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11.3.8
Processor Decoupling Recommendations
Intel recommends proper design and layout of the system board bulk and high frequency
decoupling capacitor solution to meet the transient tolerance at the processor package balls. To
meet the transient response of the processor, it is necessary to properly place bulk and high
frequency capacitors close to the processor power and ground pins.
11.3.8.1
Transient Response
The inductance of the motherboard power planes slows the ability of the voltage regulator to
respond quickly to a current transient. Decoupling a power plane may be broken into several
independent parts. The closer to the load the capacitor is placed, the more series inductance is
bypassed. By bypassing the inductance of leads, power planes, etc., less capacitance is required.
However, areas closer to the load have less room for capacitor placement and therefore, tradeoffs
must be made.
The processor causes very large switching transients. These sharp surges of current occur at the
transition between low power states and the normal operating states. The system designer must
provide adequate high frequency decoupling to manage the highest frequency components of the
current transients. Larger bulk storage capacitors supply current during longer lasting changes in
current demand.
All of this power bypassing is required due to the relatively slow speed at which a DC-to-DC
converter may respond. A typical voltage converter has a reaction time on the order of 1 to 100 µs
while the processor's current steps may be at shorter than 1 ns. High Frequency decoupling is
typically done with ceramic capacitors with a very low Equivalent Series Resistance (ESR).
Because of their low ESR, these capacitors may act very quickly to supply current at the beginning
of a transient event. However, because the ceramic capacitors are small, (i.e., they may only store a
small amount of charge, thus Bulk capacitors are needed too). Bulk capacitors are typically
polarized with high capacitance values and unfortunately higher Equivalent Series Inductance
(ESL) and ESR. The higher ESL and ESR of the Bulk capacitor limit how quickly it may respond
to a transient event. The Bulk and high frequency capacitors working together may supply the
charge needed to stay in regulator before the regulator may react during a transient.
11.3.8.2
High/Mid Frequency and Bulk Decoupling
System motherboards should include high and mid frequency and bulk decoupling capacitors as
close to the socket power and ground pins as possible. Decoupling should be arranged such that the
lowest ESL devices (0612 reverse geometry) are closest to the processor power pins followed by
the 1206 devices (when used), and finally, bulk electrolytics (organic covered tantalum or
aluminum covered capacitors). System motherboards should also include bulk decoupling
capacitors as close to the processor socket power and ground pins as possible.
recommended decoupling solutions for V
decoupling solutions for the V
11.3.8.3
Processor Core Voltage Plane and Decoupling
Due to the high current (up to 32 A) requirements of the processor core voltage, the V
fed from the VRM by means of multiple power planes that provide both low resistance and low
inductance paths between the voltage regulator, decoupling capacitors, and processor V
pins. To meet the V
ESR of the decoupling solution should be equal to or less than 3 m Ω .
Design Guide
®
®
Intel
Pentium
M Processor and Intel
CC_CORE
supply rails.
CCP
transient tolerance specifications for the worst-case the maximum
CC_CORE
®
E7501 Chipset Platform
Platform Power Delivery Guidelines
, while
Table 89
list the recommended
Table 88
lists three
is
CC_CORE
CC_CORE
201
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