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DP83867IR: resistor for RBIAS

Guru 15510 points
Part Number: DP83867IR
Other Parts Discussed in Thread: TIDA-00204

Hi,

I have a few questions about DP83867IR.

In DP83867IR datasheet page.12 Pin Function table,
the description of RBIAS pin is written as follow:
*************************************************
Bias Resistor Connection. A 11 kΩ +/-1% resistor
should be connected from RBIAS to GND.
*************************************************

So, my customer are connecting 11kohm +/-1% resistor.
But the board are having linkup issue.

On DP83867EVM and TI Design(TIDA-00204) schematic,
10kohm resistor are connected from RBIAS to GND.
So, my customer changed the resistor from 11kohm to 10kohm at their board,
then the linkup issue have been solved.

Our question is which resistor are correct for RBIAS? 11kohm or 10kohm?

best regards,
g.f.

  • Hi g.f.,

    The datasheet was updated at a later date to change RBIAS to 11K, so that is the value that should be used. Is customer using 64pin QFP device? Can you share the schematics, crystal part number, and magnetics part number?

    -Regards,
    Aniruddha
  • Hi Aniruddha,

    Thank you for the reply.
    I understood that 11kohm should be used.

    They are using DP83867IRPAP(64pin QFP).
    It's difficult to share schematics in this open community, but I could share following information.

    I don't know part number of crystal/OSC.
    As far as I know, they are using external oscilator(25MHz +/- 15ppm, 1.8V LVCMOS).
    They are connecting this oscilator to DP83867IR XI pin via external buffer.

    They are using following magnetic:
    Manufacture: "TE connectivity"
    Part#: 1-2301994-0

    This magnetic's each center tap are not independent and
    connected to each other internally.
    They are connecting center tap to GND via 0.1uF and also connecting to 1.8V power supply.
    (DP83867IR VDDIO is 1.8V)

    I understood that this type of magnetic is not recommended for DP83867,
    so I told to my customer to use magnetic which each center tap is independent
    and which can decoupled to GND independently.
    But they have no time to rework their board, so they are still using this magnetic.

    I also told to my customer that connecting center tap to 1.8V is not required
    because DP83867 implements a voltage mode line driver but maybe they are still connecting to 1.8V.

    best regards,
    g.f.
  • Hi g.f.,

    Your observations are correct.

    The center taps must be separated for proper operation.
    Also, they should not connected center tap to 1.8V supply. This is a voltage mode line driver design.
    There should not be a supply on the center tap.
  • Hi Ross,

    Thank you for the reply.
    I understood.

    best regards,
    g.f.