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DP83869HM: VDDIO backfeeding VDDA2P5 through IC

Ariel Briner
Prodigy 10 points
Part Number: DP83869HM
Other Parts Discussed in Thread: DP83869

Hi,

I have a design using 4 DP83869HM's in 2 supply configuration with VDDIO being 3.3V.

Upon power up all 3 rails (1.1V, 2.5V and 3.3V) come up simultaneously.

However, once the 3.3V rail exceeds ~2.7V it starts backfeeding the 2.5V rail through the DP83869HM's (see attached scope shot) eventually pushing the 2.5V rail up to 3.1V. I've verified this is the path the power is taking by disconnecting one DP83869HM from the 2.5V rail and probing directly at its VDDA2P5 pins (pin 3 and pin 9). When doing this I see 3.1V on the pin.

Further, using my multimeter's diode test mode between the DP83869HM's VDDIO and VDDA2P5 pins I get a result showing a 0.2V diode drop, consistent with the above results where the 2.5V rail is pushed up to 3.3V - 0.2V = 3.1V

I'm terribly confused why this is happening as it seems to be deeply wrong behaviour. Obviously it is also preventing my design from working correctly.

I would appreciate help root causing and resolving this issue.

  • 0
    TI__Expert 5425 points

    Hi Ariel,

    1. Would you be able to provide a schematic of your system? If it cannot be displayed on a public forum on E2E, you can email it to me instead.
    2. Can you check if there are any soldering issues on the board?

    Regards,

    Adrian Kam

  • 0 in reply to Adrian Kam
    Prodigy 10 points

    I've emailed you the schematic sheet containing the DP83869HM.

    Soldering appears perfect. Please see attached images as an example.

  • 0 in reply to Ariel Briner
    TI__Expert 5425 points

    Hi Ariel,

    After going through the schematic, I did not see any issues with the power supply portions. This seems to be a board level issue, especially since you measure 3.1V after disconnecting a PHY from the 2.5V rail. I would suggest checking your board and layout.

    Regards,

    Adrian Kam

  • 0 in reply to Adrian Kam
    Prodigy 10 points

    To clarify: I measure 3.1V on the DP83869HM‘s  VDDA2P5 pins even when those pins are completely disconnected from anything else and each other. 


    By doing this I’ve eliminated any way the rest of the board could be affecting the DP83869HM‘s VDDA2P5 pins, so the issue is definitely coming from within the chip. 
    I’ve also taken another PCBA that’s never been powered up and measured the same 0.2V diode forward voltage between 3.3V and 2.5V rails. 


    My next step will be to desolder one of the DP83869HM‘s (sadly I don’t have any extra that weren’t assembled on boards) so I can probe directly at the pins of the chip to see this same 0.2V diode forward voltage. 

    Can you recommend any other tests I could perform to identify why the chips are acting like this? Presently they’re unusable and are threatening to ruin our whole build. 

  • 0 in reply to Ariel Briner
    TI__Expert 5425 points

    Hi Ariel,

    We have done extensive testing on our DP83869 devices and EVMs and have never encountered this issue. Even though the VDDA2P5 pin is disconnected from the rail, there could be some route on the board/setup that you are currently not aware of that connects the VDDA2P5 pin to the VDDA3P3 supply.

    Your next step of probing directly at the pin to try and see the 0.2V forward voltage is a good one. I would also suggest double checking the board/setup to make sure that such a route I mentioned above does not exist.

    Regards,

    Adrian Kam