DATASHEET

SMT172

last update

maart 20, 2017

reference

V14

page

7

thermal conducting property of single crystalline silicon, we can assume the temperature difference within

the sensor die to be negligible. However the thermal property of the package material, the shape and the

size of soldering pads, the neighbouring components on the PCB as well as the presence of dedicated

thermal sinks are all affecting the die temperature that the sensor is measuring. Therefore a good thermal

path between the die and the object under measurement should be carefully designed and considered.

When measuring temperature of solid or liquid targets, it helps to have a good thermal contact between the

sensor and the target. This can be achieved with metals and thermal paste. When measuring air

temperatures, it is important to isolate the sensor from the rest of the measurement system, so that the

heating from the surrounding circuit components has only a small influence on the sensor temperature.

Self-Heating

All electronic circuits consume power, and all power becomes heat. Depending on the thermal resistance

to the environment and the related thermal mass on the heat path, this heat will cause an extra

temperature rise of the sensor die and will influence the final reading. Although the ultra-low power

consumption of SMT172 sensor minimizes this effect greatly, it is always important to take this into account

when designing a temperature measurement system. Design considerations like optimal thermal contact

with the environment and powering down the sensors whenever possible (see SMTAS08) are all useful

techniques to minimize this effect.

Thermal response time

The thermal response time of the temperature sensor is determined by both the thermal conductance and

the thermal mass between the heat source and the sensor die. Depending on the packaging material and

the immerging substances, this can vary in a wide range from sub-second to hundreds of seconds. The

following table illustrates the time constant (the time required to reach 63% of an instantaneous

temperature change) of TO-18 packaged sensors.

Conditions of installation

Time constant (s)

(TO-18)

In an aluminium block

0.6

In a bath filled with oil that is stirred

constantly

1.4

In air that moves at 3m/s:

-

Without heat sink

-

With heat sink

13.5

5

In non-moving air:

-

Without heat sink

-

With heat sink

60

100