Extreme Overclocking

Replacing the standard cooling system with something more efficient, for example with a water-cooling solution, and pulling up the voltages are the traditional means of increasing the overclockability of a graphics card.

The graphics card under question has GDDR3 memory chips from Samsung, known for their low overclockability which doesn’t grow much even after a voltage increase. So I didn’t tamper with the memory voltage on the reference card at all – it wasn’t worth the trouble.

The overclockability of the graphics processor is another matter, as we deal with NVIDIA’s first GPU made with the 0.11-micron tech process. The thinner tech process and the smaller transistor count in comparison to the GeForce 6800 series should render this chip capable of working at higher clock rates.

Let’s check it out. The core voltage regulator is based on the ISL6534 chip from Intersil. It is a dual-channel pulse-width controller capable of driving a line regulator. One of the channels supplies power to the GPU. Unlike the regulators on GeForce FX 5950 Ultra or FX 5900 Ultra cards, which have digital inputs for setting the output voltage level and capable of adjusting the output voltage “on the fly”, this chip uses a resistor devisor for setting the output voltage.

Curiously enough, the NVIDIA engineers made this regulator change the output voltage “on the fly”, too. Receiving the control signals from the GPU’s registers, two transistors attach resistors with preset resistances to the devisor, thus adjusting the voltage value. Voltage regulators on NVIDIA GeForce 5900 XT cards employ the same idea, by the way.

In order to lift the voltage of the graphics processor, you only must reduce the resistance of one of the divisor’s resistors. That’s exactly what I did:

You can see the controller chip of the voltage regulator in the top left corner of the snapshot, while the output voltage control scheme and the two resistors of the devisor are in the center. The arrows point to the spots I soldered an additional variable resistor to.

By changing the resistance of the variable resistor and evaluating the overclockability of the card at different GPU voltages, I found the maximum GPU frequency at which the card was stable – 640MHz. Thus, the frequencies of the reference graphics card were 640/1100MHz at extreme overclocking.

The voltage of the graphics processor was 1.233v at Windows XP startup, 1.562v after the load of the graphics card driver, in 2D mode, and 1.629v in 3D applications. Before the modification, these voltages were 1.233v, 1.431v and 1.477v, respectively.