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ML4665CQ Folha de dados(PDF) 7 Page - Micro Linear Corporation |
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ML4665CQ Folha de dados(HTML) 7 Page - Micro Linear Corporation |
7 / 13 page 7 ML4665 SYSTEM DESCRIPTION Figure 1 shows a schematic diagram of the ML4665 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 or Jabber 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 ML4665 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 IOUT 115 Ω The ML4665 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 detecting the start of idle the transmitter switches to a 1MHz output idle signal. The output current is switched through the TxOUT pin during the on cycle and the VCCTx pin during the off cycle as shown in 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. If not driving an optical LED directly, a differential output can be generated by tying resistors from VCCTx and TxOUT to VCC as shown in figure 3. The minimum voltage on these two pins should not be less than VCC – 2V. VCCTx TxOUT IOUT Figure 2. Fiber Optic LED Driver Structure. |
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