1
\$\begingroup\$

I have a datasheet of a CD74HCT86E XOR gate. Under Absolute Maximum Ratings, it says

DC Output Source or Sink Current per Output Pin, IO

For VO > -0.5V or VO < VCC + 0.5V . . . . . . . . . . . . . . . . . . . .±25mA

How should I interpret this? I'm thinking of two options:

  • Make sure that the sink or source current never exceeds 25mA. However, then I'm wondering why there are specifications on the output voltage: I should think these voltages follow from the current draw and the output impedance - you can't have a current independent of output pin voltage

  • Make sure VO never goes below -0.5V or above Vcc+0.5V. As long as this is the case, the device will not source or sink more than 25mA. Do not exceed these voltages, or the device will sink/source more than 25mA, and magic smoke will happen.

Which is it?

\$\endgroup\$
3
  • \$\begingroup\$ Simple answer: Never design to the "Absolute Maximum Ratings". In general you would be better off to ignore these altogether to make sure you don't accidentally design your circuit in such a way as to constantly stress the part. Instead, look at the "Electrical Characteristics" or "Operating Characteristics" in the datasheet. Those are the values you need to design to. \$\endgroup\$
    – DerStrom8
    Feb 16 '18 at 17:48
  • \$\begingroup\$ @DerStrom8 I'm not designing to be anywhere close to these ratings. I was just wondering what would happen if one accidentally grounded an output pin for a short moment of time, and I couldn't figure it out from the datasheet. \$\endgroup\$
    – Sanchises
    Feb 16 '18 at 17:51
  • \$\begingroup\$ Just making sure =) \$\endgroup\$
    – DerStrom8
    Feb 16 '18 at 17:56
3
\$\begingroup\$

I believe your first bullet is correct. The voltage range specification is simply saying that if operating within the specified range, the pins can handle +/- 25mA. However, if you look at the two lines above it, if you're operating above or below the specified range, the input/output diodes become the weakest link and they have a current limit of +/- 20mA before being damaged.

\$\endgroup\$
1
  • 1
    \$\begingroup\$ Thanks for your answer - I think as a beginner, it's important to read the datasheets rather than just wing it, but then at least I should be able to understand the datasheet. My confusion stemmed from the fact that I didn't see the voltage limitations as a complement to the diode ratings. \$\endgroup\$
    – Sanchises
    Feb 16 '18 at 18:04

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.