9.2.2 Detailed Design Procedure

The temperature measurement accuracy of the TMP43x depends on the remote and local temperature sensor being at the same temperature as the system point being monitored. Clearly, if the temperature sensor is not in good thermal contact with the part of the system being monitored, then there will be a delay in the response of the sensor to a temperature change in the system. For remote temperature sensing applications that use a substrate transistor (or a small, SOT23 transistor) placed close to the device being monitored, this delay is usually not a concern.

The local temperature sensor inside the TMP43x monitors the ambient air around the device. The thermal time constant for the TMP43x is approximately 2 s. This constant implies that if the ambient air changes quickly by 100°C, it would take the TMP43x about 10 seconds (that is, five thermal time constants) to settle to within 1°C of the final value. In most applications, the TMP43x package is in thermal contact with the printed circuit board (PCB), as well as subjected to forced airflow. The accuracy of the measured temperature directly depends on how accurately the PCB and forced airflow temperatures represent the temperature that the TMP43x is measuring. Additionally, the internal power dissipation of the TMP43x can cause the temperature to rise above the ambient or PCB temperature. The internal power dissipated as a result of exciting the remote temperature sensor is negligible because of the small currents used. For a 5.5-V supply and maximum conversion rate of eight conversions per second, the TMP43x dissipate 1.82 mW (PD_IQ = 5.5 V × 330 μA). If both the ALERT/THERM2 and THERM pins are each sinking 1 mA, an additional 0.8 mW is dissipated (PD_OUT = 1 mA × 0.4 V + 1 mA × 0.4 V = 0.8 mW). Total power dissipation is then 2.62 mW (PD_IQ + PD_OUT) and, with an θ_JA of 150°C/W, causes the junction temperature to rise approximately 0.393°C above the ambient.