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When a datasheet specifies something like output current, as with the 74FCT244 or for example, is this referring to the aggregated output current between all pins or the capacity of a single pin ?

  • 74FCT244

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  • Other device example

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Also, it specifies above for the 74FCT244 that with an output HIGH, the max current is -15mA, while with output LOW the associated current shall be no more than 64mA. Clearly one of these refers to the capacity of the device to source and sink current, but the relationship between current sign and output voltage appears backwards to me. Namely, the datasheet says with a HIGH output then it appears to allude to sinking 15mA (indicated by the negative sign). Similarly, the LOW output voltage has a capacity of 64mA .. sourcing? (positive sign).

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2 Answers 2

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First, these ratings are all per pin.

The question of output current sign is confusing, but it's a hangover from the days of TTL. The current being produced comes from the inputs of other logic chips, and it became standard practice to referred to sinked (sunk?) current as "positive", since that was the dominant current flow in TTL (TTL gates require much more current being sunk to produce a low than they do sourced current to produce a high, and this larger current level is what everybody had to worry about). So the sign for sourced current obviously had to be the opposite. Accidents of history, and all.

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  • \$\begingroup\$ Positive sign for sunk current is also the standard in circuit theory. \$\endgroup\$
    – The Photon
    Jan 23, 2015 at 15:54
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These ratings are per pin. I was also hoping to find in the datasheet a different number for max cumulative output current. Didn't find such number. One would have to assume that the chip can sink up to 0.5A combined on all outputs.

As an aside, here's another IC (originally from this discussion), where output current per pin and combined output current are mentioned separately. Also notice that IO,total,max < IO,max npins in this example.

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