This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

SN74AHC240: Input Signal Beyond "min" ABS MAX for <1ns

Part Number: SN74AHC240

The SN74AHC240 Datasheet lists the Vin max as -0.5V to 7V,
with a Note 1 saying these can be exceeded as long as the current into the pin is kept below the ABS MAX values of -20mA.

Given the above statement, I assume the 20mA limit is "constant current limit of 20mA" and not a transient rating, correct?

The real question is:
"Is an undershoot on the input (pin2 / pin4) that reaches beyond -0.5V (max = -0.7V) for <1ns, acceptable given the datasheet phrasing?"

Personally, I think 1ns is too fast for the ESD structure to respond so it doesn't look like an issue from that side either, but if you have any comments?

Thanks,
Darren

  • We treat the maximum rating as absolute - ie you should never exceed 20mA, for any length of time. I agree that the wording in the datasheet is not great.

    I do understand that short pulses carry very little energy, which is how our devices can survive ESD strikes of 2000 V -- however we do not rate logic device clamp diodes for transient events based on energy -- if a customer wants to count on this not being an issue, it would be up to them to take on that risk.

    I would recommend to avoid getting anywhere near the abs max values. If a system is operating a logic device at the abs max limit, then the design should be updated to avoid that.

    What is being asked is different from that subject -- ie the question is about input voltage, not input current. Most likely, the customer hasn't measured input current and only has seen a voltage waveform.

    With only 1ns of time below -0.5V, it is extremely unlikely that the diode has had time to reach 20mA of current. Quite often, it takes a few nanoseconds for the diode to even start conducting at all.

    If this is a concern for the customer, a series resistor can be added to prevent the current from ever reaching 20mA -- only 10 ohms is required with such a small overshoot.

    -

    To be clear - if this device were to fail, and that failure came across my desk, I would point out the overshoot as a possible cause. Adding a resistor is cheap and effective for preventing issues from this type of issue.