Table 92. Gmch System Memory I/O Smvref Calculation; Table 93. Effects Of Varying Resistor Values In The Divider Circuit - Intel 852GME Design Manual

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Table 92. GMCH System Memory I/O SMVREF Calculation

PURPOSE
VOLTAGE Nominal (V)
TOLERANCE (±V)
Vmax(V)
Vmin(V)
Imax (A)
GMCH VREF REQUIREMENTS: the GMCH core is called "VCCSM" =+2.5 V ± 5%. SMVREF is ("VCCSM"±
NOTE:
5%)/2 ± 2%. Whether the 2.5 V is 5% high or low, the voltage needs to be divided by 2 with a 2% accuracy.
Therefore, use 1% resistors or better.
As shown in Table 92, the max current required by the GMCH for the SMVREF input is 0.010 A. This
is too big of a load for a resistor divider. Some sample calculations are shown in Table 93; it is
impossible to maintain regulation within 2% using a resistive divider without using a resistor so small
that the 2.5-V current requirement becomes prohibitive. Hence, a buffer is required due to the 10-mA
current requirement of the GMCH SMVREF.

Table 93. Effects of Varying Resistor Values in the Divider Circuit

Rdivider
( )
1
10
100
1000
10000
100000
1000000
NOTES:
1. Rdivider: This is the resistor value selected to form the divider. Assumes both top and values are equal as
required for divide by 2.
2. Leakage: This is the amount of leakage current which needs to sourced from the 2.5-V supply, across the
divider's top resistor (Rtop) and out to the GMCH SMVREF input or the DDR VREF input. This current does not
go across the bottom resistor.
3. Rtop Vdroop: This is the resulting voltage droop across Rtop as a result of the leakage current.
4. I(2.5) total = 2.5 V/2R. This is the total current through divider. This is calculated to consider the amount of
current & power used as a DC current through the divider.
5. The implementation of a buffer is also required by the DDR. The same VREF may be used for both GMCH and
the DDR as well.
® ®
Intel 852GME, Intel 852GMV and Intel
Name
GMCH DDR SUPPLY VOLTAGE (I/O),
STATIC
VCCSM
2.500 "± 5%"
0.125
2.625
2.375
Ivccsm (max)
1.400
Leakage
(A)
0.01
0.01
0.01
0.01
0.01
0.01
0.01
®
852PM Chipset Platforms Design Guide
Platform Power Delivery Guidelines
1
VCCSM
Rtop Vdroop I(2.5) total = 2.5 V/2R
(V)
0.01
0.1
1
10
100
1000
10000
SMVREF
GMCH REFERENCE SUPPLY
VOLTAGE, STATIC
SMVREF = (VCCSM ± 5%) / 2
1.250 "± 2%"
0.025
1.275
1.225
Isdref (max)
0.010
(A)
1.25
0.125
0.0125
0.00125
0.000125
1.25E-05
1.25E-06
221