• State Resolved
  • Locked Locked
  • Replies 4 replies
  • Answers 2 answers
  • Subscribers 22 subscribers
  • Views 1558 views
  • Users 0 members are here
Support feedback
Options
Options
Related

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.

Original question:

SN74LVC2G17: Input supply above 6.5V with current limit possible ?

SN74HC14: Input voltage above VCC with current limit safe ?

Goeran Eggers
Prodigy 70 points
Part Number: SN74HC14


The recommended operating conditions in the datasheet mentions the supply voltage VCC as maximum voltage for the inputs. Nevertheless, according to the absolute maximum ratings higher voltages are allowed as long as the max. input current is within +/- 20mA.

We have an application where there is a 10k resistor in front of the input and voltages up-to 24V might be applied to the other end of the 10k resistor. The 74HC14 gets +3V3 supply and when we connect 24V to the 10k resistor the voltage at the input is about 4.3V (measured - basically fits to the +3V3 supply + internal protection diode forward voltage drop). The current into the input is around 2mA. So, these measurements are all well below the abs. maximum ratings but, above the recommended operating conditions.

Question:

- Is it safe to operate the 74HC14 as described above  - even for long term ? Or do you we have to expect de-grading or early malfunction ?

Many thanks in advance.

  • 0
    Guru 319310 points

    The absolute maximum ratings are long-term limits.

    Please note that in your circuit, the 2 mA current flows out of VCC. If the rest of the circuit does not consume this current, and if the power supply cannot sink the current, then the power supply voltage will float up.

  • 0 in reply to Clemens Ladisch
    Prodigy 70 points

    Ok, thanks. This is not an issue at all as the current consumption at the +3V3 rail is > 50mA.

    Nevertheless, would be good to get in addtion some statement from TI support directly that operating the device above the recommended operating values but, well below the limiting values will not cause any early failure or early degradation of performance over time.

    Many thanks in advance.

  • 0 in reply to Goeran Eggers
    TI__Genius 16735 points

    Hi Goeran,

    This is the footnote under the ABS max table and this best summarizes my answer:

    Stresses beyond those listed under Absolute Maximum Ratings may cause permanent damage to the device. These are stress ratings only, and functional operation of the device at these or any other conditions beyond those indicated under Recommended Operating Conditions is not implied. Exposure to absolute-maximum-rated conditions for extended periods may affect device reliability.

    What this means is that although the device will work at Abs Max Ratings, we cannot guarantee anything about the device if used beyond the recommended operating conditions.

    Thanks!

    -Karan

  • 0 in reply to Karan Kotadia
    Prodigy 70 points

    Thanks for your feedback. Good to here, that the device will work in the described area of operation. I fully understand, that the characteristics will change - we will have input current, supply current in "wrong" direction etc.