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SN74LVC244A: Octal Buffer or Driver

Max Pom
Intellectual 350 points
Part Number: SN74LVC244A

Hello,

The item (as many other items of TI as I've noticed) datasheet declares the following:

"OE should be tied to VCC through a pullup resistor to ensure the high impedance

state during power up or power down; the minimum value of the resistor is determined by the current sinking

capability of the driver."

Is it necessary to connect the OEn to a pullup resistor? it requires some logic to pull it low after power up, which complicates the design.

What voltage value can be generated at the output if don't do so (can it damage the receiver device)?

Thanks,

Max

  • 0
    Guru 316350 points

    You need a pull-up resistor on OE only if you actually need to ensure the high-impedance state during power-up.
    If you do not care about the actual signal during power-up up, you can connect it directly to GND.

    There is no guarantee how the output signal behaves during power-up; it might be low or high or switch between them.

  • 0 in reply to Clemens Ladisch
    Intellectual 350 points
    Thanks, that's what I thaught, just wanted to be sure that the unknown value during power up can be only high or low, and not something very high (overshoot) that can damage the receiver, which is not tolerant..
  • 0 in reply to Max Pom
    TI__Guru* 97046 points
    Hi Max,
    The output will only be undefined when the supply is below the recommended operating range in the datasheet -- ie below 1.65V. So we're really talking about the question: "How much output voltage can you get from a 1.65V supply, including ringing?"

    In the real world, I'd expect the ringing at 1.65V to be very minimal, probably zero, since drivers get weaker with lower supply voltages... but if we assume the worst case overshoot of Vcc/2, then the output could have up to 2.475V on it from ringing.

    Of course, if your system had low enough resistance (damping) and high enough output inductance to cause that kind of overshoot at 1.65V, I'd hate to see what happens during normal 3.3V operation.