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LTC1535CSW Datasheet(PDF) 10 Page - Linear Technology |
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LTC1535CSW Datasheet(HTML) 10 Page - Linear Technology |
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10 / 16 page  10 LTC1535 1535fa Driver Output and Slow Slew Rate Control The LTC1535 uses a proprietary driver output stage that allows a common mode voltage range that extends be- yond the power supplies. Thus, the high impedance state is maintained over the full RS485 common mode range. The output stage provides 100mA of short-circuit current limiting in both the positive and negative directions. Thus, even under short-circuit conditions, the supply voltage from the open-loop DC converter will remain high enough for proper communication across the isolation barrier. The driver output will be disabled in the event of a thermal shutdown and a fault condition will be indicated through the RE weak output. The CMOS level SLO pin selects slow or fast slew rates on the RS485 driver output (see Figures 15, 16, 17, 18 for typical waveforms). The SLO input has an internal 100k pull-up resistor. When SLO is low, the driver outputs are slew rate limited to reduce high frequency edges. Left open or tied high, SLO defaults to fast edges. The part draws more current during slow slew rate edges. Monitoring Faults on RE The RE pin can be used to monitor the following fault conditions: low supply voltages, thermal shutdown or a time-out fault when there is no data communication across the barrier. During a fault, the receiver output, RO, defaults to a high state (see Table 2). Open circuit or short-circuit conditions on the twisted pair do not cause a fault indica- tion. However, the RS485 receiver defaults to a high output state when the receiver input is open or short- circuited. The RE pin has a weak current drive output mode for indicating fault conditions. This fault state can be polled using a bidirectional microcontroller I/O line or by using the circuit in Figure 11, where the control to RE is three- stated and the fault condition read back from the RE pin. The weak drive has 100 µA pull-up current to indicate a fault and 50 µA pull-down current for no fault. This allows the RE pin to be polled without disabling RE on nonfault conditions. Both sides contain a low voltage detect circuit. A voltage less than 4.2V on the isolated side disables communication. Figure 11. Detecting Fault Conditions RE POLL FAULT FAULT INDICATED WHEN RE IS THREE-STATED VCC 1535 F11 POLL FAULT BUFFER RE RO DI DE GND LTC1535 FAULT VCC APPLICATIO S I FOR ATIO |
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