In a matter of fact, DDR SGRAM chips manufactured for video cards require
three voltages to be supplied: Vdd (for memory cells), Vddq (for input/output
buffers) and Vtt (for termination circuits). Under any circumstances Vddq must
not exceed Vdd, and Vtt is ½ of Vddq typically. Sometimes manufacturers
of graphics hardware implement both Vdd and Vddq through a single power output,
but this isn't good because Vddq should be separated from Vdd for better noise
immunity aiming faster clock speeds. Vtt may be obtained from Vddq through
simple resistor divider or separate regulator. It should be apparent that the
second way is better but more expensive to implement.
Sapphire Radeon 9600XT employs a linear stabiliser to generate Vdd and Vddq.
It's based upon an LM324 quad comparator/amplifier by ON Semiconductor
, 136Kb), a TL431-compatible shunt
regulator (PDF datasheet
, 68Kb) and two ANPEC 3055L
(APM3055L – PDF datasheet
, 126Kb) N-channel
field-effect transistors. The stabiliser may be found in the upper right corner
on the front side of the card. The TL431 supplies 2.5V reference voltage to
negative inputs of the 3rd and 4th LM324 comparators. The 3rd positive input
listens to Vdd through a resistor divider (R110 and R111), the 4th positive
input — to Vddq in the same manner (R114 and R115). The 3rd output is
connected to the gate of the APM3055L located on the back side of the card, the
4th output — to the gate of the APM3055L which may be found on the front
side of the card. Both of them are fed with +3.3V. The easiest way to adjust Vdd
and Vddq is to play with their positive input resistor dividers.
As you can see, both dividers are identical and set to deliver (0.2 ÷
3.3) × 2.5 + 2.5 = 2.65V to both Vdd and Vddq. It's better to leave
200Ω upper resistors as they are because of their small size. For example,
the 2.2kΩ lower resistors would give 2.73V, 1.5kΩ — 2.83V, and
1.0kΩ — 3.00V. Don't forget about the Vddq less-or-equal to Vdd rule
and watch the memory chips' temperature. A separate linear generator by RichTek
called RT9173A (PDF datasheet
, 159Kb) is responsible for
Vtt. It may be adjusted as well, but there is probably no reason why you should do
so. Vtt scales up and down with Vddq, and that's all right.
Oh yes, another story about capacitors. There are three liquid electrolytic
1000µF/6.3V capacitors by TEAPO installed, one per Vdd, Vddq and Vtt.
There are also six 22µF tantalum capacitors installed to improve
high-frequency response, two on Vdd (C285, C286) and four on Vddq (C281, C282,
C283, C284). Another set of two 10µF MLCCs may be found behind the
graphprocessor on the back side of the card (C38 and C39). In general, this
configuration is good enough for use with linear regulators. However, the author
has decided to replace the Vddq electrolyte with the Nichicon HC 470µF/10V
pulled recently from the graphprocessor's stabiliser, and to get rid of the Vdd
electrolyte in favour of a Nichicon PW 1000µF/10V (-10mV impedance and
+330mArms peak current compared to the TEAPO one). Once again, those two
10µF MLCCs have accommodated two 10µF MLCCs on their tops. All these
upgrades aren't of must-have importance, but it's better to be safe than sorry.