
AP-523
E
6
Figure 2. Physical Power Distribution
Figure 1 shows the ideal power model. However, in real
systems, the power distribution scheme typically appears
as in Figure 2. This system has physical components
such as cables, connectors, the PCB, and the component
packages.
To completely model this system, one must include the
inductance and resistance which exists in the cables,
connectors, PCB, and the pins and body of components
such as sockets and capacitors. A more detailed model
showing these effects is shown in Figure 3. In the past,
voltage drops due to inductance (V = Ldi/dt) and
resistance (V = IR) have been nearly negligible relative
to the tolerance of components in most systems. This has
caused the creation of simple rules for decoupling. For
example, with the current at 1 Amp, and the tolerance at
250 mV (5% of 5V), one could easily ignore the effects
of 25 m
W
of resistance in the distribution path. However,
at 10 Amps, this IR drop is equal to the 250 mV
tolerance. Similarly, 250 pH of inductance can typically
be ignored in a power distribution system, unless current
transients of 1 Amp/ns exist, as they do when using the
Pentium Pro processor. The Ldi/dt drop in this case is
also equal to 250 mV.
CPU Package
+
-
Power
Supply
C
DIE
C
PKG
C
HF
C
BULK
L
BOND
L
PIN+SOCKET
L
BOARD
L
CABLE
R
CABLE
R
BOARD
Die
Figure 3. A Detailed Power Distribution Model
The high value of the Pentium Pro processor current and
the high rate of change of the current must both be taken
into account for a successful design. The requirements of
the Pentium Pro processor are described in Section 3, and
meeting these requirements is discussed step by step in
Section 4.
3.0.
PENTIUM
PRO PROCESSOR
POWER REQUIREMENTS
This section describes the issues related to supplying
power to a Pentium Pro processor using approximate