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SN74AVC4T774: Different behaviour of unused pins over production batches?

Jeroen de Bruijn
Prodigy 10 points
Part Number: SN74AVC4T774

We are using the SN74AVC4T774PW in one of our designes and have different behaviour over the prints that are on the device.

We have a flaw in our design where we have the output of unused pins of the IC connected to a defined level (GND) instead of the input, which is floating. On most IC's this doesn't seem to cause different behaviour to where the input is connected to a defined level. In those cases the current drawn on the VCC_HOST rails of the attached schematic is a couple of µA, but in case where this does cause problems the current is somewhere between 40 mA and 110 mA.

I've found that IC's with the red marking starting with 85 generally have the µA current draw with the output of the unused pins connected to GND IC's with the red marking starting with 79 have the mA current draw.

Now, I know for sure that I should actually connect the input to a defined level (GND) and leave the output floating, but is there an explanation for why the marking seems to make a difference.

  1. What does this marking mean, is it the production date? I couldn't find any information on the marking in the product datasheet. Could you reference a document where I can find information on what the marking means? See the attached image for the red and blue marking of which I don't know the meaning. For the marking at the top WT774, I know that it's the product number.

  2. Is there any explanation for this difference between IC's?

  3. Can we be sure that the IC's that draw the µA's, without having fixed it (unused outputs are still connected to GND) don't start drawing mA's over time or with changing temperatures?



  • 0
    TI__Mastermind 45855 points
    Hi Jeroen,

    1. To answer this question, I will link this helpful application report: www.ti.com/.../szza020c.pdf
    The first 4 or so pages will answer in great detail, figure 6 will be an example of the device you are using. In short, those markings indicate different lot codes and date codes.

    2. Yes. Since the devices are from different lots then they will have slightly different performance due to process variation. This is quite common. It isn't a large difference, but in circumstances like this one the differences can be noticed.

    3. Unfortunately no. On top of process variation, temp and voltage variation can also cause this devices performance to shift. There's no guarantee that those devices won't start drawing a lot of current at any moment.