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CD74ACT02: Recommended method for preventing VCC backfeed

Part Number: CD74ACT02
Other Parts Discussed in Thread: SN74AHCT02,


Does TI have a recommended method for preventing input voltage from backfeeding VCC through the protection CMOS protection diode in cases of VCC < Vin?



  • Hi Eric, and welcome to the forums!
    The best way to prevent Vin from powering the supply rail is to select a logic gate that has over-voltage tolerant inputs - aka inputs without positive clamp diodes. The SN74AHCT02 is a direct replacement for the CD74ACT02 (in most use cases) and it does not have positive input clamp diodes.

    Another, less ideal, option would be to place a Schottky diode between the supply and the Vcc pin - only allowing current to flow into the logic gate and not out from it. You could also add an external load switch to completely disconnect the supply pin when the device is not powered.