Hi Neil and all,
The layout problem is far from "simple". I'm doing layouts that must pass CE EMV tests. Therefor I went to a design training, held by Mitsubishi engeneers, and spoke a lot to other experienced engeneers. I am not an engeneer myself, and I still feel like an amteur, having a "dangerous halfway knowledge" (the same as my english, obviously).
So here are just a few hints I can throw in, to startle the discussion a bit more:
- the idea to put Cs of different values in parallel to cover a wider range of frequ to suppress is wrong. Instead it makes a rather unpredictable network of Cs Ls and Rs that build passes for different freq ranges, or might even swing. I have seen that on spectrum analysers at mitsubishi, its amazing. So I simply use a 10uF tantalum close to the reg. Than 100nF ceramics x7r closest to each supply pin to GND.
- Grondplane is a must, and more: its a miracle. If you cut the GP by any signal, even VCC, it might start to swing in higher GHz ranges. So the rule is: no wires on GP. That is impossible to follow, I admitt. But you must try to keep the wires few an short. Better jump a lot with vias, think more about placing parts and swap pins if possible and nessesary. Do not connect islands of GP using vias from the toplayer. The GP must be casted in one piece.
The top layer gets a GP also, but it will be full of wires cutting through, no problem. But connect top an bottom GP using vias a lot. Make a ring of vias around the reg.
- VCC: try to design a wide trunk with a branch for every single supply pin. Avoid going from one chip to the next.
That is not always possible, but try to get close to the idea. I think a star-shaped layout was even better, but not very practical.
- get rid of the diodes, you do not need them, the make the network more confused and in moments of higher currents (consumption, interferences in the clr network) they let the voltage drop...
In your case you might have build a receiver, getting signals from the outer world that interfere with your clr network, making things so unpredictable.
I am on holiday right now, so I might have forgotten other rules due to tyrolian white wine and knoedels.
In two weeks, back home, I could show you one or two PCB examples, if you like.
Marc
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