Current return question

[alexg]

Hi All, quick simple question to those who knows with possible follow up, here is a little crude(abstract) diagram below but hopefully it should work to explain the question:

For example I have 2(or more)  layers PCB with a hole in the middle (rectangular cutout in the middle), one of the layers in the PCB is designated for GND, just once ground plane for everything and all GND pins from all the devices in the system are going to that plane through vias.

This is very fundamental circuit but I am trying to understand the flow of the current in this system so that later I can better design mixed system with analog and digital grounds separate etc...

Anyways, in the attached picture the way this works is that I have a power source (i.e. the battery) at the top/right corner of the board, then I have a switching regulator at the low/right corner that power from the battery (using path A as it seems to be the shortest way to power source) and converting it to desires value and then sends it to the microprocessor in the top/left corner (path B).

My question is: will the return current for the microprocessor be along path "B" or along Path "C"? I have a mental clog about this simple thing and no one to ask around, since microprocessor takes power from voltage regulator's output capacitors then maybe it will probably be the return path as well, or since I dump ground pins directly to the ground plane maybe then return path will be directly into the battery since it seems to the the shortest way between microprocessor and battery?

Can someone help please?

[Wimberleytech]

Well, my first inclination is to keep current loops tight.  Therefore, avoid C as a return path. 

[basinstreetdesign]

The short answer: the return current from the uP will mostly take path B to its source in the regulator.   

The longer answer: All current is generated by an induced difference in potential between two points.  In the case of the battery those two points are the ends of the battery.  In the case of the regulator it is its two output terminals.  When a current flows to a load it must flow back to the source or else Kirchoff's current law would be violated.  This forms a loop of current in the wiring/copper plane.  By virtue of its shape, this loop has an inherent inductance.  It also has resistance as has any other path that may be present and that the current to and from the load may take.  If the current is flowing through a copper sheet where there may be a large number of possible paths then the return current will choose a path that will minimize the inductance and resistance involved.  This is especially true if the load current involves high(er) frequencies, such as with digital logic.  So the return current will mostly be found as close to the path of the source current as possible in order to reduce the loop area and therefore its inductance.  If the source path is a wire directly above a ground plane then the return current will be found in the ground plane immediately under the wire.  Return current at (close to) 0 Hz will return through a combination of any and all paths through wires/copper plane with proportions determined by the parallel resistances presented by those various available paths.

Sorry if that answer is too long but hope that helps.   
Tim

[alexg]

The short answer: the return current from the uP will mostly take path B to its source in the regulator.   

The longer answer: All current is generated by an induced difference in potential between two points.  In the case of the battery those two points are the ends of the battery.  In the case of the regulator it is its two output terminals.  When a current flows to a load it must flow back to the source or else Kirchoff's current law would be violated.  This forms a loop of current in the wiring/copper plane.  By virtue of its shape, this loop has an inherent inductance.  It also has resistance as has any other path that may be present and that the current to and from the load may take.  If the current is flowing through a copper sheet where there may be a large number of possible paths then the return current will choose a path that will minimize the inductance and resistance involved.  This is especially true if the load current involves high(er) frequencies, such as with digital logic.  So the return current will mostly be found as close to the path of the source current as possible in order to reduce the loop area and therefore its inductance.  If the source path is a wire directly above a ground plane then the return current will be found in the ground plane immediately under the wire.  Return current at (close to) 0 Hz will return through a combination of any and all paths through wires/copper plane with proportions determined by the parallel resistances presented by those various available paths.

Sorry if that answer is too long but hope that helps.   
Tim

Thank you, just want I needed.