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SN74ALVC244: Maximum output voltage without Vcc

E _
Prodigy 10 points
Part Number: SN74ALVC244
Other Parts Discussed in Thread: SN74AUC244, SN74LV244A, SN74LVC244A, SN74LVC541A

Tool/software:

Hi,

I'm using the buffer SN74ALVC244 to drive some other IC input pins on an existing PCB.

The IC is already active before the buffer Vcc is on.

The IC pins can't  be left floating. The IC can be configured to have internal PU resistor (not PD).

Can I connect the internal PU to the buffer's output when the buffer is off?

According to the buffer's datasheet, the output voltage range is VCC + 0.5, but it "may be exceeded if the input and output current ratings are observed."

I am not sure if the buffer has a clamping diode only to GND or also to VCC? The condition for output clamp current is "Vo<0".

Is there a more suitable buffer for this case, with the same specs? (same Tpd, footprint)

Thanks

  • 0
    Guru 318700 points

    You need a buffer with the partial-power-down feature. The fastest would be the SN74LV244A at 3.3 V or the SN74AUC244 at 2.5 V/1.8 V.

  • 0 in reply to Clemens Ladisch
    Prodigy 10 points

    * About the Clamping diodes, I saw the app note ssztcn0 .

    It says that since the absolute maximum ratings table show a max value of (Vcc + 0.5), it means that there is a clamp diode to Vcc.

    But later, it says that in case of 2 clamp diodes, to Vcc and to GND, the current rating should have a +/- symbol. But the datasheet only shows -50mA value (and the condition is Vo<0).

    So what is correct? Is there a clamp diode to Vcc or not?

    * About the recommendation for SN74LV244A (3.3V):

    For SN74ALVC244, the Tpd for 3.3V is 2.8ns max (-40 to 85, and 50pF I think - from figure 1 table).

    For SN74LV244A, the Tpd for 3.3V is 13.5ns max (-40 to 125, 50pF).

    This is an existing PCB, so I prefer to avoid significant changes.

    Thanks

  • 0 in reply to E _
    Guru 318700 points

    The SN74ALVC244 definitely has a clamping diode; its absence would be advertised with the Ioff feature.

    The missing clamping current rating might be an error in the datasheet. In any case, allowing current to flow into the power supply might power up the buffer.

    I do not know whether the propagation delay actually matters in your application. These are the fastest buffer with Ioff.

  • 0 in reply to Clemens Ladisch
    Prodigy 10 points

    I found the part SN74LVC244A which has the Ioff feature as recommended, and also low Tpd (5.9ns max).

    There is also the part SN74LVC541A with Ioff and low Tpd (5.1ns max) but doesn't have the same functionality and pins.