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TUSB8040A

Calum Mackinnon
Genius 4155 points
Other Parts Discussed in Thread: TUSB8040A

I have the datasheet and the application guide for the TUSB8040A. Normally on USB lines, the host has 15K pulldowns on both the DP and DM lines. I don’t see this on the reference design, and I don’t see a mention of it on the datasheet. They may be internal, but it doesn’t explicitly say that.

  • 0
    TI__Guru 60870 points

    Hi,

    Yes, the 15K pulldowns are internal to the hub.

    Regards,

    J Nirchi

  • 0 in reply to JMMN
    Genius 4155 points

    Hey - That's good to know - is that documented anywhere?

    Cheers

    Calum

  • 0 in reply to Calum Mackinnon
    TI__Guru 60870 points

    Hi Calum,

    Starting a few years back it became standard in the industry for new hub silicon to have internal pull-downs, so we didn't call it out explicitly in the documentation.  In the design guide and EVM guide, you can see reference schematics where external pull downs were not required.

    Regards,

    J Nirchi

  • 0 in reply to JMMN
    Genius 4155 points

    One more question.

    Are there are internal source termination resistors I don’t see those on the reference schematic either.

    I looked at section 2.2 of the design guide talks about the impedance of traces - http://www.ti.com/lit/an/slla329/slla329.pdf 

    It says to layout the traces at 90 ohms, but what is the output impedance of the driver?  I guess it makes sense that each output would be 45ohms for a total source impedance of 90 ohms differential, to allow for matching the line without extra resistors, but it doesn’t say that. 

    The reason I ask is because I am going to run one of my USB signal over a cable with an impedance of approximately 180 ohms (differential, about 90 single ended) , so my source impedance (source impedance of the chip + external series termination) needs to be 90ohms.

     Cheers

    Calum

  • 0 in reply to Calum Mackinnon
    TI__Genius 14317 points

    Calum,

    Based on the evidence (EVM schematic and Data Sheet), it seems that the source termination (45 ohms series on each line) is built in to the device to match the 90 ohm differential Zo of the traces. This means that you would need additional 45 ohm series resistors on each line to match your 180 ohm diff. Zo. However this would be in violation of the USB specs.

     JMMN will have to verify this.

  • 0 in reply to Jeffery Pfarr
    TI__Guru 60870 points

    The differential impedance (90 ohm) and common mode impedance (~30 ohm) are determined by the USB specification which is available here:  www.usb.org.   We typically don't document industry standard requirements in the datasheets.

    Using a non-specfication cable is not ideal, but we've seen lots of applications that run successfully over non-standard cables.  I would not try to add any series resistors to  match the cable.

    J Nirchi