In the lexicon of acronyms frequently bandied about by geeks, gamers, and computer professionals, TDP doesn’t come up too frequently. Phrases like “How many GHz?” “How many MB of L3?” and “What’s the FSB?” are far more likely to come up in conversation than, “What’s the TDP on that chip?”
What is TDP?
But TDP, or thermal design power, is something that those with energy concerns, noise requirements, or overclocking aspirations should definitely pay attention to. The TDP of any processor describes the amount of heat dissipated during nominal operation. That heat emission is largely the product of a processor’s operating voltage and electrical efficiency; those, in turn, are dictated by clock speeds, fabrication size, and overall architecture.
For example, chips like the Intel Atom and AMD Athlon Neo are designed to use minimal power by drawing a low voltage and running at a low clock speed. Typically armed with single cores and minimal cache, these processors are intentionally simple so they top out around 15W TDP.
At 2.5 and 15W TDP, the Atom and Neo CPUs dissipate very little heat.
Compare that to high-end desktop CPUs, which currently top out at four cores, have several megabytes of total cache, and run higher clock speeds at higher voltages. The TDP for most desktop chips is between 65W and 125W, which is the reason why heatsinks have gone from the simple aluminum extrusions of the Pentium days to the heatpipe-laden monstrosities of today. Higher TDP means greater cooling needs and therefore more massive coolers.
At 130W+, today's leading desktop chips are furnaces in their own right.
Why low-TDP processors matter
Those familiar with overclocking might know that after manufacturing, CPUs go through a process called binning. It’s at this point that the manufacturer stresses the chip, so they can set a clockspeed and voltage that guarantees stable operation. Higher speed chips are not designed any differently from slower ones in the same family. Instead, lower speed chips have been down-clocked to meet market demands, or have small imperfections that prevent them from meeting the highest official clockspeeds.
While there are chips that are expressly designed with a low TDP in mind, there are also standard CPUs that happen come off the production line with lower voltage requirements. Once binned as one of the “efficient” models, they’re configured with a lower default voltage. These low-TDP parts give the same performance as the regular parts, but they consume less power and operate at lower temperatures.
Because these low-power parts run at reduced voltages and emit less heat, they can be especially valuable to enthusiasts. Home theater PCs and silent PCs both benefit from the lessened cooling requirements of a chip binned for efficiency. High-efficiency chips are also good for overclockers, as there is more room to increase the voltage before hitting the architectural limits of the CPU.
But the majority of these chips are sold to the major computer manufacturers. OEMs eat up a huge amount of the low-power chips for their small form factor and green computing units. When AMD released the 35W Athlon 64 X2 3800+, the entire supply was absorbed by OEMs like HP who put them in their Slimline series of desktops.
Final thoughts
Unlike gamers, who might be willing to spend their money on a 140W CPU, bigger companies can get a huge benefit from more efficient chips. The computing requirements for desktop apps has long been eclipsed by even the cheapest systems, so phasing out old systems in favor of efficient ones makes financial sense. The slight price premium for efficient systems will be quickly recouped by the lower power draw and cooling requirements. What might seem like pennies for a single system stacks up quickly when dealing with dozens, hundreds, and even thousands of computers.
Proving the business case for energy efficient chips, AMD is preparing to release a host of new energy-efficient offerings this week. The new chips will include two dual core (Athlon II X2), two triple core (Athlon II X3), and two quad core (Athlon II X4) CPUs which shed 30% less heat than other Athlon IIs of the same clockspeeds.
In a perfect world, the TDP envelope would continue to shrink until we can all go back to the simple fanless heatsinks of yesteryear. In the mean time, 45W quad core CPUs are a very awesome start.
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