I’m working on designing the power plane in Layer 5 of a multi-layer PCB. According to the datasheet, VDD_DDR and VDDCORE should be solid power planes. I’m currently planning to route multiple power rails in this single layer and wondering:
I have a few questions:
- When multiple power rails are placed on the same layer, should priority be based on the amount of current each rail carries? For example, should VDDCORE, VDD_DDR, 5V, or VIN be given more area depending on their current load?
- 5V_VIN is the input voltage from USB. Since it carries significant current, should it also have a solid plane region?
- What are the key design considerations when splitting a single power plane across different power rails?
Any guidance or insights would be really helpful. Thank
Rayan, you’re posting so often, can you please try to select the relevant category?
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You’re asking all the right questions, this is exactly the kind of thing that can trip up a layout if it’s not thought through early.
When you’re placing multiple power rails on the same layer, current definitely plays a big role in how much area each rail should get. VDDCORE and 5V_VIN in your case are carrying the most current by far, so yes they should get priority in terms of copper. Wider pours or even isolated regions will help with both voltage drop and heat dissipation. VDD_DDR is lower current, so it doesn’t need as much area, and the 3.3V rails look relatively light too.
Since 5V_VIN is your USB input and pushing almost 3 amps, I’d strongly consider giving it a solid region. If you can’t dedicate an entire plane to it, then at least make the pour as wide and unbroken as possible. Neck-downs and long skinny traces will work against you here, especially with that kind of current.
When it comes to splitting a single plane across different rails, one of the biggest things to watch for is isolation. You want to make sure the different voltage regions are clearly separated so there’s no risk of accidental shorting, especially during assembly or rework. And while you’re at it, try to think through the return paths, splitting planes can sometimes create odd current loops if you’re not careful about where the signals are referenced.
It’s also worth thinking about where your decoupling caps will land. You’ll want plenty of vias tying each power region to the corresponding caps to keep inductance down.
This is definitely one of those “measure twice, cut once” situations, getting the power plane strategy right up front will save you a ton of trouble down the line.
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Thanks for letting me know. I’ll choose the right category from now on
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