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HCT at voltages below the data sheet?

Ira Faberman
Prodigy 20 points

Hi!

I am being asked to lower the supply voltage from 5V to 3.6V on a circuit that has an HCT component in it. In bench testing, the chip appears to work properly, but there is no supporting documentation for HCT at 3.6V. I assume that this is because the HCT inputs are designed to handle TTL and would not be used in a low supply system. The general specifications on the HCT imply that the same die is used for HCT as for CH parts which can be used at 3.6V. Thus, I am not surprised that it works, but I need some sort of confirmation that it will work if the supply = 3.6V and the Vin is 0 - 3.3V, which is almost CMOS input level.

I am hoping for a practical answer rather than an answer limited by the data sheet.

Thanks!

  • 0
    Guru 243820 points

    I'm not TI, but I can guarantee you that they will not give you a guarantee that it will work.

    When run at 5 V, HCT devices have a higher power consumption than equivalent HC devices because the lower input voltage causes both input transistors to be turned on partially. Running with a lower VCC might increase this effect. In any case, your single test cannot rule out that some other chip's gate threshold voltage happens to be out of range.

    If you are not using 5 V (TTL), there is no longer any need to use HCT. Just replace the chip with a HC one (or one from any other appropriate CMOS family).

    HCT and HC certainly use different dies; HCMOS Design Considerations (SCLA007) says:

    To make the inputs of the HCT devices TTL-voltage compatible, the input transistor geometries were changed.
  • 0 in reply to Clemens Ladisch
    TI__Mastermind 22075 points
    Hey all,

    I do work at TI and I can verify what Clemens has said. Thanks Clemens!

    Thanks,
    Daniel
  • 0 in reply to Daniel Hartung
    Prodigy 20 points

    Thanks to you both for your help and timely replies. Fortunately, the problems seems to have been resolved in another way, eliminating the daunting task of changing chips in units already in the field.

    Best regards -