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Original question:

SN74LVCH8T245: Input leakage current

SN74LVC8T245: Power on sequence

toshi-e2e
Genius 4830 points
Part Number: SN74LVC8T245

Hi all,

The SN74LVC8T245 datasheet has been revised from Rev B to Rev C, and the Power Supply Recommendations section has been updated with the following specifications:

"The recommendation is to first power-up the input supply rail to help avoid internal floating while the output supply rail ramps up. However, both power-supply rails can be ramped up simultaneously."

Since the SN74LVC8T245 is already being used in a product that is already in mass production, and the output side is turned on first in the power supply sequence, please let me know if there are any concerns.

Regards,
Toshi

  • 0
    Guru 317060 points

    Floating inputs are undefined, so the corresponding outputs might output the wrong value or even oscillate.

    What is the state of /OE during power up?

  • 0 in reply to Clemens Ladisch
    Genius 4830 points

    HI Clemens-san,

    Thank you for your answer.

    I will check the /OE state during power up.

    If /OE is High, can the phenomenon you pointed out be prevented?

    Also, the SN74LVC8T245 has the Ioff function, but does that mean it doesn't work properly?
    The power-up sequence is described, but will the Ioff function work when one of the power supplies is turned off?

    Thanks and regards,
    Toshi

  • 0 in reply to toshi-e2e
    Guru 317060 points

    When /OE is high, the outputs are guaranteed to be disabled, even during power-up/-down. (This requires that /OE is powered, i.e., that VCCA is not powered up after VCCB.)

    The Ioff function ensures that powered-off outputs are Hi-Z. This is the opposite of your case (where the output side is powered up first).

  • 0 in reply to Clemens Ladisch
    Genius 4830 points

    HI Clemens-san,

    Just to reconfirm, will the Ioff function work normally if the power supply on the input side is turned off after turning on both the power supplies on the input and output sides?
    Should I pay attention to the recommended power supply sequence only when turning on the power for the first time?

    Thanks and regarsds,
    Toshi

  • 0 in reply to toshi-e2e
    Guru 317060 points

    The Ioff function works only on the side where the power is down. (It is not an active circuit; it's a diode that blocks current flowing from the output pin to the power supply.)

  • 0 in reply to Clemens Ladisch
    Genius 4830 points

    On page 1 of the datasheet, Ioff's functionality is described as follows: It is not explained that only the power OFF lower side is set to Hiz.
    Have the specs changed?


    "The Ioff circuitry disables the outputs, preventing damaging current backflow through the device when it is powered down. The VCC isolation feature ensures that if either VCC input is at GND, all outputs are in the high-impedance state."

  • 0 in reply to toshi-e2e
    Guru 317060 points

    Only the first sentence describes Ioff. And it says that it prevents current backflow, which can happen only when the voltage at the output pin is higher than the supply voltage.