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Tell me about tolerant functions

The word “tolerant”(*) means being able to endure specified conditions. Some CMOS logic IC families provide input- and output-tolerant functions and output power-down protection(*).
Electronic devices contain multiple ICs that sometimes operate in different voltage domains (e.g., 3.3-V and 5-V domains). In this case, you need CMOS logic ICs with input- and output-tolerant functions as well as a level-shifting function (described later). Many mobile devices incorporate a partial power-down function, which shuts down unused logic blocks to save power and prolong the battery operating time. To realize partial power-down, output power-down protection is necessary.

Input-tolerant function: While V_CC is active (within the operating voltage range) or V_CC = 0, this function prevents current from flowing into an IC* when the input voltage is higher than the supply voltage (V_CC) . However, it is not permitted to apply a voltage higher than the maximum operating voltage to the input pin.

The tolerant function of an output pin is more complicated. The output circuitry of CMOS logic ICs has a totem-configuration consisting of a pair of P-channel and N-channel MOSFETs. Therefore, most CMOS logic ICs do not permit external voltage application to the output pin while they are active. If voltage is applied to the output, it is short-circuited to the power supply or GND, possibly leading to the destruction of the IC.

Output circuit of a typical CMOS logic IC　When the output is High　( Vexternal > VCC )　When the output is Low

*) In some cases, tolerant and power down protection are used interchangeably. Input tolerant is sometimes called input power down protection, and output power down protection is sometimes called output tolerant.

Output-tolerant function: In the case of CMOS logic ICs with Disabled mode, this function prevents current from flowing into an IC via an output pin when it is in the High-Z state. However, it is not permitted to apply a voltage higher than the supply voltage to the output pin when it is enabled (High or Low). If voltage is applied to the output, the output is short-circuited to the power supply or GND, possibly leading to the destruction of the IC.
In the case of open-drain ICs as well, current does not flow into an IC from an output pin even if a voltage higher than the supply voltage is applied to the output when the output MOSFET is off. Therefore, although not specified, the open-drain output can be regarded as having an output-tolerant function.
However, even in the case of CMOS logic ICs with an output-tolerant function, it is not permitted to apply a voltage higher than the maximum operating voltage to the output.

Power-down protection(*): This function prevents current from flowing into an IC via an output pin when a voltage in the operating voltage range is applied to the output while the power supply is off (V_CC = 0 V). Power-down protection works regardless of whether the IC has Disabled mode.

The following shows the equivalent circuits for typical CMOS logic ICs that contain diodes on both the input and output sides. The diodes on the input side are inserted for the purpose of ESD protection. Those on the output side are unintentional parasitic diodes. The diode between the input and the power supply and the one between the output and the power supply might turn on if a voltage higher than VCC is applied or voltage is applied when the IC is off. In this case, the IC might be destroyed by the resulting large current. Although the output is not directly connected to a power supply line, it might be connected to another bus output in a bus system.

Equivalent input/output circuit for a CMOS logic IC without input-tolerant and power-down protection functions

Correspondence list by input tolerant function/output power down function by series
The range of voltage that can be applied to an IC is from 0 V to the maximum operating voltage even when an input-tolerant or output power-down protection function is available.

