80220 Datasheet, PDF (49/91 Pages) LSI Computer Systems – 100BASE-TX/10BASE-T Ethernet Media Interface Adapter

80220/80221
Table 21. TP Transformer Specification
Parameter
Specification
Transmit Receive
Turns Ratio
2:1 CT
1:1
Inductance, (µH Min)
Leakage Inductance, (µH)
350
0.05-0.15
350
0.0-0.2
Capacitance (pF Max)
15
15
DC Resistance (Ohms Max)
0.4
0.4
Table 22. TP Transformer Sources
Vendor
Part Number
Valor
ST6129
NanoPulse
NPI 6254-30
PCA
EPF 8025G
Belfuse
S558-5999-96
or S558-5999-98
Pulse Engineering
H1038/H1039
YCL
PT 163042
5.4 TP TRANSMIT OUTPUT CURRENT SET
The TPO± output current level is set by an external resistor
tied between REXT and GND. This output current is
determined by the following equation where R is the value
of REXT:
I_out
Where I_ref
= (10K/R) _* I_ref
= 20 mA (100 Mbps, UTP)
= 16.3 mA (100 Mbps, STP)
= 50 mA (10 Mbps, UTP)
= 40.8 mA (10 Mbps, STP)
For 100 Ohm UTP (unshielded twisted pair cable), REXT
should be typically set to 10K Ohms and REXT should be
a 1% resistor in order to meet IEEE 802.3 specified levels.
Once REXT is set for the 100 Mbps and UTP modes as
shown by the equation above, I_ref is then automatically
changed inside the device when the 10 Mbps mode or
UTP120/STP150 modes are selected.
Keep REXT close to the REXT and GND pins as possible
in order to reduce noise pickup into the transmitter.
Since the TP output is a current source, capacitive and
inductive loading can reduce the output voltage level from
the ideal. Thus, in actual application, it might be neces-
sary to adjust the value of the output current to compen-
sate for external loading. One way to adjust the TP output
level is to change the value of the external resistor tied to
REXT. A better way to adjust the TP output level is to use
the transmit level adjust register bits accessed through the
MI serial port. These four bits can adjust the output level
by -14% to +16% in 2% steps as described in Table 3.
5.5 CABLE SELECTION
The 80220/80221 can drive two different cable types: (1)
100 Ohm unshielded twisted pair, Category 5, or (2) 150
Ohm shielded twisted pair.
The 80220/80221 must be properly configured for the type
of cable in order to meet the return loss specifications in
IEEE 802.3. This configuration requires setting a bit in the
serial port and setting the value of some external resistors,
as described in Table 23. The Cable Type Select bit in
Table 23 is a bit in the MI serial port Configuration 1
register that sets the output current level for the cable type.
R_TERM in Table 23 is the value of the termination resistors
needed to meet the level and return loss requirements.
The value for R_TERM on the TPO± outputs is for the two
external termination resistors connected between VCC to
TPO±; the value for R_TERM on the TPI± inputs is for the sum
of the four series resistors across TPI±, as shown in
Figures 11-13. These resistors should be 1% tolerance.
Also note that some output level adjustment may be
necessary due to parasitics as described in the TP Output
Current section.
Table 23. Cable Configuration
Cable Type
Cable Type
Select Bit
(16.7)
R_TERM (Ohms)
TPO± TPI±
100 Ohm UTP, Cat. 5 UTP
200 100
150 Ohm STP
STP
300 150
5.6 TRANSMITTER DROOP
The IEEE 802.3 specification has a transmit output droop
requirement for 100BaseTX. Since the 80220/80221 TP
output is a current source, it has no perceptible droop by
itself. However, the inductance of the transformer added
to the device transmitter output as shown in Figures 11-13
will cause droop to appear at the transmit interface to the
TP wire. If the transformer connected to the 80220/80221
outputs meets the requirements in Table 21, the transmit
interface to the TP cable will meet the IEEE 802.3 droop
requirements.
44-499
MD400159/E