Temperature problems solved for you (IV) Ambient temperature monitoring

In the previous three articles, we have introduced the basic principles of temperature sensors and how to monitor circuit board temperature. In many applications, ambient air temperature monitoring is critical to control environmental conditions or ensure safe operating conditions. Accurately and quickly measuring ambient temperature is often challenging because the sensor may not be fully exposed to the external environment and may be affected by self-heating of other components in the system. TI's high-precision, low-power single-channel and multi-channel temperature sensors are available in compact packages for faster thermal response.

Layout Considerations for Accurately Measuring Ambient Temperature

Measuring ambient temperature using surface mount devices can be challenging because heat transfer from other power-hungry electronic components can affect the sensor's temperature reading.

To accurately measure ambient temperature, good layout techniques must be used, such as understanding the primary thermal paths, isolating the sensor package, and placing the device away from interfering heat sources. The figure below shows a simple thermostat design that uses these techniques.

Temperature Sensor Thermostat Design Thermal Radiation and Printed Circuit Board (PCB) Layout

In the figure above, passive airflow generated by the system's self-heating draws outside air over temperature sensor A. The sensor is placed at the air intake away from the primary heat source (the central processing unit) and is thermally insulated to ensure more accurate measurements.

Heat radiation and PCB layout

You must first understand which components radiate the most heat to avoid routing near heat sources. The following figure is a thermal image captured using Mentor Graphics' FloTHERM thermal analysis tool, which shows the temperature distribution in the air near a heat source.

Heat radiated on the test board

If the component is placed inside an enclosure, the heat distribution may be more concentrated. Remember to place the temperature sensor away from heat sources to avoid erroneous temperature readings in open air scenarios and inside enclosures. The following table lists the recommended distances between the sensor and the heat source for various heat source temperatures.

If the sensor is close to a heat source, it is best to create an isolation island and maximize the air gap between the sensor and the heat source. The larger the air gap, the better the ambient temperature measurement. However, when the sensor is farther away, the gap does not provide additional shielding. However, the gap can improve the thermal response time of the sensor.

Heat dissipation gap when the slot width is 0.8mm

The upper graph shows that the temperature reading is approximately 38.5°C when the cut is 0.8mm wide, while the lower graph shows that the temperature reading is approximately 35.5°C when the cut is 1.8mm wide. These images show how a larger isolation gap affects the ambient temperature reading.

Heat dissipation gap when the slot width is 1.8mm

Isolation Island PCB Layout

When designing a PCB for a temperature sensor, it is important to employ good layout techniques. The image above shows a PCB layout with isolated islands and outline routing, while the image below shows an alternative design with vias around the area where the temperature sensor is mounted.

Perforated PCB Layout

On these two small boards, the dimensions are so small that only sensors and bypass capacitors can be deployed; the smaller the thermal mass of the isolation island, the better the thermal response. These designs greatly reduce the amount of heat transfer from other components.

Temperature difference

In applications where higher measurement accuracy is required, consider using a differential temperature design. This type of design adds an additional sensor next to the high temperature component, however, this design requires a model for the correlation between ΔT and ambient temperature, and this model will vary depending on the system application. A differential temperature design takes into account the effects of self-heating, providing a more accurate algorithm to estimate the ambient temperature.

Device Recommendations

The TMP112 and TMP116 are digital temperature sensors designed for high-precision, low-power applications such as environmental monitoring and thermostat control. The TMP112 has an accuracy of ±0.5°C over the 0°C to 65°C range, while the TMP116 has an accuracy of ±0.2°C over the -10°C to 85°C range.

Both temperature sensors feature high linearity, require no calibration, and have programmable alarm functions. The TMP112 is available in a compact 1.60mm x 1.20mm small outline transistor (SOT)-563 package, while the TMP116 is available in a 2mm x 2mm thin small outline no-lead (WSON) package.

Click here to quickly locate the TI analog column and view the latest and most comprehensive information on TI sensor products. At the same time, in the next few articles, we will focus on the design considerations of various applications, evaluate the trade-off between temperature accuracy and application size, and discuss sensor placement methods.

Temperature problems solved for you (I) Basic principles of temperature sensing

Temperature Problems Solved for You (II) System Temperature Monitoring

Temperature Problems Solved for You (III) High-Performance Processor Mold Temperature Monitoring