7

ML4668

SYSTEM DESCRIPTION

Figure 1 shows a schematic diagram of the ML4668 in an

internal or external 10BASE-FL MAU. On one side of the

transceiver is the AU interface and the other is the fiber

optic interface. The AU interface is AC coupled when

used in an external transceiver or an internal transceiver.

The AU interface for an external transceiver includes

isolation transformers, some biasing resistors, and a

voltage regulator for power.

The fiber optic side of the transceiver requires an external

fiber optic transmitter and fiber optic receiver. The

transmitter uses a current driven output that directly drives

the fiber optic transmitter. The receive side of the

transceiver accepts the data after passing through a fiber

optic receiver, which consists of a module containing a

pin diode and a transimpedance amplifier.

AU INTERFACE

The AU interface consists of 3 pairs of signals: DO, CI and

DI (Figure 1). The DO pair contains transmit data from the

DTE which is received by the transceiver and sent out

onto the fiber optic cable. The DI pair contains valid data

that has been either received from the fiber optic cable or

looped back from the DO, and output through the DI pair

to the DTE. The CI pair indicates whether a collision has

occurred. It is an output that oscillates at 10MHz if a

collision, Jabber or SQE Test has taken place, otherwise it

remains idle.

When the transceiver is external, these three pairs are AC

coupled through isolation transformers, while an internal

transceiver may be capacitively coupled. Tx+, Tx– is

internally DC biased (shifted up in voltage) for the proper

common mode input voltage.

The two 39

Ω 1% resistors (or one 78Ω 1% resistor) tied to

the Tx+ and Tx– pins will provide the proper termination.

The CI and DI pair, which are output from the transceiver

to the AUI cable, require 360

Ω pull down resistors when

terminated with a 78

Ω load. However, on a DTE card, CI

and DI do not need 78

Ω terminating resistors. This also

means that the pull down resistors on CI and DI can be

1k

Ω or greater depending upon the particular Manchester

encoder/decoder chip used. Using higher value pull down

resistors as in a DTE card will save power. Refer to

Application Note 13 for a more detailed explanation of

the AUI pull-down resistors.

The AUI drivers are capable of driving the full 50 meters

of cable length and have a rise and fall time of typically

4ns. In the idle state, the outputs go to the same voltage to

prevent DC standing current in the isolation transformers.

TRANSMISSION

The transmit function consists of detecting the presence of

data from the AUI DO input (Tx+, Tx–) and driving that

data onto the fiber optic LED transmitter. A positive signal

on the Tx+ lead relative to the Tx– lead of the DO circuit

will result in no current, hence the fiber optic LED is in a

low light condition. When Tx+ is more negative than Tx–,

the ML4668 will sink current into the chip and the fiber

optic LED will light up.

Before data will be transmitted onto the fiber optic cable

from the AUI interface, it must exceed the squelch

requirements for the DO pair. The Tx squelch circuit

serves the function of preventing any noise from being

transmitted onto the fiber. This circuit rejects signals with

pulse widths less than typically 20ns (negative going), or

with levels less than –250mV. Once Tx squelch circuit has

unsquelched, it looks for the start of idle signal to turn on

the squelch circuit again. The transmitter turns on the

squelch again when it receives an input signal at Tx+, Tx–

that is more positive than –250mV for more than

approximately 180ns.

At the start of a packet transmission, no more than 2 bits

are received from the DO circuit, and are not transmitted

onto the fiber optic cable. The difference between start-up

delays (bit loss plus steady-state propagation delay) for

any two packets that are separated by 9.6

µs or less will

not exceed 200ns.

FIBER OPTIC LED DRIVER

The output stage of the transmitter is a current mode

switch which develops the output light by sinking current

through the LED into the TxOUT pin. Once the current

requirement for the LED is determined, the RTSET resistor

is selected. The following equation is used to select the

correct RTSET resistor:

RTSET= 52mA









115

Ω

The ML4668 transmitter output will drive up to 100mA,

which requires RTSET to equal 60

Ω. The transmitter enters

the idle state when it detects start of idle on Tx+ and Tx–

input pins. After detection, transmitter switches to a 1MHz

output idle signal.

The output current is switched through the TxOUT pin

as shown in figure 2. Since the sum of the current in these

If not driving an optical LED directly, a differential output

voltage on these two pins should not be less than

TxOUT

Figure 2. Fiber Optic LED Driver Structure.