What is power-down protection?
To reduce power consumption, a system with two voltage domains (VCC1 and VCC2) might provide partial power-down mode in which the subsystem operating from VCC1 is shut down. Suppose, for example, that the 74VHC Series is used in the VCC1 domain. The 74VHC Series has an unintended parasitic diode between the output and power supply. Therefore, when VCC2 > VCC1, this parasitic diode turns on. In this case, the IC might be destroyed by the resulting large current. Device destruction can be prevented by using an IC with neither an input nor output parasitic diode (e.g., 74VHCT, 74LCX, and 74VCX Series).
These series provide power-down protection.