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I'm having a small gap in understanding with a project I'm working on. The project is reliability related and requires looking at the absolute maximum values for some 54ALS family logic (Max Vcc, junction temp, Output Current, Power Dissipation) and doing some derating calculations on those values.

Just as a basic test to see if I can accurately find these values, I pulled up the datasheet for the 54ALS08 from TI. The only real absolute max I was able to pull from this was Max Vcc is 7V.

Digging a little deeper, TI's website referenced a standard military datasheet for the component: 5962-86842. That one helped a little more giving me:

Max power dissipation, per device: 22mW Junction Temp: 175C.

Cool! I'm getting close. How the heck do I determine the maximum output current though? I know for logic-low I don't want to be going over 4mA it's going to impact the logic function of other gates down the line. But, I'm more interested in the current source/sink that's going to damage the IC. Searching around I found a similar post asking about this that referenced 74HC08 (HC logic family version of this). That datasheet seemed more straight forward. Icc under Table 4 (absolute maximum values) is 50mA.

Last things, "max power dissipation, per device". I'm assuming this means that for this particular IC (quad gate) I shouldn't exceed 88mW for the entire chip in addition to 22mW from each individual gate?

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  • \$\begingroup\$ If I recall rightly, aluminum is sputtered onto the Silicon at 400 degree Centigrade. \$\endgroup\$
    – analogsystemsrf
    Feb 14, 2019 at 17:21
  • \$\begingroup\$ @analogsystemsrf ...which is relevant because??? \$\endgroup\$
    – Elliot Alderson
    Feb 14, 2019 at 17:47
  • \$\begingroup\$ If your goal is reliability, you should definitely not be using the absolute maximum numbers. Those are not recommended for operation in any case. \$\endgroup\$
    – Chris Stratton
    Feb 14, 2019 at 18:14
  • \$\begingroup\$ @ChrisStratton I wasn't sure about this either when I was asked to dredge this up. Seemed odd to derate from max values that you shouldn't be hitting anyway, thought using recommended operating conditions and derating from there made more sense. I'll try to ask for some more clarification. \$\endgroup\$
    – bladpart
    Feb 14, 2019 at 18:20

1 Answer 1

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Data sheet has this row

IO# VCC = 5.5 V, VO = 2.25 V −20 −112 −30 −112 mA

and a note explaining the -112mA

The output conditions have been chosen to produce a current that closely approximates one half of the true short-circuit output current, IOS.

So for absolute max, there you are. I'd be sure to design for well under that tho. Other (later) datasheets do better, even showing Abs Max current on the VCC & Gnd pin (70mA on HC595 for example, so many of them running in Arduino project are definitely being overstressed driving LEDs!)

74AC family is much better, with +/- 24mA outputs.

Many logic inputs are very light loads tho, in the range of uA for CMOS for example.

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  • \$\begingroup\$ Awesome! I was originally picking the Ios but I had a colleague tell me this wasn't the correct approach since in practice we're never shorting an output to ground. Was throwing off my confidence. I was starting to look at old ALS manuals. Table 1 on 4-3 here says 16mA for standard outputs on the 74ALS versions. Another said 2* the rated IOL (i.e. 2* 4mA). Wasn't sure if that was a better value to use. \$\endgroup\$
    – bladpart
    Feb 14, 2019 at 17:46

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