The following figure shows level shifting from 5 V to 3.3 V and from 3.3 V to 5 V.
Down translation (High-to-low voltage level translation):
The input-tolerant function allows level shifting from a higher voltage to a lower voltage such as from 5 V to 3.3 V. For example, the input-tolerant function allows 5 V to be applied to an input without drawing extra current even when VCC = 0 V.
For down translation using the input-tolerant function of a CMOS logic IC, ensure that the maximum low-level input voltage is within the VIL range.
Up translation (Low-to-High voltage level translation):
Level shifting from a lower voltage to a higher voltage such as from 3.3 V to 5 V can be performed if the high-level output voltage (VOH) of the driving IC satisfies the high-level input voltage (VIH) of the driven IC because VOH is almost equal to the supply voltage (VCC).
Since the minimum high-level input voltage (VIH) of typical CMOS logic ICs is specified as VCC × 0.7, VIH is 3.5 V when VCC = 5 V. Therefore, if the VOH of the driving IC is 3.3 V, the minimum VIH of the driven IC is not satisfied. In this case, consider using a CMOS logic IC with a TTL input.
The following shows the specifications of the TTL input of the 74HCT240. Since its minimum high-level input voltage (VIH) is 2.0 V, the 74HCT240 allows up translation from 3.3 V to 5 V
The 74VHCT240 cannot be used if the supply voltage of the preceding IC is lower than 2 V. In that case, it is necessary to use a level shifter.
There is also a method of Up translation using a product with open drain output. Due to the pull-up resistor Rpull, the output swings to the VCC level.
The following documents also contain related information:
Application notes
CMOS Logic ICs Selection Guide
Basics of Bus Switches
CMOS Logic ICs Usage Considerations
Basics of CMOS Logic ICs
Applications of Low-Voltage One-Gate Logic ICs to Level Shift Circuits
CMOS Logic Ics TC74VCX/LCX Series
CMOS Logic Ics TC74VHC/VHCT/VHC9/VHCV Series
Glossary of CMOS Logic Ics
FAQs
What are the differences among the general-purpose logic IC series?
Tell me about tolerant functions.
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What options do I have to perform level shifting from 1.8 V to 2.5 V and vice versa?
What is the difference between level-shifting bus switches and bus buffers?
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CMOS Logic ICs with input tolerant function VHC / VHCT / VHCV / VCX / LCX
One-Gate Logic ICs (L-MOS) with input tolerant function TC7SH/WH / TC7SET / TC7SZ/PZ/WZ / 7UL
CMOS Logic ICs with TTL input ACT / HCT / VHCT
CMOS Logic ICs with Open-Drain output
One-Gate Logic ICs (L-MOS) with TTL input TC7WT / TC7SET
Dual Supply Level Shifters
Dual Supply Bus Switch
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