Electronic – What 7400-series logic variants include input clamp diodes

7400esdlevel-shifting

When prototyping, I often use spare 7400-series chips I have lying around. I have a good variety of chips, including many 74HC and 74AHC parts, as well as older varieties like 74LS.

From time to time I like to implement "quick and dirty" 5V to 3V3 level conversion, and one easy way to do this is with by interfacing a 5V signal to a 3V3-powered chip with a current-limiting resistor. The idea here is that the "input protection" clamp diodes present on the inputs of a logic chip will conduct any excess current on the input to VDD, limiting the input voltage to the supply voltage (3V3), accomplishing 5V-to-3V3 conversion on the input with only a single resistor.

I've looked around the web, and I haven't been able to find any resource indicating what 7400-series variants include these diodes on their inputs. Of course I can look at individual datasheets, but I have a suspicion that certain variants (e.g. 74HC) usually include them, while other variants (e.g. 74AHC) may not. (these just are made-up examples)

Does anyone know what the general situation is, or if there is any definitive answer for this? And does it depend on the manufacturer?

Best Answer

NXP's Logic selection guide says (p. 18):

Devices with overvoltage-tolerant inputs do not have input clamp diodes on digital inputs, and can be used to interface to higher-voltage systems without using external current-limiting resistors.

The following families, in alphabetical order, have digital inputs that are overvoltage-tolerant and can be used to interface with subsystems operating at a higher supply voltage: AHC, ALVC, ALVT, AUP, AVC, AXP, CBTLV(D), LVC, and LVT.

over-voltage-tolerant CMOS input

(So AC(T) and HC(T) do have ESD diodes to VCC.)

TI's Logic Guide, Toshiba's Logic ICs catalog, and Fairchild's Logic Selection Guide also mention voltage tolerance or downtranslation as features of these families.

TI's application report Input and Output Characteristics of Digital Integrated Circuits at 3.3-V Supply Voltage shows what actually happens when you go above VCC (apparently, (A)LVC have Zener diodes that clamp at a higher voltage).

For devices with ESD diodes, TI's application report 3.3 V I/O Considerations for Hercules™ Safety MCUs in Automotive and Industrial Environments suggests to use not a simple resistor but a voltage divider (section 3.2).

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