Output drive vs. absolute max output current

Question

In many Texas Instruments datasheets (e.g. this one) there are two different stated values for current per output:

1. Output drive, stated as 4 mA at \$V_{cc}=5 V \$, page 1.
2. Continuous maximum current per output, stated as 25 mA, page 6.

My interpretation here is that if the device is operated at \$V_{cc}=5 V \$, TI guarantees that it can provide 4 mA per output. However, along with the second line it means that each output might also be able to provide \$4 mA < I_{output} < 25 mA \$ without damage, but the manufacturer doesn't recommend more than 4 mA at this supply voltage. Is this correct?

Answer 1

On page one, the datasheet is telling you that the output can supply 4mA and still respect V_OH(min). In other words, it can supply 4mA and still provide an output voltage that will be recognized as a logic level high by other IC's. It may be able to supply more than 4mA but the output voltage will sag. If you are not feeding the output to a logic device, but are instead using it to drive an LED at 5 or 8 mA, that sag may be acceptable to you. (Although I would look for a higher drive IC, personally, as this one is weak-ish for an LED).

On page six, you are looking at absolute maximum limits. It can supply 25mA without immediate damage (although damage may occur if you are at the absolute maximum for a long time). In general, you never design anything to be close to the absolute maximum for the IC in normal operation. This table, absolute maximums, is something you study so you can stay away from it. It is a "don't do this" table.

Absolute maximums tell you conditions that may cause permanent damage. No guarantee of correct behavior is implied.

You may also want to study tables 7.5, 7.6, and 7.7 to see how the guaranteed minimum value of V_OH(min) varies with VCC and temperature range.

Answer 2

RdsOn is the FET switch analog parameter which limits risetime in a std load pF value and voltage drop or rise guarantee with somewhat standardized test currents. If you compute RdsOn for Vol/Iol and Voh at the specified Vdd you will understand how it affects risetime into a given load capacitance from the RC=tau value.

All logic devices are analog

In this case 4mA load applies to the worse case tolerance of -10% Vdd for a 5V supply, namely 4.5V.

• RdsOn rises with lower Vdd, lower temperatures
• I usually call all 74HC family logic “50 ohm logic” at 5V but as you see there are tolerances on Vdd, batch tolerances and temperature tolerances.

Then I call 3.6V ARM or 74ALC family Cortex logic as 25 Ohm logic even though these are understood to affect risetime and voltage drop from each rail under a std. load current.

As stated in the absolute maximum 32 mA intended as a safety limit. I understand it is limited by a safe holding current before possible fusing of the gold wire bond to the substrate.