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SN74LVC16T245: Using signals higher than the Vcc when shifting between 1.8V and 3.3V

Cayton Jones
Prodigy 20 points
Part Number: SN74LVC16T245

Hi,

I'm trying to add an adapter PCB to existing hardware that I have.  In the original design, I was shifting 1.8V (Vcca) to 3.3V(Vccb), and vice-versa.  In the modified design, I can't bypass the level shifter, but my signals coming from the Vcca side are now 3.3V.  The 3.3V source is 1.8V tolerant for any signals shifted down, but can the SN74LVC16T245 handle 3.3V signal levels on the A side when Vcca is tied to 1.8V?  I didn't see this clarified in the datasheet.

Thank you,

Cayton

  • 0
    TI__Guru* 96591 points
    Hi Cayton,
    I have asked our translation expert to get back with you - he will probably see this tomorrow morning.

    My thought is that the 1.8V input on a 3.3V side could cause excessive shoot-through current (CMOS devices generally don't like to be held close to 1/2*Vcc on an input). The other side is probably safe since this device is over-voltage tolerant (as indicated in the datasheet Absolute Maximum Ratings table).

    Is there any reason that you can't get the supplies from their respective sides directly so they always match the incoming signal?
  • 0
    TI__Guru 56100 points
    Cayton,

    If I understand correctly, currently you do not need a translator, however you cannot bypass it.
    Vcca is 1.8V, and Vccb is 3.3V and the incoming signals are at 3.3V on the A side. I would suggest to connect Vccb and Vcca together if possible so that essentially the device is acting as a buffer for the signal.
    The device is overvoltage tolerant on the inputs and hence you can have higher than Vcc input on the Input lines( but not output)
  • 0 in reply to ShreyasRao
    Prodigy 20 points

    This is exactly what I needed to know.  Thank you very much!