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Original question:

TXB0104-Q1: VCCA is on, and VCCB stay off

TXB0104: Confusion regarding VCCA ≤ VCCB condition and VCC isolation feature

js
Prodigy 90 points
Part Number: TXB0104
Other Parts Discussed in Thread: TXU0304, TXU0104, TXU0204

Tool/software:

I need a level shifter for GPIO lines where one side (VCCB = 5V) can be powered off while the other side (VCCA = 3.3V) is on. 

I have implemented TXB0104 in my design because it says in datasheet:

VCC isolation feature: if either VCC input is at GND, all outputs are in the high-impedance state. 

But it also says VCCA must not exceed VCCB.

But when the VCCB is powered off, VCCA will exceed VCCB. 

The question is, if it is ok to have VCCB be powered off while VCCA is on, because these two conditions (VCCA ≤ VCCB condition and VCC isolation feature) contradict each other?

  • 0
    Guru 316350 points

    When VCCA > VCCB, a current can flow into VCCA and out of VCCB. This is not a contradiction because VCCB is not an I/O pin.

    Do you really need an auto-bidirectional level shifter? Consider the TXU0104/TXU0204/TXU0304.

  • 0 in reply to Clemens Ladisch
    Prodigy 90 points

    Yes, I have already switching to TXU0204. But with TXB0104, I could later change the I/O directions if needed, which is now not the case with TXU0204.

    Still, would be nice to know why is it called VCC isolation feature, if the current can still flow from VCCA to VCCB. Because it gives the impression that it is safe to have VCCB = 0V, while actually it is not. In which condition, will this feature be useful then?

    Thanks!

  • 0 in reply to js
    TI__Mastermind 39866 points

    Hi,

    The VCC Isolation feature is designed to help prevent the I/Os from influencing one another while minimizing leakage currents at the A/B ports when VCCA/ VCCB powers down to 0V. This feature is in multiple translators across our portfolio, however with the TXB, additional care must be taken place such that VCCB should not be less than VCCA during normal operation. 

    Regards,

    Jack