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.
To illustrate the VOH/VIH logic compatibility use an example case where the CD74HCT405x, with a VIH minimum equal to 3.5 V, is driven by either a 5-V CMOS or 5 V TTL driver. Figure 1-1, shows that the 5-V CMOS driver can work because the minimum VOH of 4.44 V is greater than the 3.5 V VIH minimum of multiplexer. However, the 5-V TTL driver minimum VOH of 2.4 V is less than the 3.5 V VIH minimum and might not be high enough to register a logic HIGH on the mux’s logic control inputs under certain conditions.
Alternatively, by selecting the TMUX405x, a device with 1.8-V logic compatible inputs, we can see from the following how this VOH/VIH compatibility issue is resolved and the logic threshold required to register a HIGH on the mux’s inputs is met under all conditions. See 1.8-V Logic for Multiplexers and Signal Switches application note, for additional information on 1.8-V logic compatible inputs.
Similarly, the low-level logic compatibility can be determined the same way through an understanding of the system and a side by side comparison of the VOL/VIL thresholds. To examine the VIL thresholds of the multiplexer, we will first need to discuss the VOL output voltage that comes from the driver. The VOL voltage is the voltage that is output by the driver when a low logic state is the desired outcome. Typically, there will be some maximum value provided in the driver’s data sheet and will be associated with certain conditions to achieve these output levels (for example, current). The voltage driven to produce a logic Low is ideally close to 0 V, but due to leakage currents, 0 V won’t necessarily be achieved. The leakage and actual voltage level on the node will be determined based on the architecture of the output driver. In a well-designed system, this VOL level will be close to 0 V varying only by mV’s. Because of this, often times system designers will overlook the VIL threshold and mainly focus on the VOH/VIH levels instead with the understanding that the VIL threshold compatibility will not play a significant role when it comes to the overall logic compatibility of the device in most scenarios.
To illustrate the VOL/VIL logic compatibility, use the TMUX405x with 1.8 V logic compatible feature with the minimum VIL equal to 0.8 V driven by a 5-V CMOS or 5 V TTL driver. Figure 1-2 shows that both the 5-V CMOS and 5-V TTL driver works because the maximum 0.5 V VOH is less than the maximum 0.8 V VIL. The TMUX405x read this as a low input, indicating that the old driver used with the existing CD405xB or CD74HCx405x are logic-low compatible with the new TMUX405x
Figure 2-1 shows the typical standard logic output levels based on the different architecture and supply voltage levels. Now that you know how to evaluate the VOH/VOL and VIH/VOH logic compatibility, you can see the TMUX405x supports all four logic standards. The CD74HC405x, CD74HCT405x, and CD405xB do not support the 1.8-V CMOS drivers.
Device | Configuration | Key Features |
---|---|---|
TMUX4051 | 1-channel 8:1 | ±12-V signal range, 1.8-V compatible logic |
TMUX4052 | 2-channel 4:1 | |
TMUX4053 | 3-channel 2:1 | |
CD4051B | 1-channel 8:1 | ±10-V signal range, CMOS compatible inputs |
CD4052B | 2-channel 4:1 | |
CD4053B | 3-channel 2:1 | |
CD74HC4051 | 1-channel 8:1 | ±5-V signal range, CMOS compatible inputs |
CD74HC4052 | 2-channel 4:1 | |
CD74HC4053 | 3-channel 2:1 | |
CD74HCT4051 | 1-channel 8:1 | ±5-V signal range, CMOS, and TTL compatible inputs |
CD74HCT4052 | 2-channel 4:1 | |
CD74HCT4053 | 3-channel 2:1 |
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