ENC424J600 datasheet(13/168 Pages) MICROCHIP

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 2010 Microchip Technology Inc.

DS39935C-page 11

ENC424J600/624J600

2.4

Ethernet Signal Pins and External

Magnetics

Typical applications for ENC424J600/624J600 devices

require an Ethernet transformer module, and a few

resistors and capacitors to implement a complete

IEEE 802.3 compliant 10/100 Ethernet interface, as

shown in Figure 2-5.

The Ethernet transmit interface consists of two pins:

TPOUT+ and TPOUT-. These pins implement a

differential pair and a current-mode transmitter. To

generate an Ethernet waveform, ordinary applications

require the use of a 1:1 center tapped pulse

transformer, rated for 10/100 or 10/100/1000 Ethernet

operations. When the Ethernet module is enabled and

linked with a partner, current is continually sunk

through both TPOUT pins. When the PHY is actively

transmitting, a differential voltage is created on the

Ethernet cable by varying the relative current sunk by

TPOUT+ compared to TPOUT-.

The Ethernet receive interface similarly consists of a

differential pair: TPIN+ and TPIN-. To meet IEEE 802.3

compliance and help protect against electrostatic dis-

charge, these pins are normally isolated from the

Ethernet cable by a 1:1 center tapped transformer

(available in the same package as the TX transformer).

Internally, the PHY uses a high-speed ADC to sample

the receive waveform and decodes it using a DSP. The

PHY implements many robustness features, including

baseline wander correction (applicable to 100Base-TX)

and automatic RX polarity correction (applicable to

10Base-T).

Four 49.9Ω, 1% resistors are required for proper

termination of the TX and RX transmission lines. If the

board layout necessitates long traces between the

ENCX24J600 and Ethernet transformers, the termina-

tion resistors should be placed next to the silicon

instead of the transformers.

On the receive signal path, two 6.8 nF 10% capacitors

are used. These capacitors, in combination with the

49.9

 termination resistors, form an RC high-pass filter

to reduce baseline wander. For best performance,

these capacitors should not be omitted or changed.

The various remaining capacitors provide DC current

blocking and provide stability to the common-mode

voltage of both of the differential pairs. The TPIN+/-

pins weakly output a common-mode voltage that is

acceptable to the internal ADC. For proper operation,

do not attempt to externally force the TPIN+/-

common-mode voltage to some other value.

The 10Ω 1% resistor provides a current path from the

power supply to the center tap of the TX transformer.

mentioned

previously,

the

TPOUT+/-

pins

implement a Current mode drive topology in which the

pins are only capable of sinking current; they do not

produce a direct voltage. This current path through the

transformer generates the transmit waveform. The 10Ω

resistor reduces the amount of heat that the PHY would

have to dissipate, and therefore, must have a power

rating of 1/12W or better.

FIGURE 2-5:

TYPICAL ETHERNET MAGNETICS CONNECTIONS

ENCX24J600

TPOUT

TPIN

3.3V

RJ-45

1:1 CT

1000 pF, 2 kV

 75

49.9

, 1%

49.9

, 1%

49.9

, 1%

49.9

, 1%

0.01

F

0.01

F

6.8 nF, 10%

, 1/12W, 1%

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