Paralleled LM4780TA PC board design - How's this look? - diyAudio
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Old 30th March 2004, 03:03 AM   #1
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Default Paralleled LM4780TA PC board design - How's this look?

Basically, this is the datasheet's both-op-amps-paralleled circuit, converted into a double-sidede PC board layout. Red layer's the top, green's the bottom. The design's right off the datasheet and parallels both amps in a single LM4780 to produce higher output power without the tremendous increase in dissipation common in bridged designs. The tradeoff is the need for precision resistors in the voltage gain control, so that both amps' outputs are within a percent or less of each other.

Since it's easy to miss a little detail, I'll put this to additional eyes... Anyone see any obvious problems I need to address?

oO
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Old 30th March 2004, 06:55 AM   #2
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please note that the trace resistance must be kept within 0.1 milliOhm (0.0001 ohms) if you want to have the 0.1% matching listed on national... yep. 0.1 milliOhms isn't too much.

also, any traces that solder to the top of the board will be hard to solder onto the top of the board on pins under the IC.

that board is impressively sized (small) it looks like. depending on C4 and whatever terminal block you have, they may overlap, if no terminal block is used this is OK.

i'd have to say i'm most worried about the output traces. pretty small in a situation where every 0.1mOhm matters. and they are also pretty small and they will be higher current traces.

The IC looks pretty far from the edge, is this correct?

Cin should probably be longer, to accomadate a metalized-poly cap.

c11 and c12 are going to be larger value caps, they seem a bit small or closely spaced.
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Old 30th March 2004, 02:27 PM   #3
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the design is a bit too complicated imho, you should probably redo this in Eagle PCB.
however, nice
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Old 30th March 2004, 09:54 PM   #4
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theChris:

Yeah, I saw that tenth-miliohm matching requirment and thought "yeah right, like hobbyists are gonna have gear capable of resolving that small a difference." I'm not going to even waste time trying to reach that kind of precise with the equipment at my disposal (although I do have a calibrated Fluke DMM that I think can read that low).

As for the top and bottom soldering issue, the editor's ExpressPCB and I'm planning to have them produce a few of the boards. They'll pre professionally-made and all holes will be plated through, so that's not an issue. They'll even take care of all the vias in the groundplanes.

C4 shouldn't be an issue as my intention for the interconnects is to solder wires directly. I've had problems with contact corrosion in connectors before (I live across the street from a large saltwater bay) so for me it's better to solder everything and risk having to clip a wire later.

The backplate of the IC will be right at level with the edge of the board. I can trim the edge lightly if it misses by a few mils.

Already debating widening the mounting path for Cin, as you mentioned. I can move Rin over a little if need be to allow the cap to have some landing area on the board. Personally I'd rather leave caps out of the signal path but if ya gotta have it, well...

Also thought about the Ci pair. 47uF @ 50VDC in Tantalum is plenty small enough to fit, but I'm not sure tants are suited to amplifier DC coupling. I'll hafta do some research on that. If worse comes to worse I can float the caps off the board and lean them over the Ri pair.

As you've noted, the board's design goal is smallest size possible with proper design and groundplane layout to keep coupling and oscillation and so forth in check. To make the traces that carry current wider may run me into problems with coupling since I'll have to ditch some shielding (especially the B amp's output around Rf2 and Rsn2), so I debated making the traces thicker in the vertical plane instead by soldering some wire along the traces. I've used that trick before with supercompact power supplies and it works nicely, esp. if one uses solid bus wire and preshapes it properly before soldering it down.

I have some 24-ga. copper sheet too - I could make SERIOUS current capacity traces if I wanna go crazy.



Tensop:

Of course it's complicated - it's crawling with groundplanes.

But, since I'm going to have a replicator make 'em right off the drawing I can make it complex. (No way in hades I'd want to cook these up by hand - I don't have -that- much time. )

oO
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Old 31st March 2004, 08:12 AM   #5
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not sure if you saw my milled PCBs that i plan to sell. mine has no explicit mouning holes for the 0.1ohm resistors as i can't see it ever working to a point that i could _garuntee_. in anycase it's set up to allow many options. parallel is possible, but still, i can't see anywat to get the matching to 0.1% without extreme measures.

your's is also smaller. i got around the Cin issue by mounting the capacitors in a staggard fasion and assuming 5mm leadspacing, though they could be made larger, such will not fit perfect.

in an ironic update, i just realized that my planned components didn't fit. so i had to change that. in an ironic turn of events, i now have space for bigger caps on the board...

also,i don't like the idea of tantalum input caps. especially in a split supply board.
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Old 2nd April 2004, 04:30 AM   #6
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Hehehe, I redesigned my layout as well. It will stack slightly easier (two complete paralleled pairs in a 3.8 x 2.5" board) and the traces that will carry appreciable current are quite a bit wider. As an added plus I got room for a larger input capacitor and more clearance for the two Ci caps. And to sweeten the pot I got a lot better groundplane layout out of the deal.

Now, as for this whole match to within 0.1% stuff, I did some digging and since nobody bothers trying to get that close a match for multiple paralleled amps I'm not going to either. As long as they're within a close enough range that one isn't feeding back into the other (and the Rout resistors are there to help balance things) there shouldn't be any real-world issues.


Hey theChris, who's milling your boards? I would like to find a board maker that can mill 'em in small qty.

oO
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Old 2nd April 2004, 08:45 AM   #7
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my school mills boards. sadly they won't be able to fit me in until monday. the school projects take precedence over entheusists projects...
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Old 8th April 2004, 04:55 AM   #8
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Aaaaaand here's the latest revision, with some fixes...

oO
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Old 21st April 2004, 12:10 AM   #9
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Aaaand, here are the boards, fresh from ExpressPCB. I panellized six of them to get their $59 prototyping price, which for my purposes is more than adequate.

Click the image to open in full size.

Each board is 1.875" by 2.5", and will parallel both op-amps in a single 4780. If built with 0.1% resistors for the gain and feedback control and 1% or better resistors for output balancing, it SHOULD be possible to parallel several of these boards together - without any design or parts changes - to drive signals into almost-dead-short impedances without releasing the magic smoke.

But alas, I won't know that until I get the parts in from Mouser and build up a couple.

This is about as compact a board as is practical while leaving enoug space for components AND allowing for sufficient groundplane action to reduce loops and noise.


My gameplan is to build little amp "modules" with their own heatsink/fans (tunnel HSFs for Intel's Xeon processors - which are meant for 100-watt thermal loads!) and everything in one nice neat package. Need more power or a lower operating impedance? Add a module or two!



More on the design as I test 'em...

oO
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Old 25th April 2004, 01:29 AM   #10
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Update...

Parts to build two are on order from Mouser. When I get a pair built and tested I'll post my thoughts on them.

I am seriously considering a production run of the boards if the basic design holds up. Already have an idea on how to make 'em easily connectable in parallel, which could prove useful.

oO
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