This section shows the different techniques for calculating the junction temperature under various operating conditions. For more details, see Semiconductor and IC Package Thermal Metrics.

There are three recommended ways to derive the junction temperature from other measured temperatures:

1. From package temperature:
   Equation 1. $T_J={\psi }_{\mathrm{JT}}\times P+T_{\mathrm{case}}$
2. From board temperature:
   Equation 2. $T_J={\psi }_{\mathrm{JB}}\times P+T_{\mathrm{board}}$
3. From ambient temperature:
   Equation 3. $T_J=R_{\mathrm{\theta JA}}\times P+T_A$

P is the power dissipated from the device and can be calculated by multiplying current consumption with supply voltage. Thermal resistance coefficients are found in Section 7.8.

Example:

Using Equation 3, the temperature difference between ambient temperature and junction temperature is calculated. In this example, we assume a simple use case where the radio is transmitting continuously at 20dBm output power. Let us assume the ambient temperature is 105 °C and the supply voltage is 3.3 V. To calculate P, we need to look up the current consumption for Tx at 105 °C in Figure 7-11. From the plot, we see that the current consumption is 68 mA. This means that P is 3.3 V × 68 mA = 224.4 mW.

The junction temperature is then calculated as:

As can be seen from the example, the junction temperature is 5.3 °C higher than the ambient temperature when running continuous Tx at 105 °C and, thus, well within the recommended operating conditions of 115 °C.

For various application use cases current consumption for other modules may have to be added to calculate the appropriate power dissipation. For example, the MCU may be running simultaneously as the radio, peripheral modules may be enabled, etc. Typically, the easiest way to find the peak current consumption, and thus the peak power dissipation in the device, is to measure as described in Measuring CC13xx and CC26xx Current Consumption.