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HCT/TTL output current and supply current

Discussion in 'Electronic Basics' started by Grant Stockly, Oct 25, 2007.

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  1. I am trying to compare some standard series and LS parts to HCT parts.

    I don't understand how the maximum ratings are calculated.

    For example, the Phillips data sheet for the 74HCT125 lists +/- Io at
    35mA. It lists +/- Icc and +/- Ignd as 70mA.

    Then it lists the power dissipation per package at 750mW. If the
    maximum current for Icc or Ignd is 70mA, how do you get 750mW for a
    maximum supply voltage of 7V?

  2. Lord Garth

    Lord Garth Guest

    TTL is a current sinking family. Outputs sink current from an external
  3. Phil Allison

    Phil Allison Guest

    "Grant Stockly"

    ** Says " Per Output Pin ".

    So 140mA is possible with all four outputs in use.

    ** Cos something in the common Vcc & Ignd lines will not tolerate 140 mA

    ** 140mA at 7 volts = 980mW.

    The max dissipation figure ( 750mW) is * theoretically * well possible under
    overload conditions - assuming the full 7 volts is dropped across the
    conducting MOSFETS.

    ....... Phil
  4. Rich Grise

    Rich Grise Guest

    You use some care in your design so that the total dissipation is
    less than 750 mW, like don't load them to the limit. A TTL input
    shouldn't need more than a few mA to get pulled high (although, I'd
    use a series resistor to limit _base_ current), and when pulled low,
    it only draws leakage, which will^H^H^H^Hshould be negligible. :)

    If you're trying to use them for power drivers, then, as I said,
    use care in your design, or use ULN2003 or so power buffers.

    Hope This Helps!
  5. Thanks for the advice that everyone has given me. I am trying to
    choose a modern part to replace some bus drivers on a new S-100 card
    design. I'm trying to find a suitable replacement for the 8T97/74367
    and a new implementation using the '125 parts. I can get old parts
    but they are not as reliable as the new ones it seems.

    The HCT parts look like they can drive enough TTL loads to be a good
    match for the project. I don't expect that any of the pins will see
    any huge amount of current except for one, but that won't happen very
    long or very frequently.

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