Mobile Intel Pentium 4 Processor - M and Intel 845MP/MZ Chipset Platform Design Guide

Mobile Intel
®
Pentium
®
4 Processor-M and Intel
®
845MP/845MZ Chipset Platform
178 Design Guide
R
Table 64. MCH-M DDR I/O
NAME VCCSM
1
SDREF
PURPOSE
MCH-M DDR SUPPLY VOLTAGE
(I/O), STATIC
MCH-M REFERENCE SUPPLY
VOLTAGE, STATIC
VCCSM SDREF =( VCCSM± 5%) / 2
VOLTAGE Nominal (V) 2.500 "± 5%" 1.250 "± 2%"
TOLERANCE (+/-V) 0.125 0.025
Vmax(V) 2.625 1.275
Vmin(V) 2.375 1.225
Ivccsm (max) Isdref (max)
Imax 1.400 0.010
NOTE: MCH-M VREF REQUIREMENTS: the MCH-M core is called "VCCSM" =+2.5 V ±5%. SDREF is ("VCCSM"±
5%) /2 ±2%. This means that whether the 2.5v is 5% high or low, we need to be able to divide that voltage
by 2 with a 2% accuracy. This basically means to use 1% resistors, or better.
As shown in Table 64, the max current required by the MCH-M for the SDREF input is 0.010 A. This is
too big of a load for a resistor divider. Some sample calculations are shown in the table below. It is not
possible 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 MCH-M SDREF.
Table 65. Effects of Varying Resistor Values in the Divider Circuit
Rdivider
(ohms)
Leakage
(A)
Rtop Vdroop
(V)
I(2.5) total=2.5v
/
2R
(A)
1 0.01 0.01 1.25
10 0.01 0.1 0.125
100 0.01 1 0.0125
1000 0.01 10 0.00125
10000 0.01 100 0.000125
100000 0.01 1000 1.25E-05
1000000 0.01 10000 1.25E-06
Rdivider: This is the resistor value selected to form the divider. Assumes both top and values are equal as
required for divide by 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 MCH-M SDREF input or the DDR VREF input. This current
does not go across the bottom resistor.
Rtop Vdroop: This is the resulting voltage droop across Rtop as a result of the leakage current.
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.