Where there is power there is heat, and where there is heat there is often a need to sense temperature (the most widely sensed physical variable). What we call temperature is our measurement of a material’s thermal energy, and there are many sensors available to measure it ranging from very low cost and limited range to sophisticated and specialized units. In some cases, the decision as to which sensor to use is difficult because there are so many viable options, while other times only one or a few will make the cut, so to speak. Not surprisingly, what to use is a function of the expected temperature itself (high, low, and span), required accuracy and resolution, cost (of course), and other factors.
Basic diodes and the diode junction of transistors have been used for temperature sensing since the development of the physics-based relationship among temperature, voltage, and current of the junction (Reference 1). Think back to your introduction to semiconductor devices, and you’ll hopefully remember this exponential graph (Figure 1).
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| Figure 1. | The basic diode-junction current/voltage curve versus temperature is highly nonlinear and can be a hindrance or used as a positive effect. |
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It makes clear the classic equations for a forward-biased diode:
where IS is the reverse saturation current, V is diode’s forward voltage drop, η is an ideality factor (a constant between 1 to 2), and VT is the thermal voltage of diode which, in turn, is given by:
where T is the absolute junction temperature in Kelvin, q is the electron charge, and k is the Boltzmann’s constant.
You may be thinking: I already know this, so enough of the physics, please. On the other hand, if this is unfamiliar to you, it would be a good idea to go online for a brief refresher or tutorial.
This temperature dependency of the diode junction is both a curse and blessing. It has a severe impact on the basic performance of the semiconductor device as currents and voltages change, of course, and temperature coefficient (tempco) is a carefully studied data-sheet number. IC designers resort to many clever topologies to minimize its effect or, even better, work out schemes so the changes it provokes will cancel themselves out.
While this temperature sensitivity is a hindrance to the performance of discrete devices and ICs, it can also be leveraged for temperature sensing. Many analog and digital devices use a basic on-chip junction to sense their own die temperature and even invoke shutdown if the die gets too hot. This eliminates the need for a separate sensor and is a cost-effective solution to self-monitoring.
However, when you want to interface with several external diodes used as sensors, the interface can get complicated with respect to multiplexing and A/D conversion. Fortunately, IC vendors have recognized the challenges of using multiple diodes and have created some unique interfaces for use with these sensors. That is what intrigued me about the recently released EMC1812 family of low-voltage diode-sensor ICs from Microchip Technology (Figure 2). Depending on the specific family member selected, these ICs handle between one and four external temperature-sensing diodes plus an on-chip sensing diode.
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| Figure 2. | The Microchip Technology EMC1812 family provides more than just the analog interface to one or more diodes as temperature sensors; it also includes digitization, processor interface, and some basic data analysis to offload the processor. |
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The ICs in this family do much more than just provide basic diode interface and digitization with an SMBus/I2C-compatible interface. They can implement a temperature rate-of-change calculation and then provide preemptive alerts if that rate exceeds user-set limits. They also improve the diode performance as a temperature sensor by including a resistance error-correction feature which automatically eliminates the temperature error caused by series resistance, providing greater flexibility in routing of thermal diodes wiring. They also incorporate beta compensation to eliminate temperature errors caused by low and variable beta of transistors which are common in current fine-geometry processors; and they determine the optimal sensor external diode/transistor settings.
An IC such as this one transforms the diode junction, used as a low-cost but challenging temperature sensor, which needs a significant level of analog and digital I/O support. Instead, the diode is far easier to interface, while reducing the need for the system processor to constantly evaluate readings, check alarm conditions, and more. It’s yet another example of how an interface IC can modernize the use of an old sensor into one which is compatible with today's I/O and processor needs.
