put the ps on the amp board, do I really need the 100uf decoupling cap? I would use the .1, though. I didn't put it on my pcb due to size. If I could just use the .1 cap, that would be easy to add.

Also, about the gnd. gnd carries no voltage, right? So does it really have to be thick?


If you didn't notice on the pcb, the pads on the top right are for input (this amp is for a sub, so connection to the input/crossover/volume board), the bottom center is for speaker, and the top left is power in. The pad connecting to the speaker pin is a jumper over the + voltage wires


I couldn't find your pdf file, but I only looked quickly.


Also, how much do traces limit power? The original amp I designed with the 3886 used all thin traces, not much wider than the pins themself. I designed that one manually using rub-on traces from radioshack.

Will using the thicker traces improve sound quality also?

Thanks, mike

crown300 17th January 2004 02:19 PM

Your PCB layout is a great first effort and you are applauded for your interest and work.
however............
IMO, the design will most likely oscillate at a very high frequency, maybe not at idle, but during power peaks. This will show itself as a lot of heat when running at a 'normal' level. I would not want to use my best speaker during your testing.
Please, please take the time to read the earlier GC threads. There is a lot of time saving information there.
Ground:
Note on page 1 of the LM3886 datasheet that the 8 ohm 'load' is connected to Ground. (please note that this is Power Supply ground - not earth ground.) On page three (near the bottom) there is a symbol "I out" which is "Output Current Limit". note the value is 7 Amps. This current has to flow through your speaker and "return" to the power supply (during audio peaks). Also the LM3886 needs to sense this current, hence it needs to be between the power supply and the load. This current path will cary high peak Amps and a small trace will have a (relatively speaking) resistance causing voltage drop. (Mr Ohm at work!).
Part deaux: if this 'ground' (with its varing output load voltage) has connected to it some of the input parts, they will see a voltage change where there is no change to the actual input signal. Instant oscillation. Meltdown.
So a GC PCB layout needs to account for this.
PS, IMO, most "automatic" PCB layout programs do not make power traces wide enough.

peranders 17th January 2004 04:31 PM

Quote:

Originally posted by soundNERD
I couldn't find your pdf file, but I only looked quickly.
You just have to click at "downloads" in the left frame or you can click here

soundNERD 19th January 2004 02:30 AM

I looked at peranders pcb. I don't need anything that fancy, all I need is a small amp to power a small 8-inch subwoofer.

So, I would like to use the PCB I designed, but just modify it to not oscillate.

Lets say that I thickened up the PCB traces somehow (or left them as is, and covered them with a layer of solder, or solder a bare wire across all of them). Would that be enough for the speaker? If I made them as wide as the + and - supply wires, would that be enough?

What would be the cause of the HF oscillation? Is it the gnd trace? Or something else. If it isn't the ground, how could I fix it?

Thank you for the help, Mike

peranders 19th January 2004 07:42 AM

Quote:

Originally posted by soundNERD
Actually, I changed my mind. I think I will just put the ps on the amp board. That will eliminate the need for the decoupling caps.
One of the most importantthings are good ground and good power supply decoupling.

I think you should take a look at AN-1192 and the recommended design. Change then the design if you really know what you are doing.

If you remove the "fancy stuff" from my design see what's left and where it's located. Check also Thomas Madsen's LM3886 "Das Modul".

Consider AN-1192 as "the law" at the moment.

The casue of oscilllations is often not sufficiently low impedance (at the MHz region) between supply voltage and ground sensed at the LM3886.

BTW: I will order my LM3886 monster pcb's today. :) EDIT: Done!

skyraider 19th January 2004 04:57 PM

quote:
'The casue of oscilllations is often not sufficiently low impedance (at the MHz region) between supply voltage and ground sensed at the LM3886.'


can you explain further?


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