Extreme-Overclocking of NVIDIA GeForce FX 5950 Ultra: Methods and Technology
Overall, SAPPHIRE RADEON 9800 XT ULTIMATE Edition turned to be not the best choice for extreme overclocking. We’ve got low frequency gains, although spent a lot of time and effort. You will see the performance gains from our volt-modding later, right now we will deal with a solution based on GeForce FX 5950 Ultra.
This graphics card, as well as GeForce FX 5900 Ultra, uses Intersil ISL6569ACR chip in its voltage regulator.
This chip, just like the FAN5240 from Fairchild, allows adjusting the output voltage “on-the-fly” by changing the type of the input signal at the digital VID0..VID4 inputs. We discussed this chip in greater detail in our article about extreme overclocking of the NVIDIA GeForce FX 5900 Ultra and the ATI RADEON 9800 PRO. The graphics card uses the flexibility of the chip to send a voltage of 1.1V to the GPU at startup, 1.2V in 2D and 1.6V in 3D applications. Just like with the GeForce FX 5900 Ultra, we used the OFS input of the ISL6569ACR controller to increase the Vcore here.
According to the technical documentation, when there is a resistor with a resistance R between the OFS input and the “Ground”, the output voltage of the regulator goes up by V=(R*100mkA)/10.
The PCB of the GeForce FX 5950 Ultra carries an R554 resistor with zero resistance, through which the OFS input of the regulator is grounded. To increase the GPU voltage, I soldered up a variable resistor with a resistance of 22kOhm (through wires, for convenience) instead of the zero one.
I increased its resistance thus raising the GPU voltage. I preferred to stop at 10kOhm, which resulted in an increase of 0.1V for the Vcore in all operational modes. In other words, it was 1.2V at startup, 1.3V in 2D and 1.7V in 3D.
Vmem modding was somewhat more complicated. An impulse regulator on the HIP6012CB chip from Intersil supplies power to the internal circuitry (VDD) of the graphics memory chips. The chip is connected in the following manner:
The output voltage of the regulator is defined by the resistances of R2 and R3 resistors according to the formula: V=1.27*(1+R3/R2). To boost the regulator’s output voltage, we can reduce the resistance of R2 resistor by soldering up another resistor in parallel to it.
I used a 5.6kOhm resistor, soldering it through thin wires for convenience:
This modification helped me to increase the voltage of the memory chips internal circuitry from 3.2V to 3.44V.