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

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 output.

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.

Bob

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?

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.

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.

Tam

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

-- Erik

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!

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

Lefty

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

You look at the data sheet.

Good Luck! Rich

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

Cheers! Rich

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).