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SN75LVPE802: about PCIe Reference Schematic

Tsukui Yusuke
Guru 21930 points
Part Number: SN75LVPE802
Other Parts Discussed in Thread: DS125BR111, DS80PCI102

Hi,

Let me talk about the PCIe Reference Schematic of  SN75LVPE802.
The following circuit examples are described in the data sheet.

Please tell me the reason why the RX1P / N capacitor and RX2P / N have different capacities.
I would like to know why the value of the capacitor is different on host to device side.
I thought RX1P/N coupling capacitor should be 470nF.

Could you give me your advice.



Best Regards,
Yusuke/Japan Disty

  • 0
    TI__Mastermind 43795 points

    Hi Yusuke-san,

    I do not recommend the SN75LVPE802 for PCIe applications.  Instead I recommend the DS80PCI102 or the DS125BR111.

    For SATA applications I would remove the 330 Ohm resistors and use just a single set of AC coupling capacitors on each input or output pair.  Also, for SATA I recommend to use 10nF capacitors as outlined in the specification.

    Regards,

    Lee

  • 0 in reply to Lee Sledjeski
    Guru 21930 points

    Lee-san,

    Thank you for your response.
    "I do not recommend the SN75LVPE802 for PCIe applications."

    Could you tell me the reason why you do not recommend it?

    ”For SATA applications I would remove the 330 Ohm resistors and use just a single set of AC coupling capacitors on each input or output pair. 
    Also, for SATA I recommend to use 10nF capacitors as outlined in the specification.”

    So, that means the circuit diagram of the data sheet fig 45 is incorrect.
    Is this correct in my understanding?
    Customers want to know the recommended value of AC coupling cap and pull down resistors.



    Best Regards,
    Yusuke/Japan Disty

  • 0 in reply to Tsukui Yusuke
    TI__Mastermind 43795 points

    Hi Yusuke-san,

    The technique of using external 330 Ohm pull-down resistors to trigger a "receiver detect" in PCIe does not always work with components in use today.  If the this process for configuration fails the system will not work.

    Regards,

    Lee

  • 0 in reply to Lee Sledjeski
    Guru 21930 points

    Lee-san,

    Thank you for your kind support.
    Please excuse my lack of explanation.

    Customer's application is ”SATA Express”.
    It is written with the support it in the data sheet of  SN75LVPE802.



    On the basis of it,
    Could you please answer the question again.


    "Please tell me the reason why the RX1P / N capacitor and RX2P / N have different capacities.
    I would like to know why the value of the capacitor is different on host to device side.
    I thought RX1P/N coupling capacitor should be 470nF."

    Could you give me your advice.


    Best Regards,
    Yusuke/Japan Disty

  • 0 in reply to Tsukui Yusuke
    TI__Mastermind 43795 points

    Hi Yusuke-san,

    For a SATA-3 application make the following changes to the schematic drawing.

    1. Remove all the 330 Ohm resistors.

    2. Remove the 470nF capacitors.

    3. Change the value of all 220nF capacitors to 10nF.

    The values shown in the LVPE802 schematic were an attempt to make the device work for PCIe, they are not needed for SATA-3.

    Regards,

    Lee