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Thermal Design Power

Mobile CPU Mania
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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

TDPtyp represents the power dissipation of the processor while executing publicly available software at nominal voltages and normal operating conditions.


TDPmax > TDPtyp

Tjunction

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.

Desktop Processors

As a representative of the Desktop CPUs, I will take a closer look at the Intel Pentium III processors. Usually when a CPU is being reviewed, we take a look only at the Benchmark results. If the processor gets unstable, we increase the Vcc of the CPU a bit to get the CPU stable. With the increase of the voltage the heat dissipation of the CPU is also increased. That is part of the reason why huge coolers are so popular.

In a desktop PC this is easy. You have space inside of the system for huge coolers, and when the ambient temperature near the CPU gets too high, you get yourself an additional cooler to provide a decent airflow around the CPU-cooler.

Lets take a look at the Pentium III CPUs and its TDPmax and Tjunction .

The TDPmax of the 1133 B MHz CPU is the theoretical value, since Intel had to withdraw its CPU from the market after our publication of its defects. The main reason for the huge TDPmax of the 1133 B MHz CPU is related to the fact that Intel used the good old overclocker trick to increase Vcc of the CPU from 1.70V to 1.80V.

As described earlier, with an increased voltage you need to take much more care of TDPmax and Tjunction . To make matters worse, Intel decreased Tjunction to 62°C. This requires higher efforts to keep the CPU cool.

For all the AMD fans out there, the AMD Athlon require also massive cooling. At 1133 MHz the Athlon has a TDPmax of 63W and a Tjunction of 95°C.

On the positive side, Desktop CPUs are widely available and up to 300 US$ cheaper then a mobile CPU.

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