Heat Matters in Electronic Design. A Temperature Sensor From ADI Emphasizes Why
Components like Analog Devices' new digital temperature sensor can springboard us into a discussion on thermal issues—whether they appear in power-supply, sensor, or motor-drive circuits.
The new ADT7422 from Analog Devices offers error as low as ±0.1°C, but thermal response time—which we’ll discuss in this article—is an important part of maximizing measurement accuracy.
Thermal Design for Electrical Engineers
Components such as the ADT7422 provide a good opportunity for us to remember the importance of thermal issues in electronic design.
Though we may have to rely on mechanical or thermal engineers for complex analyses and simulations, we should be able to handle the more basic issues, which do in fact arise frequently in the design of power-supply, sensor, and motor-drive circuits.
You can find information on important thermal topics in AAC’s articles on IC packages with exposed thermal pads, PCB thermal management techniques, heat-sinking for transistors, and thermal design for linear regulators.
In this article, I want to briefly discuss self-heating and response time.
This thermal image shows local variations in PCB temperature. Image used courtesy of Gareth Halfacree. [CC BY-SA 2.0]
Ambient Temperature vs. IC Temperature
Reducing Temperature Differential
In many applications, the goal of a temperature sensor IC is to measure the temperature of the surrounding environment. This is a bit more difficult than it sounds, because a functional integrated circuit is dissipating power in the form of heat, and this heat affects the temperature of the IC and of the air immediately surrounding the IC.
Thus, an important aspect of temperature-sensor design is minimizing the difference between the local temperature and the ambient temperature.
Part of this task is the business of the IC designers. They can help us out by creating temperature sensors that are highly efficient and that can effectively transfer heat from the interior of the chip to the PCB.
Board designers, however, must also incorporate adequate pathways for heat dissipation. The datasheet of ADT7422 might offer some recommendations for PCB layout practices that will ensure good thermal performance, though I can’t remember if this is common with sensor ICs.
In any case, though, you can use standard layout practices—thermal vias, large copper areas, careful stencil design for exposed-pad packages—to help move heat from the sensor to the circuit board.
The power-supply portion of this PCB layout demonstrates the use of thermal vias and generously sized copper pours.
Improving Response Time
Facilitating heat dissipation into the circuit board reduces the IC-to-PCB temperature differential. But the goal is to reduce the IC-to-ambient temperature differential, and thus we must consider the physical characteristics of the PCB as well.
A PCB with a relatively large amount of thermal mass will tend to maintain its current temperature despite changes in ambient temperature. This is where response time comes into play.
Even if we manage to achieve a very low error under steady-state temperature conditions, slow response time will result in poor accuracy if the ambient temperature changes quickly (I’m assuming here that we expect our sensor to capture these changes—if you’re interested only in average temperatures, response time is not a major concern).
The ADT7422 datasheet clearly indicates the solution to slow response time: “mount the sensor on the smallest PCB possible.” If the thermal mass of a large PCB is making your measurements less responsive to short-duration temperature variations, you could consider creating a separate, smaller PCB for the sensor and transferring measurement data via board-to-board cabling.
Temperature Sensing for Medical, Commercial, and Industrial Applications
The ADT7422 offers accuracy up to ±0.10°C for temperatures from 25°C to 50°C and ±0.25°C from −20°C to +105°C; the full operational temperature range is −40°C to +125°C. No user calibration is required, and measurements are digitized with 16-bit resolution and delivered via I2C.
Efficient MCU interfacing is facilitated by two user-configurable interrupt signals: one for a “critical overtemperature” condition and one for (presumably non-critical) overtemperature and undertemperature conditions.
The ADT7422 block diagram. Image used courtesy of Analog Devices
As you can see in the schematic below, few external components are required, and if you already have some good I2C code from a previous project, incorporating the ADT7422 into a new design should be quite straightforward.
Diagram of a typical I2C interface connection. Image used courtesy of Analog Devices
Power Supply Noise
The datasheet specifically mentions that output noise generated by a switching regulator can negatively affect the ADT7422’s accuracy.
For those who want to minimize measurement error, Analog Devices recommends the use of an RC filter that can reduce the noise amplitude at the sensor’s VDD pin to less than 1 mV.
But don’t forget about response time! They also state that this filter should be located as far from the sensor as possible so that its thermal mass doesn’t diminish dynamic performance.
Do you have any techniques for maximizing the performance of temperature-sensor circuits? Let us know in the comments section below.