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TUSB8020B: Proper Trace Width and Spacing for Differential Pairs

Jason Singh
Prodigy 20 points
Part Number: TUSB8020B

Hi, I'm working off the Reference Guide listed on the product page for the TUSB8020B 2-Port USB 3.0 Hub. Looking at the guide, it specifies 4.4 mils for trace widths and 5 mils for spacing of the differential pairs. However, when looking at the image of the routing for the top layer, it seems that the actual traces used (for the data lines) are far smaller than 4.4 mils. I'd appreciate some clarification on what exactly is being referred to as being 4.4 mils wide.

Also, the manufacturing limits I'm bound by have a minimum trace width/spacing of 5 mils each. Could somebody point me to a reliable resource for determining the proper width/spacing to achieve the required 90 Ohm Impedance listed in the datasheet? I'm quite new to designing multilayer PCBs and to differential signals, so as much design advice as possible would be greatly appreciated.

  • 0
    TI__Guru 60870 points

    Hi Jason,

    The 4.4 mil trace width may be correct for that particular application, but it isn't a universally applicable value.  The trace width and spacing needed to obtain 90 ohm differential impedance will vary depending on the board stack-up and PCB material among other factors.  There is usually a way to calculate the correct trace width and spacing in the layout tool itself. 

    The layout photos shown in the documentation show the connections between the devices, not the exact trace widths. 

    Regards,

    JMMN

  • 0 in reply to JMMN
    Prodigy 20 points
    JMMN,

    Thanks for the reply, that actual makes a lot of sense.

    I've also been researching Multilayer fabrication techniques for the past few hours and found some tools to help me out. For future reference, multi-circuit-board eu provides a great guide for building standard stackups and the Saturn PCB Toolkit really helps to calculate appropriate values for the target impedance for many common materials. I've linked them below.

    www.multi-circuit-boards.eu/.../series.html
    saturnpcb.com/pcb_toolkit.htm

    Thanks,
    Jason Singh
  • 0 in reply to Jason Singh
    TI__Guru 60870 points
    Thanks! Those are very useful.