The most common measure of package thermal performance is thermal impedance (θ_JA) measured (or modeled) from the chip junction to the air surrounding the package surface (ambient). The mathematical expression for θ_JA is:

Where:

T_J = chip junction temperature
T = ambient temperature
P = device power dissipation

Factors that can influence the measurement and calculation of θ_JA include:

• Whether or not the device is board mounted
• Trace size, composition, thickness, and geometry
• Orientation of the device (horizontal or vertical)
• Volume of the ambient air surrounding the device under test and airflow
• Whether other surfaces are in close proximity to the device being tested

Due to the charge profile of supercapacitors, maximum power dissipation is typically seen at the beginning of the charge cycle when the voltage is at its lowest.

Device power dissipation, P, is a function of the charge rate and the voltage drop across the internal PowerFET. P can be calculated from the following equation during charging:

The thermal loop feature reduces the charge current to limit excessive IC junction temperature. It is recommended that the design not run in thermal regulation for typical operating conditions (nominal input voltage and nominal ambient temperatures) and use the feature for nontypical situations such as hot environments or higher than normal input source voltage. With that said, the IC will still perform as described, if the thermal loop is always active.