Hardware Reference
In-Depth Information
1.8V, and 65W at 1.8V would equate to 36.1A of current at that voltage (volts × amps =
watts).Ifthevoltageregulatorused+5Vasafeed,65Wwouldequatetoonly13Aat+5V.
That would assume 100% efficiency in the regulator, which is impossible. Therefore, as-
suming 80% efficiency (which is typical), there would be about 16.25A actual draw on
the +5V due to the regulator and processor combined.
When you consider that other circuits on the motherboard also use +5V
power—remember that ISA or PCI cards are drawing that power as well—you can see
how easy it is to overload the +5V lines from the supply to the motherboard.
Although most motherboard VRM designs up through the Pentium III and Athlon/Duron
use +5V-based regulators, most systems since then use +12V-powered regulators. This is
because the higher voltage significantly reduces the current draw. Using the same 65W
AMD Athlon 1GHz CPU as an example, you would end up with the current draw at the
various voltages shown in
Table 18.12
.
Table 18.12 Current Draw at Various Voltages
As you can see, using +12V to power the chip results in only 5.4A of draw, or 6.8A as-
suming 80% efficiency on the part of the regulator.
So, modifying the motherboard VRM circuit to use the +12V power feed would seem
simple. But as you'll recall from the preceding text, the ATX 2.03 specification has only
a single +12V lead in the main power connector. Even the short-lived auxiliary connector
had no +12V leads, so that was no help. Pulling up to 8A more through a single 18-gauge
wiresupplying+12Vpowertothemotherboardisarecipeforameltedconnectorbecause
the contacts in the main ATX connector are rated for only 6A using standard terminals.
Therefore, another solution was necessary.
Platform Compatibility Guide
The processor directly controls the amount of current drawn through the +12V connector.
Modern motherboards are designed to support a wide range of different processors;
however the voltage regulator circuitry on a given motherboard may not have been de-
signed to supply sufficient power to support all processors that might otherwise fit in the
socket. To help eliminate the potential power problems that could result (including inter-
mittent lockups or even damage such as damaged components or burned circuits), Intel
