Thermal Design Power
The thermal design power is the maximum amount of power the thermal solution is required to dissipate. The thermal solution must be designed to dissipate the TDP (thermal design power) without exceeding the maximum Tjunction specification.
CPU vendors provide the values TDPmax and TDPtyp to the designers.
TDPmax represents the total power dissipation of the processor while executing a worst-case instruction mix at nominal voltages and normal operating conditions.
You can also calculate the TDPmax with the following equation, if the CPU vendor does not want to provide the higher TDPmax:
TDPmax = Vcccore max * Icccore
The Intel Pentium III processors incorporate an on-die diode that monitors the die temperature (junction temperature). Modern Desktop Pentium III Motherboards monitor this thermal diode of the CPU.
This Tip is only for the very crazy people among us (Note: At your own Risk!):
Make sure that your Pentium III Motherboard monitors the thermal diode of the CPU. Enter the Motherboard BIOS and go to the Hardware Monitor Setup. Lift the CPU fan a bit and you can monitor the CPU temperature rise within seconds, until the Motherboard BIOS enters the ACPI modus or the CPU shuts itself down. In most cases the Motherboard BIOS enters the ACPI mode (hangs up the system) to prevent further damage to the CPU.
Don't do this with AMD Athlon CPUs! AMD Athlon CPUs do not have this feature inside the CPU. If your fan stops, is not mounted correctly or if it is not pressed to the CPU very tightly, your CPU will get fried within seconds beyond recovery!
When the junction temperature reaches 135 °C, the Pentium III processor will stop executing all instructions. This is signaled to the system with the THERMTRIP# (Thermal trip) signal. The processor will remain stopped until RESET# goes active via Restart or Reset-Switch.
The goal of every thermal design must be to dissipate TDPmax and avoid reaching Tjunction with a safe margin.