LXT9782 datasheet(48/78 Pages) INTEL

48 / 78 page

LXT9762/9782 — Fast Ethernet 10/100 Multi-Port Transceiver with Serial MII

Datasheet

• Avoid vias and layer changes as much as possible.

• Keep the transmit and receive pairs apart to avoid cross-talk.

• Route the transmit pair adjacent to a ground plane. The optimum arrangement is to place the

transmit traces two to three layers from the ground plane, with no intervening signals.

• The output stage of the transmitter shown in Figure 24 on page 50 is designed to match the

100

Ω characteristic impedance of an unshielded CAT5 twisted-pair wire. The external resistor

that is typically required for impedance matching is integrated in the transmitter of the

LXT97xx. The internal termination provides a constant current reference in both 10BASE-T

and 100BASE-TX applications and meets all IEEE transmitter requirements such as return

loss, while reducing external component requirements. It has no impact in fibre designs.

• Some magnetic vendors are producing magnetics with improved return loss performance. Use

of these improved magnetics increases the return loss budget available to the system designer.

• Improve EMI performance by filtering the TPO center tap. A single ferrite bead may be used

to supply center tap current to all ports. All ports draw a combined total of

520 mA so the

bead should be rated at

780 mA.

3.1.6.1

Magnetics Information

The LXT97x2 requires a 1:1 ratio for the receive transformers and a 1:1 ratio for the transmit

transformers. The transformer isolation voltage should be rated at 1.5 kV to protect the circuitry

from static voltages across the connectors and cables. Refer to Table 16 for transformer

requirements. Before committing to a specific component, designers should contact the

manufacturer for current product specifications, and validate the magnetics for the specific

application.

3.1.7

The Fiber Interface

The fiber interface consists of a PECL transmit and receive pair to an external fiber-optic

transceiver. The transmit and receive pair should be DC-coupled to the transceiver, and biased

appropriately. Refer to the fiber transceiver manufacturer’s recommendations for termination

circuitry. Figure 25 on page 51 shows a typical example.

Table 16. Magnetics Requirements

Parameter

Min

Nom

Max

Units

Test Condition

Rx turns ratio

–

1 : 1

––

Tx turns ratio

–

1 : 1

––

Insertion loss

0.0

0.6

1.1

Primary inductance

350

––

µH

Transformer isolation

–

1.5

–

Differential to common mode rejection

––

.1 to 60 MHz

––

60 to 100 MHz

Return Loss

-16

––

30 MHz

-10

––

80 MHz

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