Hardware Reference
In-Depth Information
+5V as a feed, 65W would equate to only 13A at +5V. That would assume 100% efficiency in the
regulator, which is impossible. Therefore, assuming 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 assuming 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 wire supplying +12V power to the
motherboard is a recipe for a melted connector because 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 designed 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 intermittent lockups or even damage such as damaged
components or burned circuits), Intel created a power standard called the
Platform Compatibility
Guide (PCG)
. The PCG was marked on most Intel boxed (retail) processors and motherboards
introduced from 2004 through 2009. It was designed for system builders to use as an easy way to
know the power requirements of a processor and to ensure that the motherboard can meet those
requirements.
The PCG is a two- or three-digit alphanumeric value (for example, 05A), where the first two digits
represent the year the particular specification was introduced, and the optional third character stands
for the market segment. PCG designations in which the third character is A apply to processors and
motherboards that fall in the low-end market (requiring less power), whereas designations whose
third character is B apply to processors and motherboards that fall in the high-end market (requiring
more power). Motherboards that support high-end processors by default also support low-end
