T_J is the average temperature of the semiconductor die when the converter is operating. The power dissipated within the regulator will cause the junction temperature to rise above the ambient within which it is operating. The thermal design must keep this temperature below T_J-max. The maximum safe junction temperature is fixed at the time that the regulator is designed, and cannot be changed. Considerations of current density in the metallization on the IC die, place limitations on the maximum junction temperature. Higher temperatures require heftier metal interconnects and therefore a larger and more expensive regulator. Exceeding T_J-max will usually not cause damage, since most regulators incorporate thermal shutdown protection to shut off the converter when the temperature exceeds T_J-max. The user must decide how far below the T_J-max specification is reasonable for the application. Many companies have general guidelines for the maximum junction temperature they allow for a given circumstance; and of course these must be followed. The guideline is simple: the lower the junction temperature the higher the reliability of the device. Calculations of reliability versus time and temperature are possible, but beyond the scope of this discussion. Conversely, selecting too low a temperature will limit the maximum ambient temperature or the power dissipation that can be achieved in the application; see Equation 1.