The datasheet for the ST Micro L6917BD Voltage Regulator is here:
http://www.st.com/stonline/books/pdf/docs/8162.pdf
The mod is connecting to pin 9 of the chip, which is the FB terminal.
Pin 9 is on the side with the indented "dot" in the top of the package
- the dot marks pin 1. By adding or subtracting current from the summing
junction FB, it is possible to change the output voltage. (Note: This
method isn't available on all voltage regulators - it just happens
that the connectivity of this chip exposes the necessary resource for
you to play with.) By connecting the variable resistor from pin 9 to
ground, current is pulled from the summing junction, and to replace
it, the chip increases its output voltage. The output voltage is tied
to FB via a resistor as well. So, the mod pulls on the junction and the
output voltage rises in response (this is closed loop feedback).
In this case, the maximumoc web page states to take a 25000 ohm variable
resistor and connect it from pin 9 to ground. My advice to you, is to
use two resistors - a fixed one and a variable one - connected in series.
The reason for this, is it is too tempting to keep turning the knob on
the variable resistor. If you do that without a protection resistor, the
voltage could be raised too high and kaboom!
| L6917BD |
| dot |
|*_1_2_3_4_5_6_7_8_9_______|
|
Fixed
Resistor
|
+
|
Variable (use either one of the end terminals
Resistor and the center one)
|
Ground
By looking at page 30 of the ST micro datasheet, you can see that the
resistors used in the feedback network- the voltage divider network
and the shaping network, are using fairly small resistance values.
This suggests to me that the 25K variable pot is too big - what will
happen is, the "interesting" area of pot rotation will be so small,
it will be easy to overshoot the mark when dialing it. The voltage
change you desire will be over a small portion of the adjustment
range.
To figure out what the value should be, I would start with a selection
of fixed resistors. Say 20K, 10K, 5K, and so on. First connect
the 20K resistor from pin 9 to ground. Using a voltmeter, measure
Vcore when the resistor is connected and then disconnected (in the
same session). With the 20K resistor, the change in voltage should
be quite small. Next try the 10K resistor, and see how much the voltage
changes. (Note: Keep your fingers off the contacts when connecting the
resistor, because your body resistance will screw up the experiment.)
What should happen, is the increase in voltage seen with the 10K
resistor should be twice the amount seen with the 20K resistor. Based
on this, you should be able to extrapolate how much resistance will
be needed to reach your desired voltage boost. That value of resistance
will become the "Fixed Resistor" in the picture I drew above.
Say for example:
1) No resistor from pin 9 to ground 1.65V measured
2) 20K resistor from pin 9 to ground 1.75V measured (delta = 0.1V)
3) 10K resistor from pin 9 to ground 1.85V measured (delta = 0.2V)
To get to a boost of 0.4 volts, then it is easy to see that a 5K ohm
resistor would do that, so your fixed resistor becomes 5K ohms (and 5.1K
ohms is the nearest value at the store).
The variable resistor will be larger than thix fixed resistance. If
the minimum boost is to be 25% of the maximum boost, then the variable
resistor will be 3 times the value of the fixed resistor. By including
the fixed resistor in series, you are guaranteeing that even if the
variable pot is turned until it reads zero ohms, the fixed resistor
limits the boost to 0.4V or whatever you designed it to be.
Note that, once you touch the board with a soldering iron, the warranty
is voided.
Also note, that the other mod, the chipset voltage mod,
is pretty dangerous as well. At least one of the first people to
try this, borked his board (I think I read about some of that over
on nforcershq). And, to the best of my knowledge, no one really knows
what the Nvidia stated voltage spec is for this voltage. (So, the degree
of danger to the chipset is unknown, unless someone knows what the
nominal value is supposed to be).
Have fun,
Paul
