Why does my processor require more voltage to power 1 core vs all 4 cores? (i5-2500k?

[ensign_lee]

Just out of curiosity, why does my CPU seem to use more voltage when only 1 core is being used (1 at 100%, the rest at like 0%) vs when all 4 are being used (at 100% each)?

I've noticed for my i5-2500k, I used 1.336V whenever I'm doing something like folding@home and using 1 core 100%, but whenever I use all 4 cores at 100%, my voltage can dro down to 1.288V. Why is that?

 

[LagunaX]

It's called VDroop.
The volts are being spread out under load to more cores but your are using more watts.
On RealTemp you can see the watts jump up under load.

 

[Concillian]

Key in understanding this: voltage is not regulated ON the CPU, there is some distance to travel.
Consider that 100W CPU power at 1.3 volts is a pretty decent amount of current. Home wiring is only 15 amp wiring, and this is 75A. Heavily overclocked CPUs may be using twice that. This means the resistance of the wires and pathways between where the voltage is regulated and the CPU comes into play.

Current increases with power usage, current increasing causes slight heating of pathways, which changes resistance of the material in those pathways. As resistance increases, more voltage is dropped through the wires and pathways and less voltage is available to pass through the CPU, therefore CPU voltage decreases.

It's spread across multiple CPU contact points, but it's a pretty significant amount of current for those pins / pads. How many contact points would it take for 75A in the equivalent to home wiring ratings? (75A would require AWG 6 or AWG 4 wire, which is about 5mm in diameter of solid copper)

 

[Mark R]

When one or more cores are idle, the core series CPUs, can engage 'turbo' mode, which upclocks the operational cores.

In order to keep the core stable at 'turbo' speed, the CPU will request a higher voltage from the mobo.

When you are using all 4 cores fully, turbo is no longer possible (or is possible only at a very low level) - these lower CPU clocks require less voltage for stability, so less voltage is requested from the mobo.

 

[ensign_lee]

yeah, I probably should have included that I'm already overclocked to 4.2 Ghz, so any "turbo" shouldn't be affecting it.

but vdroop seems to explain it pretty well. Thanks guys!