To ensure the reliability of a device that is drawing power, there is a maximum safe junction temperature (Tj) that should not be exceeded during the lifespan of the part. 

The standard for this maximum temperature is around 150 C, and devices expected to source/sink large amounts of current, or operate in high temperature environments need to have this maximum temperature considered. 

The ability for a device to dissipate heat is characterized by its thermal resistance to difference interfaces. These interfaces are the ambient air (Rth ja), the PCB (Rth jb) and the package or case (Rth jc). These values are measured in degrees celsius per watt (C/W). These values can help understand the limits of safe operation for a device for given conditions.

Our C8051F50x devices do not include this value in the datasheet, but we have characterized these values for the typical case. These are listed below. Note that these values are not guaranteed maximums, and only estimate the values across devices.