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Texas Instruments Ambient Temperature Measurement Layout Considerations Application notes
Layout Considerations for Accurately Measuring Ambient
Temperature
Introduction
Measuring ambient temperature with a surface mount
device can be challenging due to the heat transfer
from other power-hungry electronic components that
can influence the temperature reading of the sensor.
To accurately measure ambient temperature, it is
important to follow good layout techniques, such as
understanding the dominant thermal path, isolating the
sensor package, and placing the device away from
interfering heat sources. Figure 1 shows a simple
thermostat design using these techniques.
Table 1 lists the recommended distances to place the
sensor for various heat source temperatures.
Table 1. Recommended Distance Away From Heat
Source
HEAT SOURCE
TEMPERATURE
AMBIENT
TEMPERATURE
RECOMMENDED
DISTANCE
40 °C
20 °C
7.62 mm
60 °C
20 °C
15.24 mm
100 °C
20 °C
38.1 mm
The passive air flow from the self-heating of the
system draws external air over temperature sensor A.
The sensor is placed at the intake vent away from the
main heat source—the CPU—and is thermally isolated
for a more accurate measurement.
Figure 2. Heat Radiated Across Test Board
Figure 1. Temperature Sensor Thermostat Design
Heat Radiation and PCB Layouts
It is important to know which components radiate the
most heat first so that routing traces near the heat
sources can be avoided. Figure 2 shows a thermal
image captured with the FloTHERM thermal analysis
tool that shows the temperature distribution through
the air from the heat source.
If the sensor is close to the heat source, it is good
practice to create an isolation island and maximize the
air gap between the sensor and heat source. The
bigger the air gap, the better the ambient temperature
measurement. When the sensor is further away,
however, the gap does not provide additional
shielding. The gap may improve the thermal response
time of the sensor, though.
Figure 3 shows a 0.8-mm wide cutout with a
temperature reading of about 38.5°C, and Figure 4
shows a 1.8-mm wide cutout with a temperature
reading of about 35.5°C. These images show how a
larger isolation gap affects the ambient temperature
reading.
If components are placed within an enclosure, the heat
distribution may be more condensed. Remember that
the temperature sensor should be placed away from
the heat source to avoid erroneous temperature
readings in both open-air setups and enclosures.
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Layout Considerations for Accurately Measuring Ambient Temperature
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Figure 3. Thermal Air Gap With 0.8-mm Slot Width
Figure 6. Perforated PCB Layout
Differential Temperature
In applications requiring more precise temperature
measurement, users can consider a differential
temperature design. In this type of design, additional
temperature sensors are added next to hot
components as shown in Figure 1, and the difference
in temperature between temperature sensors A and B
is measured.
Figure 4. Thermal Air Gap With 1.8-mm Slot Width
When designing the PCB for the temperature sensor, it
is important to follow good layout techniques. Figure 5
shows a PCB layout with an isolation island and
contour routing, while Figure 6 shows an alternative
design that incorporates perforations around the
section with the temperature sensor.
On both mini-boards, the dimensions are small enough
to allocate only the sensor and bypass capacitor—the
smaller the thermal mass of the island, the better the
thermal response. These designs greatly minimize the
amount of heat transfer from other components.
A model of how the ΔT correlates to ambient
temperature is required, however, and this will vary
depending on the system application. This design
factors in the impact of self-heating to provide a more
accurate algorithm to estimate the ambient
temperature.
Device Recommendations
The TMP112 and TMP116 are digital temperature
sensors designed for high-accuracy, low-power
applications, such as environmental monitoring and
thermostat controls. The TMP112 offers an accuracy
of ±0.5°C from 0°C to 65°C, whereas the TMP116
offers an accuracy of ±0.2°C from –10°C to 85°C.
Both temperature sensors are highly linear, require
zero calibration, and have programmable alert
functionality. The TMP112 features a compact 1.60mm × 1.20-mm SOT563 package, while the TMP116
features a 2.00-mm × 2.00-mm WSON package.
To ensure optimal performance and device longevity,
additional layout recommendations are discussed in
the collateral listed in Table 2.
Table 2. Recommended Collateral
COLLATERAL
DESCRIPTION
Application Report
TMP116 Ambient Air Temperature
Measurement
Application Report
Precise Temperature Measurements with
TMP116
Application Report
Temperature Sensors: PCB Guidelines for
Surface Mount Devices
Figure 5. Isolation Island PCB Layout
2
Layout Considerations for Accurately Measuring Ambient Temperature
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