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Analog/PLL supplies - linear regulator required?

David Kronstein
Intellectual 370 points

On the Mistral EVM, I notice a many of the DM8148 1.8V analog supplies run off linear regulators. The datasheet mentions having filters on each analog supply, but doesn't say if a linear regulator is required. Is it possible to avoid adding extra regulators and run the analog supplies from the main 1.8V switching regulator with beads to filter out the noise?

 

Regards,

David

  • 0
    TI__Mastermind 25055 points

    David,

    Here are a few points on this topic:

    • The datasheet does contain some power supply filter information, and the recommendations in the Power-Supply Decoupling section of the datasheet should also be followed.
    • The "core" rails (CVDD, CVDD_ARM, CVDD_DSP, CVDD_HDVICP) are separate to allow for optimal power consumption. If you are not planning on using AVS or are not concerned about power consumption then you can connect the four "core" rails to the same regulator. Of course this means that all of the rails will have to run at the highest voltage required to achieve the performance needed.
    • Many of the I/O supplies are separated to reduce noise from other peripherals. If you are confident that you can isolate noise in the design then you can also group I/O supplies. We recommend that PLL supplies be driven by their own LDO if possible or at the very least have dedicated filters. It is a good practice to isolate the high speed peripheral supplies (HDMI, PCIe, SATA, VIDEO, USB) but there is some leeway if you aren't using these peripherals or if you are confident in your power supply to power these from a single LDO.

    Regards,
    Marc

  • 0 in reply to MarcPyne
    Genius 3476 points

    Marc

     

    >>We recommend that PLL supplies be driven by their own LDO if possible or at the very least have dedicated filters.

     

    Can you give us specs for the ferrite bead, assuming one per supply and a 100nF filter cap.?.  i.e impedance = ??ohms @ 100MHz

     

    thanks

    Steve

  • 0 in reply to Stephen Turner
    Genius 3476 points

    I'm going to use the following ferrite:

     

    http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=490-5203-2-ND

    in conjunction with a 100nF cap

     

    -steve

  • 0 in reply to Stephen Turner
    TI__Mastermind 25055 points

    Steve,

    I'll let you know if I can find any other recommendations on the ferrite, but that should be good.

    Regards,
    Marc

  • 0 in reply to MarcPyne
    Intellectual 370 points

    I was planning on using a 600R bead plus a 10u+0.1u cap. Would this be overkill?

     

    David

  • 0 in reply to David Kronstein
    Genius 3476 points

    David

    I would say that if you have space on the PCB, then go for it.  What is the DC resistance of the ferrite?.  I would not think you'd want to drop more than 10-20mV from the 1.8V supply.

    My problem is that since TI has not specified the current draw of the 1.8V analog and PLL supplies, its hard to guess what kind of current handling capability and DC resistance the ferrite should have.  I'm assuming none of the supplies would draw more than 100mA.

    -steve