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Original question:

DP83867IS: Misleading Power Sequence Diagram in the Datasheet & Dismatch with older versions

DP83867IS: Power Sequence Question for Three-Supply

Darren (FAE)
Expert 5483 points
Part Number: DP83867IS

I have a question regarding the thee-supply power sequencing. Please let me know if I am understanding the DS correctly.

According to page 110 of the DP83867IS Datasheet...

  1. When using the three-supply setup...
  1. You need to ramp 2.5V and 1.8V supplies at the same time, with 1.8V settling within 0~25ms of 2.5V settling. - YES?
  2. Or, you ramp 1.8V after 2.5V, as long as 1.8V still settles within 25ms of 2.5V ramp start. - YES?
  3. If the 2.5V is only powering the DP83867IS, then you can ramp 1.8V any time before ramping 2.5V...why?

Is it because if 1.8V ramps before 2.5V the 2.5V line will see the 1.8V, but if 2.5 only supplies the DP83867IS, it shouldn't be an issue?

What about the power-down sequence? Usually 1.8V is ramped down before 2.5V, but is that still true if 2.5V powers just the DP83867IS?

Thank you for your support.

  • 0
    TI__Genius 16030 points

    Hi Darren,

    Copy pasting your questions and replying in-line :

    1. You need to ramp 2.5V and 1.8V supplies at the same time, with 1.8V settling within 0~25ms of 2.5V settling. - YES?...True
    2. Or, you ramp 1.8V after 2.5V, as long as 1.8V still settles within 25ms of 2.5V ramp start. - YES?...True
    3.  If the 2.5V is only powering the DP83867IS, then you can ramp 1.8V any time before ramping 2.5V...why?

    Reason of point 3 : You are correct in your understanding. There is a on-chip diodes between 1.8V and 2.5V, so if we ramp up 1.8V when 2.5V is not powered up : 2.5V can also get charged along with 1.8V (assuming 2.5V rail is floating in the system when 2.5V is not powered up). Thus to avoid any system level impact of 2.5V ramping up when it is not supposed we have put this caution.

    Power down is also a caution as will like to avoid turning on the diode with anode at 1.8V and cathode at 2.5V (to avoid any damage to this diode because of possibly large current when 1.8V node is higher than 2.5V node). When ramping down it may happen that there is a high resistance path which is the actual cause of ramping down the supply of 2.5V and hence this current path is limited. But as it is dependent on system, as a caution it is better to maintain the voltage difference as suggested in datasheet.

    --

    Regards,

    Vikram