AN607

Rev. 1.7

39

thickness) and the specific heat capacity of PCB material is 0.6 J/(g-C), so the heat capacity is 1.5 J/C.

The time constant R1 x C2 is 1.5 J/C x 55.5 C–second/J = 83 seconds.

This time constant is independent of the PCB area—more area means lower thermal impedance but higher

thermal mass. To improve the time constant beyond this thinner PCB material would have to be used or there

would need to be fins or airflow to reduce the thermal impedance.

Turning our attention to R2 the thermal conductivity of FR4 material in plane is around 1Watt/meter-C. So, for the

example of the connection of the sensor area to the rest of the system by 1cm wide 3cm long 1.5mm thick FR4:

Or thermal impedance (1/conductance) is 2000 °C/W (this is R2). This is assuming minimal copper routing on the

connector material.

With this design since R2 is 36 times R1, so the system heating will have a fairly minor effect. For example if the

system heating is 10 °C the sensor temperature would only increase 0.3 °C.

If more thermal connection is tolerable the connector area could be made shorter or wider or the PCB area the

sensor is connector to could be made smaller.

This example was intended to illustrate the thermal design considerations for good response to ambient conditions

and insulation from the system. In some cases, it is not possible to place the sensor in sufficient thermal contact

with the environment to shield it for the thermal mass and heat sources in the system. In this case, it is often

possible to compensate for the system by placing an additional temperature sensor in the system. However, in all

cases, the thermal contact of the sensor to the environment should be maximized, and the thermal contact of the

sensor to the rest of the system should be minimized.