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SN74LVC8T245: Problem with large current consumption

Egor Borodulin
Prodigy 245 points
Part Number: SN74LVC8T245

Hello!
I have some trouble with SN74LVC8T245.

I connect it as shown at scheme.

But my device began to consume too much current, when OE is high.


After the tests on my PCB board, I connected this chip separately to eliminate any effect of the PCB and got the same results.

Tests with this IC (Ampermeter at Vccb):


1) Vcca - 3.3V, Vccb - 3.3v, DIR connected to Vcca, OE connected to Vcca

2) Vcca - 3.3V, Vccb - 5.5v, DIR connected to Vcca, OE connected to Vcca

3) Vcca - 5.5V, Vccb - 3.3v, DIR connected to Vcca, OE connected to Vcca

4) Vcca - 3.3V, Vccb - 5.5v, DIR connected to Vcca, OE connected to GND

5) Vcca - 5.5V, Vccb - 3.3v, DIR connected to Vcca, OE connected to GND

6) Vcca - 5.5V, Vccb - 5.5v, DIR connected to Vcca, OE connected to GND

Why does high current consumption occur at high OE levels and Vccb > Vcca (outputs disable)?

  • 0
    TI__Guru 62180 points
    Hi Egor,

    The device being powered up with the OE high, would isolate the A/ B channels from each other(Hi-Z).
    The input circuitry on both the channels (A & B) would still be active and would need to be defined so that it would not float to a value closer to the threshold (Vcc/2). Would it be possible to have a weak pulldown on the channels and see if it reduces the current consumption?
    Is 3.3 to 5.5V translation required or is it anything lower?
  • 0 in reply to ShreyasRao
    Prodigy 245 points
    Thanks for your reply.

    >>Is 3.3 to 5.5V translation required or is it anything lower?
    Yes, I need 3.3 to 5.5V translation. By the documentation the maximum voltage value is 6.5 V, so I decided that 5.5 still is on the upper edge of the safe range.

    >>Would it be possible to have a weak pulldown on the channels and see if it reduces the current consumption?
    I tried to add 100 kOhm pulldown resistors at IO port on both sides and current has become within the expected level. Thank you for your advice.
  • 0 in reply to Egor Borodulin
    TI__Guru 62180 points

    That is good to know, Egor.

    Thanks for the confirmation.