Datasheet Linear Technology LTC1535

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Design Notes

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  Isolated RS485 Transceiver Breaks Ground Loops &mdash DN228
  PDF, 992 Kb, File published: Apr 2, 2001
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  Isolated RS485 Transceiver Breaks Ground Loops – Design Note 228
  Mitchell Lee Previously, isolation was achieved by using at least three
  optoisolators and a separate isolated power supply. The
  LTC1535 replaces not only the optoisolators, but also the
  power supply, as it includes an on-chip DC/DC converter.
  Other features include selectable driver slew rate to reduce
  EMI and susceptibility to reflections, full-duplex pinout
  and fail-safe detection of open and shorted lines.
  The LTC1535 consists of two separate dice assembled on
  a proprietary, isolated lead frame. The lead frame includes
  integral coupling capacitors that bridge the isolation barrier
  and exhibit 2,500VRMS guaranteed standoff. Data communication takes place via the coupling capacitors, while
  an on-chip, 400kHz push-pull converter sends power to the
  isolated side through a small transformer. Total common
  mode capacitance across the barrier amounts to less than
  20pF, with the transformer accounting for about 16pF of
  the total. Figure 1 shows the complete circuit for a fully
  isolated RS485 port.
  Internally, the two halves of the LTC1535 communicate
  in a ping-pong fashion, first sending transmit data to the
  isolated side and then sending receive data back to the
  nonisolated side. The sampling nature of the internal communications link means that some jitter is introduced into
  the data; this limits the useful baud rate to approximately …

Articles

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  Isolated RS485 Transceiver Breaks Ground Loops &mdash LT Journal
  PDF, 308 Kb, File published: Nov 1, 1999
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  DESIGN IDEAS Isolated RS485 Transceiver
  Breaks Ground Loops Linear Technology Magazine November 1999 For example, if the twisted pair is
  accidentally miswired or faults to 117V
  at some remote location, the floating
  section of each LTC1535 and its associated circuitry will also carry 117V.
  An unwary user or installer could
  then come in contact with what is
  assumed to be a safe, low voltage
  circuit. Figure 6 shows how to detect
  and warn the user that a fault condition exists on the twisted pair or its
  shield. A small (3.2mm) glow lamp is
  connected between GND2 (the
  LTC1535’s floating ground) and the
  equipment’s safety “earth” ground. If
  a potential of more than 75VAC is
  present on the twisted pair or shield,
  B1 will light, indicating a wiring fault.
  Resistors R3 and R4 are used to ballast the current in B1. Two resistors
  are necessary because each resistor
  can only stand off 200V, as well as for
  reasons of power dissipation. As
  shown, the circuit can withstand a
  direct fault to a 440V, 3-phase system. …

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Manufacturer's Classification

• Interface > RS485/RS422 Transceivers