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Old 11th July 2004, 02:45 AM   #1
squadra is offline squadra  Netherlands
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Default pcb layout for lm3886

This is the pcb I have designed for the LM3886T device.
Before I have the pcb's done it would be nice if people that know a thing or two about pcb design could make some comments about this design.

Remarks:
- SMD1 & SMD2 are 100 nF caps, soldered to the bottom of the pcb.
- The hole for pin 3 of the LM is placed a little forward to make some room for the trace from Pin 1 to 5 to be wider.
- Rectifier & buffering is on a separate pcb.

Better not use an exact copy of pcb, the dimensions of the LM are wrong, as Layo doesn't have the multi-watt package.
(or maybe I just couldn't find it, after all it's my first try.
That might also explain why it took > 6 hours )

And for some reason if I scaled the file to something smaller than the original size the file's size got bigger. Therefore I attached the bigger but smaller file
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Old 11th July 2004, 10:05 PM   #2
squadra is offline squadra  Netherlands
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A few small changes:
- Found out how to create my own library elements, the lm3886 now has the correct pin layout & spacing.
- Moved GND pin to the middle, looks more like a star now.
- Moved mute resistor so it is completely below the LM avoiding shortcircuit with the heatsink.
- Added parts for Zobel network.
- Boardsize is 5 * 4 cm, 1 euro card can accomodate 8 of these pcbs.

Image now is smaller too, just 25k
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Old 11th July 2004, 10:48 PM   #3
Morbid is offline Morbid  Israel
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i think you better put also the holes not in use for the PCB in the LM3886 legs,
like every IC design, you put the full package detail...because if u dont drill that you just need to cut legs...

11 holes i assume?

if u have the schematic i am looking for a good & max.usage of this chip for max. power
thanks
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Old 11th July 2004, 11:51 PM   #4
squadra is offline squadra  Netherlands
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Quote:
Originally posted by Morbid
i think you better put also the holes not in use for the PCB in the LM3886 legs,
like every IC design, you put the full package detail...because if u dont drill that you just need to cut legs...

11 holes i assume?

if u have the schematic i am looking for a good & max.usage of this chip for max. power
thanks
There were 11 pins to start with
I am going to cut pins 2, 6 & 11, mainly because I need the extra space for wider traces.

I'm basically using the schema as in the datasheet.
Some parts values have changed, and since I don't like elco's in the signal path I have moved the DC blocking cap to the input.
Partlist for this pcb:
Rin: 47k
Rb: 1k
Ri: 1k2
Rf1: 18k
Rm: 22k
unnamed 1E for zobel network

Cs(2x): 220uF elco + 100nF SMD
unnamed 3u3 cap @ input
unnamed 100nF for zobel network
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Old 12th July 2004, 07:39 AM   #5
Morbid is offline Morbid  Israel
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ok, well still i dont recommand cutting legs....
even if they are NC in the inside schematic there can be errors.
like if someone takes this PCB layout and cut the wrong leg
or the chip itself can be less stable (phisicaly)....

u can still bridge legs 1 and 5 without hurting leg 3...

b.t.w how u make the PCB after you draw it?
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Old 12th July 2004, 09:54 AM   #6
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Squadra,

I agree with morbid, cutting of the pins is not a good idea. But if you want to do it, at least try to get the right pinnumbering on the chip, now it is quite confusing and you can bet somebody, sometime, maybe even you, will measure the wrong things or make a wiring mistake.

Question: I assume that one of the holes in the star ground is for the speaker return? Do you have a ps with a center tap? If you have two separate supplies, it would be good to run the returns of both to the star ground, then you need one more hole.

BTW Nice layout, looks pretty optimal to me.

Jan Didden
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Old 12th July 2004, 11:27 AM   #7
squadra is offline squadra  Netherlands
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Quote:
Originally posted by Morbid
u can still bridge legs 1 and 5 without hurting leg 3...

b.t.w how u make the PCB after you draw it?
Morbid,

If I don't cut pin 2 and leave 3 inline then the trace between pin 1 and 5 will be very small, too small for me.
I have returned pin 11, there was no reason to cut that one.

I will send the gerberfiles to a company that creates pcbs, I have no means of creating the pcbs myself.

Quote:
Originally posted by janneman
Squadra,

I agree with morbid, cutting of the pins is not a good idea. But if you want to do it, at least try to get the right pinnumbering on the chip, now it is quite confusing and you can bet somebody, sometime, maybe even you, will measure the wrong things or make a wiring mistake.

Question: I assume that one of the holes in the star ground is for the speaker return? Do you have a ps with a center tap? If you have two separate supplies, it would be good to run the returns of both to the star ground, then you need one more hole.

BTW Nice layout, looks pretty optimal to me.

Jan Didden
Thanks Jan,

Speaker return?
I plan on using a PS with separate grounds for V+ & V-.
Together with the speaker return I need 4 connections at the star ground.

So, again a new version
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Old 12th July 2004, 12:21 PM   #8
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Lead not input ground throuth output ground, but beside by independent line to the middle of PS. Get input cap to the middle of PCB commonly with input groung. After that all will be OK .
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Old 12th July 2004, 02:12 PM   #9
squadra is offline squadra  Netherlands
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Quote:
Originally posted by Upupa Epops
Lead not input ground throuth output ground, but beside by independent line to the middle of PS. Get input cap to the middle of PCB commonly with input groung. After that all will be OK .
I have interpreted 'the middle of PS' as the center of the star ground. Is that correct?

Your tips lead to the newest attachment

Tnx,

Pj
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Old 12th July 2004, 03:05 PM   #10
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Getting better all the time. You may want to put the input gnd pad close to the input hot pad, makes it easier to solder the screened cable from the input. Run this pad with a short track to the start gnd.

The speaker output pad seems floating, or is there a jumper? Anyway, move the speaker pad to the right side of the electrolytic. Avoids speaker current under the cap. Also, move the V+ and V- pads closer to the chip, above the elcap, makes your pcb more compact and avoids unnessesary track length.

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