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USB and digital power supply

Discussion in 'Electronic Basics' started by Michael, Aug 25, 2005.

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  1. Michael

    Michael Guest

    I'm working on a project using a PCM2902 (TI) DAC chip.

    In the data sheet, on page 27 (typical circuit connection),
    it shows pin 27 as Vddi which connects to the USB D+
    line via a 1.5K resistor.

    I read where USB data is derived from a -200mv and a +200mv above a certain
    reference on the data lines, so I guess the voltage at Vddi is always there
    but varies with this (reference + 200mv) fluctuation?

    But isn't that a (very?) crude method of powering the digital part of the
    chip?

    Wouldn't it be preferable to have a seperate supply, even a seperate
    regulator, always on....on the Vddi pin?

    It may be a stupid question....but just trying to learn.

    Thanks.
     
  2. Ban

    Ban Guest

    Yes Mike,
    you got it the other way around. VDDi is an output of a voltage regulator.
    Look at page 10, the supply is via the VBUS terminal. The resistor on the
    data line pulls up the level to signal the controller, that a device has
    been connected.
     
  3. Michael

    Michael Guest

    Ok....I see, thanks.

    All this talk of seperate analog and digital supplies and yet internally the
    chip is fed from the same bus.

    ?

    I had started to read where seperate ground runs aren't nearly as important
    as seperate power planes.

    I guess you don't have that option with this chip.
     
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