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.
It is easy to distinguish whether an IC has a power-down protection function.
Let’s look at the datasheets for ICs of the 74VHC and 74LCX Series.
See the DC Characteristics table.
Power-off leakage current (IOFF) is specified in the datasheet for the 74LCX245, which indicates that almost no leakage current flows even when power supply is off.
Therefore, the 74LCX245 has a power-down protection function.
In contrast, since the 74VHC245 has a parasitic diode between the output and power supply, a large current will flow if tested under the same conditions.
Therefore, there is no IOFF regulation and it does not have a power down protection function.
Note: The following four series provide a power-down protection function:
•74VHCT Series •74VHCV Series
•74LCX Series •74VCX Series
For the 74VHCT Series, power-down protection is specified by IOPD and IIN instead of IOFF.