ML4663 Datasheet, PDF (8/15 Pages) Micro Linear Corporation – Single Chip 10BASE-FL Transceiver

ML4663
SYSTEM DESCRIPTION
Figure 1 shows a schematic diagram of the ML4663 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 39Ω 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 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. 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 ML4663 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 9.6µs 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
I_OUT


115Ω
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 the V_CCTx pin during the off cycle
as shown in figure 2. Since the sum of the current in these
two pins is constant, V_CCTx should be connected as close
as possible to the V_CC connection for the LED.
If not driving an optical LED directly, a differential output
can be generated by tying resistors from V_CCTx and
TxOUT to V_CC as shown in figure 3. The minimum
voltage on these two pins should not be less than
V_CC – 2V.
V_CCTx TxOUT
I_OUT
Figure 2. Fiber Optic LED Driver Structure.
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