JAJSEA7B September 2017 – February 2020 TMP461-SP
PRODUCTION DATA.
The local temperature sensor inside the TMP461-SP device monitors the ambient air around the device. The thermal time constant for the TMP461-SP device is approximately two seconds. This constant implies that if the ambient air changes quickly by 100°C, then the TMP461-SP device takes approximately 10 seconds (that is, five thermal time constants) to settle to within 1°C of the final value. In most applications, the TMP461-SP package is in electrical, and therefore 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 TMP461-SP is measuring. Additionally, the internal power dissipation of the TMP461-SP 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. Equation 6 can be used to calculate the average conversion current for power dissipation and self-heating based on the number of conversions per second and temperature sensor channel enabled. Equation 7 shows an example with local and remote sensor channels enabled and 16 conversions per second; see the Electrical Characteristics table for typical values required for these calculations. For a 3.3-V supply and a conversion rate of 16 conversions per second, the TMP461-SP device dissipates 0.531 mW (PDIQ = 3.3 V × 161 μA) when both the remote and local channels are enabled.
The temperature measurement accuracy of the TMP461-SP device depends on the remote and local temperature sensor being at the same temperature as the system point being monitored. If the temperature sensor is not in good thermal contact with the part of the system being monitored, then there is a delay between the sensor response and the system changing temperature. This delay is usually not a concern for remote temperature-sensing applications that use a substrate transistor (or a small, SOT23 transistor) placed close to the device being monitored.