Ground connections in multilayer PCB layout

Thread Starter

josmA

Joined Jun 12, 2019
35
Hello,

I am trying to design a "complex" board layout and the doubts come out. I would appreciate it if you could help with them.

This is a rough blocks diagram of the PCB:
1.jpg
Stackup used: 6 Layers
Layer 1: Signal + Power (All the boost and buck converters components are located on the top layer)
Layer 2: GND ( Power Ground?)
Layer 3: Signal + Power
Layer 4: GND + Signal
Layer 5: GND ( "Normal" Ground?)
Layer 6: Signal + Power

1) My main concern is how or where to route the grounds. I have many power ground pins from boost and buck converters distributed around the PCB, as well as the common ground for all the ICs. After watching videos, it seems that splitting the ground plane (layer 2) is not a good idea, but if I use vias to connect GND signals from layer 1 to the ground plane of layer 2, I guess it will be a mess. My thought is to connect all the power ground to layer 2, and the rest of the grounds to layer 5. Is this a good way to do the layout? I am very confused. The only similar reference I could find was from ROHM:

5.jpg

It doesn´t connect the noisy ground of a buck converter input capacitor to the common ground, but connect the ground of the output capacitor to PGND and common GND, and there isn´t PGND plane below the ground of the IC controller (AGND). In this case, is it recommending to split the Power plane of layer 2?

I am sorry for the speech, to sum up, what is the right approach for GND layout?

2) I am using up to 10A in the boost converter outputs, and I am trying to avoid noise at the buck converters input (they supply voltage to sensitive sensors) which are connected to the same battery connector. Please check the pictures below, what is the right way to connect the voltage power from the battery connector to the boost converters?

2.jpg
3.jpg
Or any other recommendation?

3) The ground pins of the output capacitors of each boost converters are close to the other boost converter because of the limited size. I saw in other layouts manufacturers use a line or a bunch of vias to split sections like ground from Vin and Vout of boost converters. Is it a good idea to use vias between the boost converters? Please see the diagram below:

4.jpg

I have done many simple PCBs but I am a beginner with noise, ground, etc. and this is an important task, so any help would be much appreciated.

Thank you very much in advance
 

andrewmm

Joined Feb 25, 2011
1,757
lots of things to look at in these designs.

Generally the more grounds you have, the more likely you are to get noise problems!
this is counter intuitive, but let me explain.
A single ground, most likely , things will work,
if you have multiple grounds , you are controlling the current flow, and are more likely to make it worse.

Have as much ground on the board as possible
have all grounds joined together in as many places as possible,

Look at the local layout of the power circuits,
look where the current is flowing, and keep these routes as short as possible,
 

Lo_volt

Joined Apr 3, 2014
219
Keep your traces short and wide. For this application and since all of the motors are supplied via the battery, it makes sense to use a single power plane and a single ground plane.
Review your circuit and identify any nets that are going to carry large currents. Before you start layout, enter those into your design rules to include traces wide enough and vias large enough to carry the current.
As long as you use at least the power and ground planes, you have a good start. When you place the parts for each switching converter try to group each converter's parts together tightly and arrange them so as to keep traces short. You probably shouldn't bother trying to put signals on a power or ground plane unless absolutely necessary. It's too easy to cause a problematic split in the plane.
 
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