EEVblog Electronics Community Forum
Electronics => Beginners => Topic started by: OnCor on January 18, 2023, 01:28:44 pm
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Hi all. I've designed a 7 port USB hub on a 2 layer PCB (1oz) with signal/5V traces on the top and a ground plane on the bottom. The trace width to the power pins on the USB ports is 2mm wide which should carry around 4 amps according to online calculators I've used which would definitely cover 500 mA to all the USB devices. There are a couple of USB controller ICs that will draw power from those traces as well as a 5V fan. Everything seems to work well on my prototypes, but I do wonder if there is a better approach to providing power to components on the board.
I have to route part of the power traces as well as traces to a couple of crystal oscillators on the bottom of the board which breaks up the ground plane slightly and might not be ideal for the USB data lines. A couple of other options from the power perspective would be a) a power plane on the top of the board which would surround the high speed signal traces as well as those going to other components or b) a combo power/ground plane where the outer edge of the board would have a section for power to cover the USB ports as well as the fan and a ground plane through the remainder of the board. The first option of the power plane would prevent me from having to route power thru the ground plane and would seemingly provide all the amperage components need. However, might that cause any interference on the data lines?
The second option would also provide more current than my 2mm traces, but would surround data lines with ground plane. I would still have to route some power traces thru the ground plane on the bottom though. I believe I would also need to add vias between the top and bottom ground planes from what I've seen on other boards.
What might be the best option of the three?
I've attached some pics from KiCad to show what I'm working with and those three possibilities.
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It looks to me like you could route things a slightly differently way and get a better layout; for example, could you move Y2 to the bottom right of U2, route the traces going to on the bottom of the chip, and route VCC to that chip from the top? If you route VCC using a star topology so that there is an individual trace going to each IC, that trace doesn't need to be very thick, since it only carries power to the chip. That would eliminate the need for that bit power trace that cuts across the ground plane.
Sorry if this is not super clear—hard to describe in words—but I really do think that you could simplify the routing. I hope it helps a little!
—CC
Edit: Attached a 4-port USB 2 hub I built for myself; not the same problem, of course, but maybe it helps with some routing ideas. Cheers!
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In case it helps
This application note provides information on designing a printed circuit board (PCB) for the SMSC
USB251x/xB Family of USB 2.0 Hub Controllers including industrial grade products in the family. The
PCB only requires two layers of copper.
https://ww1.microchip.com/downloads/en/Appnotes/en562810.pdf (https://ww1.microchip.com/downloads/en/Appnotes/en562810.pdf)
This application note provides information on general printed circuit board layout considerations for Microchip’s
USB57x4, USB553x, USB25xx, and USB46xx families of Hub Controller devices. This information is broadly applicable
to any of Microchip’s USB 2.0 and USB 3.1 Gen 1 device implementations.
(uses 4 layer pcb)
http://ww1.microchip.com/downloads/en/AppNotes/00001876.pdf (http://ww1.microchip.com/downloads/en/AppNotes/00001876.pdf)
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Thanks to both of you for your input. I may be able to readjust some components on the board to avoid running power lines through the ground plane. However, I read through the first PDF provided by mariush and noticed this section:
PCB Layout Guide for USB Hubs
SMSC AN 15.17 3 Revision 1.0 (10-13-11)
APPLICATION NOTE
Figure 2 Top Layer Copper: USB Traces and Port Power
Controlled Impedance for USB Traces
The USB 2.0 specification requires that USB DP/DM traces maintain nominally 90 Ω differential
impedance. In this design, the USB DP/DM traces are 27 mils wide with 5 mils spacing. A continuous
ground plane is required directly beneath the DP/DM traces and extending at least five times the
spacing width (5 x 5 mils = 25 mils) to either side of DP and DM.
On a 2 layer board does this mean that the entire bottom layer should be continuous ground plane and there should also be ground plane adjacent to the signal lines on the top layer? Or does this simply mean that the ground plane on the bottom of the PCB should be continuous underneath and to the sides of the signal traces, but otherwise can be broken elsewhere? If it is ok to route lines through the ground plane as long as it isn't around the USB signal lines then perhaps I have a little more flexibility with my routing.
I noticed that this document also indicates the shields of each USB connector should not be connected to ground, but instead should be linked together with a trace. I did a little research on this and actually found several references like the one below that indicate the shield of each USB connector should actually be tied to a 330 ohm resistor as well as a 0.1uf capacitor to help prevent ESD as well as EMI interference. Can anyone confirm if this is the best practice?
https://arduino.stackexchange.com/questions/37508/where-to-connect-shield-in-usb-power-supply-for-arduino (https://arduino.stackexchange.com/questions/37508/where-to-connect-shield-in-usb-power-supply-for-arduino)