Although the aforementioned temperature switch designs are viable methods of implementing temperature threshold detection, the size and cost can be reduced, and accuracy can be improved. The TMP390 is an ultra-low power, dual-channel, resistor programmable temperature switch that is able to operate in temperatures ranging from –50°C to 130°C. This device functions identically to the previously-designed temperature switches, but comes in a smaller package and reduced cost, and also has improved accuracy for systems that require high accuracy. For example, Figure 1-8 shows the design of the circuit needed to provide temperature threshold detection when using the TMP390. All specifications are nearly identical as before: a hot threshold of 80°C with 10°C of hysteresis and now a cold threshold of –50°C with 10°C of hysteresis, which is a range of 10°C more than what is offered by the discrete solutions.

Note that there are far fewer components when compared to the discrete solution; this results in a much smaller layout area of 26.2 mm², as shown in Figure 1-9. At temperatures of 25°C, 85°C, and –10°C, this design results in an approximate supply quiescent current of 0.5 μA, which is considerably lower than the currents of the discrete solutions.

Figure 1-9 Temperature Switch Layout Using the TMP390