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
Design Guide 177
R
Table 63. Termination Voltage and Current Specifications
Name "Vtt" Description
PURPOSE TERMINATION SUPPLY
VOLTAGE, STATIC
DEFINITION Vtt = ("Vref")± 0.040 V (((2.5 V+/-8%)/2) ± 0.050 V) ±0.040
VOLTAGE Nominal (V) 1.250
TOLERANCE (±%) 3.2%
TOLERANCE (±V) 0.040
MAX ABSOLUTE SPEC VALUE
(V)
1.440 (((2.5 V+8%)/2)+0.050 V)+0.040
MIN ABSOLUTE SPEC VALUE (V) 1.060 (((2.5 V-8%)/2)-0.050 V)-0.040
MAX RELATIVE SPEC (calculated
from measured "VCCSM" value)
(measured Vref)+0.040 V
MIN RELATIVE SPEC (calculated
from measured "VCCSM" value)
(measured Vref)-0.040 V
Itt (max)
ABSOLUTE MAXIMUM CURRENT
REQUIREMENTS
2.400
11.7.4.1. VREF Generation
It may also be noted in Figure 95 that when the 1.25-V reference is generated from the 2.5-V supply a
buffer is used. A buffer has also been used to provide this reference to the system for the MCH-M and
memory. Specifically, this is the “VREF” signal to the memory and the “SDREF” signal to the MCH-M.
Our reference design utilizes this buffer to provide the necessary current to these devices, which the
simple resistor divider is not capable of providing. This “SDREF” voltage to the MCH-M has the tightest
tolerance in the memory system of ± 2%. Using common 1% resistors consumes 1% of this 2% tolerance.
This means SDREF must now be controlled to a 1% tolerance (i.e. be able to divide VCCSM/2 within
1%). A simple resistor divider is not a voltage regulator and is most definitely not a current source. Any
current drawn across the resistor divider used to generate this 1.25-V reference will cause a voltage drop
across the top resistor that distorts or biases this reference to a lower voltage.