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ULN2803 current limit

Discussion in 'Electronic Basics' started by David Harmon, Oct 6, 2005.

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  1. David Harmon

    David Harmon Guest

    ULN2803A: The datasheet says the 500 mA limit applies to "one device
    in the package". Does that really mean that the whole package can
    pass 4 Amps at once using 8 pins? Or do you only get the full 500mA
    while using only one device at a time.
     
  2. Chris

    Chris Guest

    Hi, Dave. Great question. Here are two data sheets for the 2803:

    http://www.onsemi.com/pub/Collateral/ULN2803-D.PDF
    http://www.allegromicro.com/datafile/2801.pdf

    The first data sheet is remarkably opaque on this really basic
    question. I would guess that's because it's kind of a "gumball" part
    -- it's been around for decades.

    Both data sheets tell you that each output is rated for 500mA.
    However, the Vce of the outut darlington is typically between 1.0V and
    1.5V for high output currents. Assuming worst case, eight outputs of
    500mA with 1.5V across each output, you've got 6 watts being dissipated
    by the IC package. Not happening in this world. The IC will glow red
    for sure.

    The second data sheet is more descriptive. It has a graph that says if
    you've got a 100% duty cycle (the output is always on), you can't even
    run two outputs at full current. This is because of power dissipation
    limitations. The maximum output per pin goes from 500 mA (one output
    on at one time) to about 470mA with two on, down to about 160mA with
    all eight outputs on at 100% duty cycle. Look at the graph on page 6.

    Actually, since these are maximums, you should derate the outputs on
    the data sheet accordingly. I would think good engineering practice
    would suggest no more than about 1 watt power dissipation maximum for
    the IC package. That would mean 500mA for 1 output, 330mA for two
    outputs, 220mA for 3 outputs, 167mA for 4, and so on. But how
    conservative you want to be is up to you. Don't exceed the maximums
    shown on the graph, though.

    If you need 500mA from more than one output, use TO-220 NPN
    darlingtons, and discrete resistors to give you 1mA of drive current at
    the base of the transistor. Almost any CMOS or TTL output can drive
    1mA. And the 500mA collector current is small enough so you won't need
    heat sinks for the transistors.

    Good luck
    Chris
     
  3. Pooh Bear

    Pooh Bear Guest

    There will be a thermal restriction.

    Essentially it's dissipation limited. It's up to you to figure out how
    that affects your useage.

    Don't forget to derate the package dissipation vs ambient temp btw.


    Graham
     
  4. David Harmon

    David Harmon Guest

  5. Jasen Betts

    Jasen Betts Guest

    That depends the amount of power disipated, as
    long as it's below the limit for the package you should be fine.

    you have the data sheet (figures for voltage drop etc...), you work it out.

    Bye.
    Jasen
     
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