The TMUX405x family of multiplexers are pin to pin to with the CD405xB and CD74HCx405x families. The multiplexers support 24 V single-sided supply or ± 12 V in dual-sided supply and feature 1.8 V logic compatibility, fail-safe logic, and the industries smallest (SOT-23) package.
Seeing the performance and features of the TMUX405x family, many designers will add the TMUX405x to enhance their systems currently using CD405xB and CD74HCx405x. When designing the new device into the system, one considerations that must be made is the logic compatibility between the previous and new devices. Legacy CD405xB and CD74HCx405x devices will typically require legacy Transistor-Transistor Logic (TTL) or CMOS logic drivers to control the logic inputs of the mux. Due to the architecture of these drivers, they will each have different output voltage levels which impacts how devices interact with these drivers. This application note's objective is to expand on this logic compatibility and show how to evaluate whether the TMUX405x 1.8 V logic compatibility feature is compatible with the legacy TTL or CMOS logic drivers currently in use.
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The fundamental concept of the threshold levels (VIH, VIL, VOH, VOL) are expanded on in this TI Precision Labs training video - Output/input logic levels (VOH, VOL, VIH, VIL). The following information builds upon the basic understanding of these thresholds based on the information provided in this video.
To check the compatibility between any device’s logic inputs and the pre-existing driver, a simple comparison of the VOH and VOL levels of the driver versus the VIH and VIL thresholds of the multiplexer in question can be done. The following is a more in-depth explanation on how to check the VIH and VIL compatibility of your TMUX405x when used in a system that has a pre-existing CD405x or CD74HCx405x designed in.