5
\$\begingroup\$

I've a 4 layer PCB in which I've to layout a power switching supply mixed with digital signal circuit.

My stack up is:

  • Top Layer: component and routing (power net and signals)

  • Internal plane 1: GND

  • Internal plane 2: +3V3

  • Bottom Layer: routing (power net and signals)

Now the question is: what's the better way to layout the GND internal plane?

I've thought of three possibilities:

  1. use all the PCB area with GND plane

  2. clear out both planes on the PCB area in which is present the power switching circuit and use the only what remains with GND

  3. split GND planes and make a dedicated analog GND internal plane only for power switching circuit

I was thinking to proceed with solution 1), but I'm wondering if the power switching circuit may inject noise to the GND plane by coupling.

Is solution 1 OK? How should I modify it?

\$\endgroup\$
  • \$\begingroup\$ I would suggest you to split internal GND plane, and dedicated GNDA for power switching, connect to the rest of the GND plane, just in one place (at the power input). That will just improve your first solution, you will still have solid ground plane, but will direct your currents on the one connected place. \$\endgroup\$ – Haris778 Mar 1 '16 at 17:26
  • \$\begingroup\$ Going with 1) is fine. From your description, it will work fine. It is actually not easy to improve on one solid ground, and many ways to make it much worse. \$\endgroup\$ – rioraxe Mar 2 '16 at 0:01
  • \$\begingroup\$ My power switching supply has a dedicated GND. So GND and GNDA aren't physically connected but separated by a transformer. This is why, at first thought, i decided not to make splitted GND planes. Moreover, the application note of the switching IC suggest to route the GND with a particular pattern, so i can't make a plane and simply connect all the gnd with vias. I was thinking to route GNDA on bottom layer. Maybe i answer to my question by myself :) \$\endgroup\$ – Gianluca Suzzi Mar 2 '16 at 9:12
1
\$\begingroup\$

As suggested, you want to separate your ground plane into regions to reduce noise coupling between the different functions of the circuit, in your case: switched supply and signals. The regions should be connected with each other in a single point only (see image below).

Google Image for "pcb ground plane switch" gave the following result:

enter image description here

(Note that the borders between the ground plane regions apply for ALL the layers. If signal traces run over multiple ground planes you might get large current loops.)

To sum up, you should go for a combination of 1) and 3).

| improve this answer | |
\$\endgroup\$
  • \$\begingroup\$ My power switching supply has a dedicated GND. So GND and GNDA aren't physically connected but separated by a transformer. This is why, at first thought, i decided not to make splitted GND planes. Moreover, the application note of the switching IC suggest to route the GND with a particular pattern, so i can't make a plane and simply connect all the gnd with vias. I was thinking to route GNDA on bottom layer. \$\endgroup\$ – Gianluca Suzzi Mar 2 '16 at 9:14
  • \$\begingroup\$ What switching supply IC are you using? Modelnumber? In case you have to keep separate grounds, you should just skip the connection between the ground plane regions. \$\endgroup\$ – Niklas Mar 2 '16 at 9:33
  • 1
    \$\begingroup\$ I "must" use the Fairchild FSEZ1317W. I'm referring to the AN-8033 My circuit is exactly as described at pag.12 and as you see they suggest to make a specific routing for GND. \$\endgroup\$ – Gianluca Suzzi Mar 2 '16 at 14:36

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.