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Common Anode v.s. Common Cathode

Discussion in 'Electronic Design' started by EEng, Nov 7, 2003.

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  1. EEng

    EEng Guest

    It has always been my contention and design technique to sink current
    where possible, rather than source it, thus I prefer common anode
    wherever possible for displays and other driven devices and I write
    all firmware routines to provide lows for execution rather than
    highs.. The reasons are obvious on most data sheets that most devices
    can sink more than they can source.

    Recently while designing a 12 character 16seg LED display
    (hostile/rugged environment) I was surprised to find that the majority
    of display drivers by almost 10 to 1 prefer common cathode so that the
    driver is sourcing. Obviously this doesn't fit with my experience.
    For the display and driver being used, an All Lamps On condition would
    require in excess of 2A. I doubt most drivers can source that.

    I spoke with the company and they informed me that they too had
    problems with this, and so now all their display drivers have built in
    10% duty cycle multiplexing, also built in. The problem with this is
    it requires a charge pump to maintain the command word and I can't
    have a charge pump anywhere inside the product (highly sensitive RF
    device). The company did manage to find one of their static devices
    that fulfill all my needs except for retention of the command word,
    but that's fine as I was planning on using a Look Up Table and EEPROM
    to store critical command data. This static device also has the
    ability to store display command/data words as long as power is
    applied and that works too, since it will be stored in EEPROM as well.

    Seemingly the best of both worlds, but I'd like a consensus opinion
    regarding preferences, not needs for Common Cathode v.s. Common Anode.
    My personal choice as stated is common anode. Other opinions and
    explanations are greatly appreciated.

  2. With a multiplexed display you have to sink *and* source ALL the
    current. So it boils down to the current level, the number of each
    driver involved (if any, as some can perhaps be directly driven from a
    microcontroller or PLD), and the cost of each.

    All told, ,my preference is also for CA.

    Best regards,
    Spehro Pefhany
  3. EEng

    EEng Guest

    Interestingly enough, we have in fact gone to a multiplexing driver
    for our 12 character 16seg display. Check out the Maxim 6954 and
    6955. For our purposes, we can't use anything with a charge pump so
    these work fine. We'll have to cascade two of them, but that's fine,
    and yes, they're being driven directly from a PIC using SPI protocols.
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