Datasheet

SZZA016B
7–233 Basic Design Considerations for Backplanes
Introduction
Since the beginning, most equipment makers have used parallel-backplane architectures to
deliver large amounts of data across one shared bus. The parallel backplane provides a physical
and electrical interconnect between various modules in a system. Each module in the backplane
communicates with other modules through the backplane bus. Typically, this bus is driven by a
backplane transceiver, primarily as the point-of-contact between backplane cards. The basic
backplane is a parallel data-transfer topology used in a multipoint transfer scheme. For example,
the TDM bus in a wireless base-station unit operates in a multipoint fashion, with high-speed
data communicating between different regions across the backplane.
With the expansion of the internet and wireless/telecom infrastructures, new end-equipment
markets have emerged that deliver faster data, integrated voice and data, or a little of both, and
need higher performance backplanes. The constant pressure to increase bandwidth requires
design engineers to choose between higher performance, more expensive backplane-optimized
transceivers to maximize the frequency, increase the bit width using older technology, or a
combination of these goals. This application report addresses some of the basic design issues
encountered in higher performance backplanes. The effects of distributed capacitance on
termination resistance and flight time are examined and the various effects of stubs and
connectors are discussed.
Backplane Design Topology – Point-to-Point vs Multipoint
Figure 1 is an example of a simple point-to-point data transfer. A driving device at point A drives
a 51-Ω transmission line. A termination resistor (R
TT
) that matches the line impedance is placed
at point B, along with a receiving device. All calculated values for the termination resistance are
ideal. The designer must use actual values that best match, or are lower than, calculated values.
In this example, the transmitter is an open-drain device that pulls the line low when turned on,
but requires the termination resistor to pull the line high when turned off.
l = 10 in. or
25.4 cm
Tx Rx
R
TT
= 51 Ω
AB
51-Ω Stripline
V
TT
Figure 1. Point-to-Point Application
When the output transistor drives the line low, a constant dc current flows from the termination
voltage, V
TT
, to ground. Too small a termination resistance can damage the driver due to
excessive currents. Assuming V
TT
= 1.5 V, V
OL
= 0.4 V, and R
TT
= 51 Ω, the constant dc current
is about 21.6 mA (I
OL(max)
= (V
TT
– V
OL
)/R
TT
). However, this current increases linearly with V
TT
and should not exceed the recommended current rating of the output driver.
In this, and the following multipoint example, the transmission line is assumed to be a stripline
trace that is 10 in. (25.4 cm) long with a natural impedance (Z
o
) of 51 Ω, which corresponds to a
characteristic capacitance (C
o
) of 3.5 pF/in. (138 pF/m).