ML4662 Просмотр технического описания (PDF) - Micro Linear Corporation

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ML4662

10BASE-FL Transceiver

Micro Linear Corporation 

SYSTEM DESCRIPTION

Figure 1 shows a schematic diagram of the ML4662 in an

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

transceiver is the AU interface and on the other is the fiber

optic interface. The AU interface is AC coupled when

used in an external transceiver or can be AC or DC

coupled when used in 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, fiber optic receiver, and the

ML4622 or ML4624 fiber optic quantizers. 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 the fiber optic

receiver and the ML4622/ML4624 fiber optic quantizer.

AU INTERFACE

The AUI 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 AC or DC coupled. For the AC

coupled interface, DO (which is an input) must be DC

biased (shifted up in voltage) for the proper common

mode input voltage. The BIAS pin serves this purpose.

When DC coupled, the transmit pair coming from the

serial interface provides this common mode voltage, and

the BIAS pin is not connected.

The two 39Ω 1% resistors tied to the Tx+ and Tx– pins

provide a point to connect the common mode bias voltage

as discussed above, and they provide the proper matching

termination for the AUI cable. The CI and DI pair, which

are output from the transceiver to the AUI cable, require

360Ω pull down resistors when terminated with a 78Ω

load. However on a DTE card, CI and DI do not need 78Ω

terminating resistors. This also means that the pull down

resistors on CI and DI can be 1kΩ 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.

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.

ML4662

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 ML4662

will sink current into the chip and the 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 TxIN± 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 9.6µs 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

IOUT





162Ω

(1)

The transmitter enters the idle state when it detects start of

idle on Tx+ and Tx– input pins. After detection, the

transmitter switches to a 1MHz output idle signal.

The output current is switched through the TxOUT pin

during the on cycle and through the VCCTx pin during the

off cycle (Figure 2). Since the sum of the current in these

two pins is constant, VCCTx should be connected as close

as possible to the VCC connection for the LED (Figure 2).

VCCTx TxOUT

IOUT

Figure 2. Fiber Optic LED Driver Structure.

7

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