Voltage Regulator-Down (VRD) 10.1 Design Guide

Vtt Requirements (REQUIRED)
R
Design Guide
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Table 3-2. Vtt Measurement Lands
Device Supply Land
Processor Vtt D25
Processor Vss D26
865 MCH Vtt F29
865 MCH Vss E29
910 MCH Vtt F29
910 MCH Vss E29
3.2 Processor-MCH Vtt Mismatch
The Intel
®
Pentium
®
4 processor Extreme Edition supporting Hyper-Threading Technology on
0.13 micron process requires the LGA775 Vtt regulator to sink and source current. This is a
requirement because the processor’s Vtt rail is connected to a reference core voltage on the die
(equal to VID) and does not use the independent Vtt supply on the motherboard. Other LGA775
processors split the Vtt rail from the reference core voltage and only require a single source Vtt
voltage regulator.
If the Extreme Edition processor is placed in a Vtt configuration without current sink capabilities,
a voltage delta will appear on the Vtt rail due to the difference in the processor Vtt (equal to VID)
and MCH Vtt (Typically 1.2 V) voltage values. When the bus is inactive, this voltage delta can
create a condition where the higher processor Vtt voltage creates a back-drive current in the MCH
termination. Since this current cannot be consumed by the MCH leakage current and the VR has
no sink capability, the Vtt voltage will increase beyond the regulation limits. As a result, the Vtt
VR may shut down due to OVP or may function with stability issues. Note, operating at Vtt
voltages above specifications will impact the impedance calibration of the active MCH front side
bus termination transistors and may lead to bus failures. The heightened voltage may also reduce
the life span of the MCH.
Early Intel reference boards were designed with a Vtt circuit that did not include sink capabilities.
If the target motherboard is to support the Pentium 4 processor Extreme Edition, additional
circuitry must be added to ensure correct operation.
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