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SN74AHC244Q: Application Question Consultation

WU YISYUAN
Expert 2165 points
Part Number: SN74AHC244Q

Hi,

    I have some question in use SN74AHC244Q.

3.3Vin for IC power supply, input is 5V square wave, output is 3.3V square wave.

Specification says that when 3V is supplied, the maximum effective level of VIL is 0.9V.
But when the VIL level is 1.4V, the output is also low. How should explain this? (Blue input, yellow output)

thanks.

  • 0
    TI__Guru 58090 points

    Hi Wu,

    To guarantee that the device accepts a voltage level as a low, it has to be <= 0.9V. Higher voltage may or may not be accepted by the device as a low( unknown).

    operating the device between Vih and vil is not a good idea for operation as well as power consumption of the device.

    Refer to the app note below:

  • 0
    Guru 276270 points

    At 3 V, VIL is guaranteed to be at least 0.9 V, and VIH is guaranteed to be at most 2.1 V. This means that the actual switching threshold could by anywhere between these two limits.

    In practice, the threshold will be pretty much exactly in the middle, but it might move due to manufacturing variations, temperature effects, or noise by other gates being switched.