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SN74HCT240: Shutdown voltage and output behavior after shutdown

Morty Xu
Intellectual 980 points
Part Number: SN74HCT240
Other Parts Discussed in Thread: SN74AHCT240

Hi Team,

The datasheet of the SN74HCT240 only mentions the recommended VCC minimum of 4.5V,

  • is there a specific shutdown voltage?
  • If the pin A input is high when the chip is powered off or not powered, what is the status of the output pin Y?

  • 0
    Guru 263860 points

    Below the recommended minimum of 4.5 V, correct operation is not guaranteed. (But I would expect behaviour similar to HC devices.)

    HCT I/Os are diode-clamped to the power rails. So when an input is high, a current will flow from the input into VCC, and the device will be (partially) powered. The output pin might be high, or low, or oscillating.

    If you want to be able to power it down while input signals are present, use a device with overvoltage-tolerant inputs like the SN74AHCT240.