
Draft 6/5/00
TP Transmit Output Current Set
A-7
Copyright 2000 by LSI Logic Corporation. All rights reserved.
common mode signal at the input. This capacitor is also shown in
Figure A.1
through
Figure A.3
.
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 A.2
all contain
common mode chokes along with the transformers on both the transmit
and receive sides, as shown in
Figure A.1
through
Figure A.3
. Common
mode bundle termination may be needed and can be achieved when the
receive secondary center tap and the unused pairs in the RJ45
connector are connected to chassis ground through 75
resistors and
a 0.01 Fcapacitor, as shown in
Figure A.1
through
Figure A.3
.
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.
A.4 TP Transmit Output Current Set
The TPO
±
output current level is set with an external resistor connected
between the REXT pin and GND. This output current is determined from
the following equation, where R is the value of REXT:
I
out
= (10K/R)
I
ref
Where
REXT should typically be a 10 k
1% resistor to meet IEEE 802.3
specified levels. Once REXT is set for the 100 Mb
its/
s and UTP modes
as shown by the equation above, I
ref
is then automatically changed inside
the device when the 10 Mb
its/
s mode or UTP120/STP150 modes are
selected.
Ir
ef
= 40 mA (100 Mbits/s, UTP)
= 32.6 mA (100 Mbits/s, STP)
= 100 mA (10 Mbits/s, UTP)
= 81.6 mA (10 Mbits/s, STP)