Overcome last-minute requirement changes with small, leaded SOT-23 thin multiplexers

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Overcome last-minute requirement changes with small, leaded SOT-23 thin multiplexers [Copy link]

We’ve all been there—a belated change in requirements throws your design into disarray. There isn’t enough time to make design changes, and the multiplexer options are slim. There are countless last-minute changes that can happen, but one problem I often encounter when working with designers is how to monitor the increased number of nodes after a microcontroller has been selected, as shown in Figure 1. In this case, the biggest challenge we face is the lack of available board space to fit additional multiplexers.

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Figure 1 : General-purpose input / output (GPIO) expansion with 8:1 multiplexer

Fortunately, a relatively simple solution is available in the form of a small 8:1 multiplexer, such as the TMUX1308.

When you think of small-footprint multiplexers, you might think the only option is a device in a quad flat no-lead (QFN) package. But there is another option: a multiplexer in a small-outline transistor (SOT)-23 package.

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Figure 2 : TI 16- pin package size comparison

Figure 2 compares the dimensions of regular 16-pin packages, and you’ll notice that the thin SOT-23 is a leaded package that is half the size of the thin small outline package (TSSOP) solution used in most designs today. You can easily replace a 16-pin TSSOP with two 16-pin SOT-23 thin devices and retain the ability to lay out in a similar area. The SOT-23 thin package also uses a 0.5mm pitch, a widely accepted manufacturing design rule, and is easy to hand solder. If you want to increase board density but require a leaded package, the SOT-23 package is a good choice.

To learn more about the advantages of small package size, read the technical white paper “Designing Compact Signal Chains with High Performance in Small Spaces.”

The SOT-23 multiplexer can also help you handle last-minute requirement changes when you need to add new system functionality late in the design process. The example in Figure 3 shows that the battery monitoring circuit selection is locked and all GPIOs are used to measure multiple negative temperature coefficient thermistors (NTCs) around the system. Late in the design, the designer wants to add a feature to store battery life information from the battery monitor in an electrically erasable programmable read-only memory (EEPROM).

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Figure 3 : Battery management circuit multiplexing between EEPROM and NTC

In this case, the lead designer is interested in using the SOT-23 package but is unsure if it will be available in time for the company to approve it. I suggest that they use a multiplexer such as the SN3257-Q1 or TMUX1574, which are available in both TSSOP and SOT-23 low-profile package options. As shown in Figure 4, since the SOT-23 low-profile package can fit inside the TSSOP package, they can place dual packages on their printed circuit board (PCB) and reduce the risk of the SOT-23 package not being approved while continuing to use the TSSOP package as a backup package. For more details on dual-sourcing PCB layout, see the article "Secondary Sourcing Options for Small Package Amplifiers" in Analog Design Journal.

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Figure 4 : Dual package layout; 16 -lead SOT-23 thin package inside a 16 -lead TSSOP package

When designing a system, it is inevitable that challenges will arise at the last minute. The components that were used to solve these problems in the past are now available in smaller package options. Smaller package options offer the size advantages of QFNs and the mechanical advantages of leaded packages. Don’t forget to include the SOT-23 thin package in your list of approved components ahead of time; it will give you another tool to accommodate last-minute changes in future designs.