80220 datasheet(49/91 Pages) LSI

49 / 91 page

80220/80221

4-49

MD400159/E

Table 21. TP Transformer Specification

Parameter

Specification

Transmit

Receive

Turns Ratio

2:1 CT

1:1

Inductance, (

µH Min)

350

Leakage Inductance, (

µH)

0.05-0.15

0.0-0.2

Capacitance (pF Max)

DC Resistance (Ohms Max)

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:

out

= (10K/R) I

ref

Where 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

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

TERM

(Ohms)

Select Bit

(16.7)

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.

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