Intel Xeon - MCH Power Sequencing Requirement; Filter Topology for VCCAHI1.2 (System Bus); Power Sequencing Requirement for MCH

Platform Power Delivery Guidelines
180 Design Guide
12.3.6 MCH Power Sequencing Requirement
The MCH has only one power sequencing requirement. The MCH requires that 1.2 V rises with or
before 2.5 V to avoid electrical overstress of oxide layers and possible component damage. This
means that at any point during system power up, the 2.5 V power plane voltage must not be higher
than the 1.2 V power plane voltage until the 1.2 V voltage is within 1.2 V regulation. This is
depicted in Figure 12-19. Notice that at no point before 1.2 V is ramped does the 2.5 V plane
exceed the 1.2 V plane’s value.
NOTE: This graph does not represent specific values or requirements on data/time frames.
A possible solution to safeguard against 2.5 V coming up before 1.2 V, is to tie the power good
signal of the 1.2 V regulator to the output enable pin of the 2.5 V voltage regulator.
If the same voltage regulator is used to derive both 1.2 V and 2.5 V, then other logic must be used.
A solution is to use a comparator to 1.2 V, and connect the output of the comparator to the output
enable signal of the 2.5 V regulator. Figure 12-20 shows this implementation.
Figure 12-18. Filter Topology for VCCAHI_1.2 (System Bus)
VCC_CPU
100 nH
1 Ω
VCCACPU
100 µF
0.1 µF
MCH
RLC Network
Within 1"
Within 1"
of Ball
Route to
ball U15
Figure 12-19. Power Sequencing Requirement for MCH
Time (t)
Voltage (V)
2.5V
1.2V
0.5
1.0
1.5
2.0
2.5