A general logic IC has a diode for protection at the input terminal and the output terminal.
Logic ICs with input tolerant and power down protection functions do not have a diode on the power supply side. This makes it possible to apply a voltage that exceeds the power supply voltage of the logic IC, convert the signal voltage level between ICs operating at different voltages, and partially power down the block connected to the bus system. TOSHIBA calls this function on the input side input tolerant and the output side power down protection.
It is becoming important for CMOS logic ICs, which act as an interface between core ICs and peripheral ICs, to be enhanced with additional functions such as input tolerant function and power down protection.
It is important for CMOS logic ICs, which act as an interface between core ICs and peripheral ICs, to be enhanced with additional functions such as input tolerant functions and power down protection.
Please also refer to the FAQ below.
Input tolerant means that a signal with a level higher than the power supply voltage (VCC) can be input within the operating range when the power is applied or at 0V. By using a product with this function, it is possible to convert the level from high voltage to low voltage.
<Example of voltage level conversion>
5V → 3V voltage conversion: 74VHC or 74LCX is used. 3V → 1.2V voltage conversion: TC74VCX is used.
As shown in the equivalent circuit of a general CMOS logic IC, a diode is intentionally inserted on the input side for the purpose of ESD protection. There is a parasitic diode on the output side. The diode between the input and the power supply may be turned on by applying a voltage of VCC or more of the power supply voltage or applying a voltage when it is off, causing a large current to flow and destroying the element. In such a case, you can prevent the destruction by using a product with an input tolerant function that does not have a diode between the input and the power supply.
In a system with two power supplies (VCCA, VCCB), there is a case where the VCCB system is shut down (partial power down) in order to reduce power consumption. For example, when the 74VHC series is used for VCCB, there is an unintentional parasitic diode on the output terminal side, so when the condition VCCA> VCCB is satisfied (Case1), the diode between the output and power supply of the system of VCCB turns on. It may cause current to flow and destroy the element. In this case, destruction can be prevented by replacing the product with a product that does not have a parasitic diode at the output in addition to the input (example: 74VHCT series, 74LCX series, 74VCX series). It can be said that these products have power down protection (Case2).