80220 Datasheet, PDF (45/91 Pages) LSI Computer Systems – 100BASE-TX/10BASE-T Ethernet Media Interface Adapter

80220/80221
5.0 APPLICATION INFORMATION
5.1 EXAMPLE SCHEMATICS
A typical example schematic of the 80220/80221 used in
an adapter card application is shown in Figure 11, a hub
application is shown in Figure 12, and an external PHY
application is shown in Figure 13.
5.2 TP TRANSMIT INTERFACE
The interface between the TP outputs on TPO± and the
twisted pair cable is typically transformer coupled and
terminated with the two resistors as shown in Figures 11-
13.
The transformer for the transmitter is recommended to
have a winding ration of 2:1 with a center tap on the 2x
winding tied to VCC, as shown in Figures 11-13. The
specifications for such a transformer are shown in Table
21. Sources for the transformer are listed in Table 22.
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 VCC to each of the TPO± outputs, and
their value should be chosen to provide the correct termi-
nation impedance when looking back through the trans-
former from the twisted pair cable, as shown in Figures 11-
13. 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.
To minimize common mode output noise and to aid in
meeting radiated emissions requirements, it may be nec-
essary to add a common mode choke on the transmit
outputs as well as add common mode bundle termination.
The qualified transformers mentioned in Table 22 all
contain common mode chokes along with the transform-
ers on both the transmit and receive sides, as shown in
Figures 11-13. Common mode bundle termination may be
needed and can be achieved by tying the unused pairs in
the RJ45 to chassis ground through 75 Ohm resistors and
a 0.01 uF capacitor, as shown in Figures 11-13.
To minimize noise pickup into the transmit path in a system
or on a PCB, the loading on TPO± should be minimized
and both outputs should always be loaded equally.
5.3 TP RECEIVE INTERFACE
Receive data is typically transformer coupled into the
receive inputs on TPI± and terminated with external
resistors as shown in Figures 11-13.
The transformer for the receiver is recommended to have
a winding ration of 1:1, as shown in Figures 11-13. The
specifications for such a transformer are shown in Table
21. Sources for the transformer are listed in Table 22.
The receive input needs to be terminated with the correct
termination impedance meet the input impedance and
return loss requirements of IEEE 802.3. In addition, the
receive TP inputs need to be attenuated. It is recom-
mended that both the termination and attenuation be
accomplished by placing four external resistors in series
across the TPI± inputs as shown in Figures 11-13. The
resistors should be 15%/35%/35%/15% of the total series
resistance, and the total series resistance should be equal
to the characteristic impedance of the cable (100 Ohms for
UTP, 150 Ohms for STP). It is also recommended that a
0.01µF capacitor be placed between the center of the
series resistor string and VCC in order to provide an AC
ground for attenuating common mode signal at the input.
This capacitor is also shown in Figures 11-13.
To minimize common mode input noise and to aid in
meeting susceptibility requirements, it may be necessary
to add a common mode choke on the receive input as well
as add common mode bundle termination. The qualified
transformers mentioned in Table 22 all contain common
mode chokes along with the transformers on both the
transmit and receive sides, as shown in Figures 11-13.
Common mode bundle termination may be needed and
can be achieved by tying the receive secondary center tap
and the unused pairs in the RJ45 to chassis ground
through 75 Ohm resistors and a 0.01 µF capacitor, as
shown in Figures 11-13.
In order to minimize noise pickup into the receive path in
a system or on a PCB, loading on TPI± should be
minimized and both inputs should be loaded equally.
44-455
MD400159/E