Type	Series Name	Series Number	One-Gate Logic Equivalent	Operating Voltage Range V_CC (V)	Propagation Delay Time t_pLH, t_pHL (ns)^*1	*Output Current (mA)^2**	Input-Tolerant Function	Output Power-Down Protection
For 5-V Systems	Standard	TC40xxB TC45xxB	TC4S TC4W	3 to 18	200 ( at 5.0V)	±0.51 (at 5.0 V)	△^*3	-
	High Speed	TC74HC	TC7S TC7W	2 to 6	23 ( at 4.5V)	±4.0 or 6.0 (at 4.5 V) (±2.0 in the case of TC7S）	△^*3	-
	High Speed	TC74HCT	TC7WT	4.5 to 5.5	28 ( at 4.5V)	±4.0 or 6.0 (at 4.5 V) (±2.0 in the case of TC7S）	-	-
	Advanced	TC74AC		2 to 5.5	8.5 ( at 4.5V)	±24 (at 4.5 V)	-	-
	Advanced	TC74ACT		4.5 to 5.5	9.0 ( at 5.0V)	±24 (at 4.5 V)	-	-
	Very High Speed	TC74VHC 74VHC	TC7SH TC7WH	2 to 5.5	8.5 ( at 5.0V)	±8.0 (at 4.5 V)	✔	-
		TC74VHC9 74VHC9	TC7SH TC7WH	2 to 5.5	10 ( at 5.0V)		✔	-
		TC74VHCT 74VHCT	TC7SET	4.5 to 5.5	9.5 ( at 5.0V)		✔	✔^*4
		TC74VHCV 74VHCV		1.8 to 5.5	8.5 ( at 5.0V)	±16 (at 4.5 V)	✔	✔
For low-voltage systems	Medium Speed	TC74LCX 74LCX	TC7SZ TC7PZ TC7WZ	1.65 to 3.6 (1.65 to 5.5 in the case of TC7SZ/PZ/WZ)	6.5 ( at 3.3V)	±24 (at 3.0 V)	✔	✔^*5
For low-voltage systems	High Speed	TC74VCX	7UL	1.2 to 3.6 (0.9 to 3.6 in the case of 7UL)	4.4 ( at 3.3V)	±24 (at 3.0 V) (±8.0 in the case of 7UL）	✔	✔

*1: Maximum propagation delay time of typical ICs (TC4001, TC74HC244, TC74AC244, TC74VHC244, TC74LCX244, and TC74VCX244) at 85°C, except for the TC4001 for which the maximum propagation delay time at 25°C is shown
*2: Those output current values are from the Operating Ranges table of the datasheet. Absolute maximum rated output current is specified separately.
*3: TC4049BF/BP, TC4050BF/BP, TC74HC4049BP/BF/BFT, 74HC4049D, TC74HC4050BP/BF/BFT and 74HC4050D have the input-tolerant function that allows level shifting from a higher voltage to a lower voltage.　　
*4: The TC7SET series does not have an output power-down protection function.
*5: The ICs of the TC7SZ series in the fSV package and TC7SZU04/TC7WZU04 do not have an output power-down protection function except those with an open-drain output.

Care should be taken as to the output-tolerant function.
Only the IC series with output power-down protection provide an output-tolerant function (that allows a voltage in the permissible range to be externally applied to the output). Even in the case of such series, the output-tolerant function is supported only by the logic functions having Output Disabled mode (i.e., a mode in which the output assumes the High-Z state regardless of the input). In other words, the output-tolerant function is available with ICs having a three-state output (such as the 74LCX244 and 74LCX373).

List of ICs with an output-tolerant function

Level shifting using an input-tolerant function:
The input-tolerant function can be used to step down the High-level voltage. For example, let’s consider how to step down an output voltage of a logic IC of the HC series from 5 V to 3.3 V using an IC of the VHC series.
For down translation, care should be exercised as to the following characteristics. (The following values are for the VHS series.)

Input voltage (shown in the Operating Ranges table) VIN(max): 5.5 V
Maximum Low-level voltage of the preceding IC and Low-level input voltage of the level-shifting IC (shown in the Electrical Characteristics table),
VIL(max) : V_CC x 0.3 V
Use the stepped-down voltage (3.3 V in this example) as the supply voltage for the level-shifting CMOS logic IC.

The maximum input voltage must not exceed the value shown in the Operating Ranges table of the datasheet. In addition, the maximum Low-level input voltage must be lower than value shown in the Electrical Characteristics table (VIL).
As long as you pay attention to the above characteristics, you can easily level-shift a logic signal as shown in the following figure.
Note, however, that the threshold voltages of 5-V and 3.3-V ICs are different. Therefore, the duty cycle of the logic signal might change as a result of level-shifting.
If this is a concern, use Toshiba’s dual-supply level shifter that provides level-shifting without affecting a duty cycle.