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

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

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
follow.

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. ### BobGuest

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?

4. ### John PopelishGuest

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/WB2TTGuest

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

6. ### Guest

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

-- 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. ### LeftyGuest

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

Lefty

9. ### Rich GriseGuest

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!
Rich

10. ### Rich GriseGuest

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

Cheers!
Rich

11. ### EeyoreGuest

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

What do you think it means ?

Graham

12. ### John PopelishGuest

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).