Intel Xeon Processor and Intel E7500/E7501Chipset Compatible Platform Design Guide

Intel
®
Xeon™ Processor and Intel
®
E7500/E7501 Chipset Compatible Platform Design Guide 193
Platform Power Delivery Guidelines
11.2.10 AGTL+ Reference Voltage
The processor GTLREF[3:0] and MCH HDVREF[3:0], HAVREF[1:0], and HCCVREF are low
current inputs (less than 15 µA each) to the differential receivers within each of the components on
the AGTL+ bus. Use a voltage divider to generate a GTLREF[3:0] of 0.63*VCC_CPU ± 2%.
R1 and R2 should be small enough values that the current drawn by the GTLREF inputs (IREF) is
negligible versus the current through R2 and R1. Equation 11-1 shows GTLREF, where “n” is the
number of IREF inputs supplied by the divider.
The worst case GTLREF should be analyzed with IREF at the maximum and minimum values
determined for the number of loads being supplied. If the number of loads can change from model
to model because of upgrades, this should be taken into account as well. Analyze Equation 11-1
with R1 and R2 at the extremes of their tolerance specifications.
Use two voltage dividers for each processor, and one for the chipset component. Assume a
maximum of 15 µA of leakage current per load. These leakage currents can be positive or negative.
The following discussion illustrates using a single voltage divider to support two GTLREF loads
assuming VCC_CPU of 1.50 V. Using an 84.5
Ω ± 1% resistor for R1 and a 49.9 Ω ± 1% resistor
for R2 in Figure 11-18 creates a static usage of 11.2 mA (1.500 V/149.9
Ω) per voltage divider. The
worst case solution for Equation 11-1 can be found with IREF at 30 µA, R1 at the low end of its
tolerance specification (83.7
Ω), and R2 at the high end of its tolerance specification (50.4 Ω),
yielding Equation 11-2. The target of 0.63 of VCC_CPU is 0.945 V (0.63 * 1.50). This resistive
setting satisfies the ± 2% specification.
Figure 11-17. GTLREF Divider
s
Vcc_CPU
R
2
R
1
GTLREF
I
S
I
REF
Equation 11-1. GTLREF
12
REF2CC
11 RR
InRV
GTLREF
+
×−
=
Equation 11-2. Resistor Tolerance Analysis
GTLREF
1.50 50.4⁄ 0.000030–
1 50.4⁄ 1 83.7⁄+
-----------------------------------------------------
0.927V==