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HELP on homework... how do you calculate fan-out?

Discussion in 'Electronic Design' started by [email protected], Feb 20, 2007.

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  1. Guest

    I need help for this problem.. I have a test tomorrow and this is the
    only problem I do not know how to do... any help would be appreciated!
    Thank you very much!

    10. Given the following specifications, answer the questions that

    74LSxx Family Devices 74Fxx Family Devices
    Max Typ Min Max Typ Min
    IOL 25mA 23mA 22mA 33mA 32mA 31mA
    IOH -23mA -21mA -20mA -30mA -27mA -25mA
    IIH 200uA 150uA 100uA 250uA 200uA 180uA
    IIL -300uA -280uA -260uA -320uA -300uA -280uA

    What is the fan-out for LS devices driving LS devices?

    What is the fan-out for F devices driving LS devices?
  2. Bob

    Bob Guest

    Is this really a homework problem? Usually we ONLY answer homework problems,
    here. I hope you're not lying.

    Anyway, the specs give you how much an LS and F output can drive (IOx) --
    both in the low and high state. The specs also give you how much input
    current is needed to drive each type of input (IIx)-- both in the low and
    high state.

    So, the fanout will involve figuring out how many inputs (worst case) can be
    driven from a single output stage (worst case). What is "worst case"? Here's
    a hint -- if you can't supply the amount of current an input (or inputs)
    need then the low and high state voltage will NOT be met. Here's another
    hint -- two inputs connected to an output will draw twice as much current
    (from that output) as compared to if only one input was connected to that

    There's no point in us giving you a formula because it won't help you (in
    the long run). This is a "common sense" kind of question. If it's not
    obvious, to you, what is really being asked then you need to go back to the
    basic concepts of what voltage and current really are.

  3. Joel Kolstad

    Joel Kolstad Guest

    Well, if you have a device that could source, say, 10mA when high and could
    sink, say, 5mA when low, and were trying to drive an input that required, say,
    1mA when driven high and 200uA when driven low, I'd say that... hmm... when
    high, you could drive 10 of these devices and when low you could drive 25 of
    these devices... so I suppose the fan-out would be the more restrictive
    case -- 10 devices.

    Surely that's enough to let you solve your homework problem?
  4. Fan out is the number of inputs (worst case limits) that can
    be driven by an output while remaining inside the voltage
    limits for a logic low and logic high state.

    Taking the LS family, for example, the worst case (weakest)
    output current that an output can sink (IOL or output
    current absorbed while maintaining a logic low voltage) is
    at least 22 mA, while pulling down to a valid logic low
    voltage. Each input may supply as much as (highest spec)
    300 uA current into that output (IIL) or input current
    supplied into a logic low voltage), while it is pulling down
    to a logic low. Since there are about 73 times 300 uA in 22
    mA, the pull down fan out is up to 73. Each output can
    supply up to 20 ma while maintaining a logic high state
    (IOH), while each input may need up to 20 uA supplied to it,
    hold it up to a logic high state (IIH). Since there are
    about 100 of 200 uA in 20 mA, the fan out for a logic high
    is up to 100. But you have to take the lower of these two
    conditions, so the 73 for the logic low rules. You would
    expect a fairly large speed reduction, if you ever came
    close to this limit, because the output also has to charge
    and discharge all the trace and input capacitances before
    stable logic level voltages are achieved.

    I hope I got all that right.
  5. Tam/WB2TT

    Tam/WB2TT Guest

    Simplified, the maximum fanout of a device is its MIN output current,
    divided by the MAX input current of whatever you are driving. Throw away
    anything after the decimal point.

    Your numbers don't look right. As I recall, the IOL for a 74LSxx is 8 ma;
    the IOH is around 1 or 2 ma. The 74F numbers don't look right either. You
    don't care what the MAX output or the MIN input is.

  6. Guest

    Thank you all for your help! I get it now. And yes, it is a homework

    -- Erik
  7. Guest

    Actually a quick question: where it says What is the fan-out for F
    devices driving LS devices, which is the input and which is the
    output? Thank you!
  8. Lefty

    Lefty Guest

    Your homework appears to be overdue...........I hated school!

  9. Rich Grise

    Rich Grise Guest

    On Mon, 19 Feb 2007 16:07:05 -0800, elyngved wrote:

    "HELP on homework... how do you calculate fan-out?"

    You look at the data sheet.

    Good Luck!
  10. Rich Grise

    Rich Grise Guest

    I'm sure the teacher will let him know. ;-)

  11. Eeyore

    Eeyore Guest

    If you don't know that, it definitely means you don't know what fan out is.

    What do you think it means ?

  12. If the middle letter is an O, it is an output spec. If the
    middle character is an I, it is an input spec. The first
    character is I, representing current. The last character
    tells you if the spec relates to the logic low state (L) or
    the logic high state (H).
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