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Original question:

SN75LVPE802: about PCIe Reference Schematic

SN75LVPE802: about PCIe Reference Schematic

daniel
Genius 5095 points
Part Number: SN75LVPE802
Other Parts Discussed in Thread: DS125BR111, DS80PCI102

Hi Team,

PCIE Gen3 application. Why the RX1P / N capacitor and RX2P / N have different capacities?

220uF or 470uF is right?

Thanks

  • 0
    TI__Mastermind 43795 points
    Daniel,
    The higher value was originally specified to help compensate for the overall reduction in AC coupling capacitance caused by the series combination of 220nF and 470nF. As stated in the previous link, the LVPE802 is not recommended for new PCIe designs. Recommended devices are the DS80PCI102 and DS125BR111.

    Regards,
    Lee
  • 0 in reply to Lee Sledjeski
    Genius 5095 points
    Dear Lee,

    Thanks for your help.
    Sorry, I don't quite understand the meaning of"compensate for the overall reduction in AC coupling capacitance caused by the series combination of 220nF and 470nF."
    In my opinion, these coupling capacitors should be symmetrical. Could you explain how to calculate to get this 470uF?
    So the 470uF needs on 802's device side RX pin, right?

    Also, Could you confirm why LVPE802 is not recommended for new PCIe designs?

    Thanks
  • 0 in reply to daniel
    TI__Mastermind 43795 points

    Hi Daniel,

    Putting capacitors in series will reduce the total capacitance.  So using a higher value will have less impact on the total capacitance.

    The LVPE802 does not  include a receiver detect mechanism.  Without this feature it is possible that the PCIe link will not properly recognize the LVPE802 and fail to complete configuration. 

    Regards,

    Lee