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DP83867IS: DP83867IS

Milos Stankovic
Prodigy 70 points
Part Number: DP83867IS

Tool/software:

Hello, on the page 112 of the DP83867IS datasheet is described power sequencing when three power supplies are used. 

1. 1V8 should be stable not later than 25 ms from the 2V5 ramp up.
2. When powering down 1V8 must be powered down before 2V5 is powered down. 
First demand is fairly easy to implement in my design. Second is a bit tricky. My question is: can both 1V8 and 2V5 be brought down simultaneously?
I plan to use the same EN signal for both power supplies and to use a soft start time for 1V8 greater than for 2V5. This is how I will satisfy demand 1.
I don't know how to make 1V8 be brought down before 2V5. 
If you have some app note, please share.

Regards,
Milos

  • 0
    TI__Mastermind 39795 points

    Hi Milos,

    1.8V and 2.5V can be brought down simultaneously. The reason for this guidance is that there is a backpowering behavior observed from VDD1P8 to VDD2P5 when in three supply mode. If the 2.5V is brought down first, there will be some residual pedestal voltage present on the rail which doesn't affect the PHY, but if the 2.5V is shared elsewhere, those devices may not appreciate much. 

    So either you can:

    a) bring both supplies down, thereby eliminating potential for backpowering

    b) ensure that 2.5V supply is only PHY-facing such that any backpowering that does take place is okay inherently

    Sincerely,

    Gerome

  • 0 in reply to Gerome Cacho
    Prodigy 70 points

    Hi Gerome,

    I think that I made the mistake. My case is two supplies, not three. Here is what I have:
    1. VDDA1P8 is left open
    2. VDDIO=1V8
    3. VDDA2P5=2V5
    4. VDD1P0=1V0

    I do not have a sequencing issue, since there is no requirement for the sequence of the supplies when operating in two-supply mode (datasheet, page 113).

    Am I right?

    Best regards,
    Miloš