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Original question:

DP83869HM: VDDIO backfeeding VDDA2P5 through IC

DP83869HM: When I connect DP83869HM and first provide all power then getting 3.1 V on 2.5V rail

Ajit Ladgaonkar
Intellectual 510 points
Part Number: DP83869HM
Other Parts Discussed in Thread: AM2634, , DP83869, DP83869EVM

Tool/software:

Hi Team,

I have used DP83869HM in circuit along with AM2634 controller. So, whenever I apply all supplies like 3.3V, 1.2V and 2.5V to board. I am getting 3.1V on 2.5V rail. This is not correct and its confusing me. I have referred schematic of DP83869HMEVM to make this circuit. I am feeding all the supply from external power supply.

I have tried by removing DP83869HM from board then after powering up. I am getting all the supplies correct. 

What could be the root cause? Please help me on this as this is driving project build. I am not able to get ethernet connection established properly.

Thanks!

  • 0
    TI__Mastermind 39795 points

    Hello,

    This is very odd. I do see that you are supplying 1.2V on-board which is higher than normal (1.1V). How many designs is this behavior showing up on?

    Could you please also provide schematic as well as scopeshot of supply rampup VDDIO, VDDA2P5, VDDA1P8, VDDA1P1 all in a single shot?

    Sincerely,

    Gerome

  • 0 in reply to Gerome Cacho
    Intellectual 510 points

    Hi Gerome,

    I have applied all the supply voltages from external power supply. I have not done any power sequencing for both AM2634 and DP83869HM. Do I need to add it? 

    I have referred schematic of TMDSCNCD263 for designinf our custom board only difference is that there is not power sequencing in our board.

    Is this issue rise because of no power sequencing?

    Thanks!

  • 0 in reply to Ajit Ladgaonkar
    TI__Mastermind 39795 points

    Hello,

    So each rail is supplied externally via bench supply (3.3V, 1.1 or 1.2V, and 2.5V)? Assuming that the PHY is in 2 supply mode (1.8V internally generated), there is a requirement where VDDIO can lag no longer than 50ms after 2.5V and 1.1V rails. 

    However, I am still surprised that the 2.5V is potentially presenting 3.1V. Is all boards showing this behavior? This may also indicate an unintentional short.

    Sincerely,

    Gerome

  • 0 in reply to Gerome Cacho
    Intellectual 510 points

    Hi,

    Yes, we have two boards and both showing same behaviour. But when I remove DP83869 IC then there is no jump in voltage. All voltages are fine.

    Thanks!

  • 0 in reply to Ajit Ladgaonkar
    TI__Mastermind 39795 points

    Hello,

    Is it possible to provide a current consumption of the 2.5V rail to the PHY? You may compare this to the TMDS board if you have this available. Else, you may also confirm this against DP83869EVM. 

    As this is unexpected behavior, I am thinking that this can be due to a short. You may see whether or not a there is continuity between rail and GND while DUT is in-place. It may also be helpful to ABA swap with a fresh DP83869 IC to ensure nothing was damaged in manufacturing of the board.

    Sincerely,

    Gerome