
Application Information
Copyright 1999-2002 by LSI Logic Corporation. All rights reserved.
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April, 2002
type of RJ-45 connector used on the secondary side (line side) of the
transformer. The pinout of these types differ slightly so that traces to the
magnetics may be kept as short and direct as possible.
The "non-stacked" RJ-45 (also referred to as harmonica) is a traditional
horizontally oriented connector consisting of four RJ-45 jacks in a single
in-line assembly.
The newer "stacked" connector consists of a two-over-two configuration
so that four RJ-45 jacks are located in the footprint area of two side-by-
side connectors. This is a significant improvement for higher density
multi-port applications and smaller system form-factors.
The L84225 pin-out has been optimized for connection to transformers
and connectors designed for the higher density stacked configuration.
The Quad Transceiver also operates with non-stacked connectors using
either transformers that map the pin-out to the non-stacked configuration,
or using trace "crossovers" on the PCB layout.
The transformers listed with "non-stacked" pinouts use crossover
connections inside the part to map the stacked pinout of the L84225 to
non-stacked RJ-45 connectors. Crossovers internal to the transformer
are not made in a controlled impedance environment, so this can impact,
somewhat, the cross-talk performance of the system.
For best cross-talk and system performance, it is suggested that stacked
connector and transformer configurations be used. Alternately, non-
stacked connectors may be used with stacked transformer types and the
crossover wiring can be put on the pc board using a ground plane to
reduce impedance mismatch.
The transmit output needs to be terminated with two external termination
resistors in order to meet the output impedance and return loss
requirements of IEEE 802.3. It is recommended that these two external
resistors be connected from VDD to each of the TPOP/N outputs, and
their value should be chosen to provide the correct termination
impedance when looking back through the transformer from the twisted
pair cable, as shown in
Figure 11
. The value of these two external
termination resistors depends on the type of cable driven by the device.
Refer to the Cable Selection Section for more details on choosing the
value of these resistors.