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SN74LVC8T245: 74LVC8T245 1.8V input driven with 1.5V logic driver

Eric S
Expert 5640 points
Part Number: SN74LVC8T245

I wanted to make sure that it was okay for regular operation to drive a 1.8V input with a 1.5V logic signal.  (Vcca = 1.8V, DIR=Vcca for uni-direction translation)
The Vih specification indicates Vcca x 0.65 = 1.17V as the minimum voltage required to ensure a logic HIGH at the output, but I just wanted to confirm that this was a totally acceptable use-case, or if there is any potential odd behaviors if the 1.8V input is driven with a 1.5V logic signal all the time.   The inputs to this device are currently mixed between 1.5V and 1.8V, so moving to the lower Vcci "AVC" option would not work, I don't believe.    THank you.

  • 0
    TI__Expert 5640 points
    Related question... It appears there is a use-case where Vcca side will need to drive the 1.5V logic device (DIR pin not constant). In this case, the AVC8T245 should be a better option with ability to use Vcca = 1.5V. However, with Vcca=1.5V, and DIR pulled-HIGH, can some of the inputs be safely driven with 1.8V logic on AVC8T245? (still mixed-voltage system with some inputs 1.5V and some 1.8V on the 8-chan device... is this a valid operating condition across temp/voltage to drive inputs with Vcca + 0.3V logic signals?)
  • 0 in reply to Eric S
    TI__Mastermind 29925 points

    Hello Eric,

    Using 1.5V input should be fine since it is well over the Vih threshold.

    I would also say that you should be fine applying 1.8V input to the AVC8T245 with VCCA at 1.5V, however, you may see some leakage current into VCCA with the I/O at a higher voltage. It will not damage the device but may increase power consumption. I will need to verify this in the lab to be sure.

  • 0 in reply to Eric S
    TI__Mastermind 29925 points
    Hello Eric,

    I have just tested the device in the lab. The output switches fine with the 1.5V input and there is no discernible additional leakage current. Operating the device under these conditions will be fine.