Platform Power Delivery Guidelines
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Intel
®
855PM Chipset Platform Design Guide
power rail that is distributed
directly
from the ATX power supply is: 5 V
SB
(5 V Standby). There are
other standby rails that are created with voltage regulators on the motherboard.
Derived power rail:
A
derived
power rail is any power rail that is generated from another power rail using an on-board
voltage regulator. For example, 3.3 V
SB
is usually derived (on the motherboard) from 5 V
SB
using a
voltage regulator.
Dual power rail:
A
dual
power rail is derived from different rails at different times (depending on the power state of the
system). Usually, a dual power rail is derived from a
standby supply
during
suspend
operation and
derived from a
core supply
during
full-power
operation. Note that the voltage on a
dual
power rail may
be misleading.
11.2. Platform
Power
Requirements
The following figure shows the power delivery architecture for an example Intel 855PM chipset
platform. This power delivery architecture supports the “Instantly Available PC Design Guidelines” via
the S3 system state. To ensure that enough power is available during S3, a thorough power budget
should be completed. The power requirements should include each device’s power requirements, both in
suspend
and in
Full-On
. The power requirements should be compared against the power budget supplied
by the power supply. Due to the requirements of main memory and PCI 3.3 Vaux (and possibly other
devices in the system), it is necessary to create a
dual
power rail.
The solutions given in this document are only examples. There are many power distribution methods
that achieve similar results. It is critical, when deviating from these examples, to consider the effect of
the change.
