IL3522 Datasheet, PDF (6/10 Pages) List of Unclassifed Manufacturers – Isolated RS485/RS422 Interface

IL3522
Power Consumption
IsoLoop Isolators achieve their low power consumption from the way they transmit data across the isolation barrier. By detecting the edge
transitions of the input logic signal and converting these to narrow current pulses, a magnetic field is created around the GMR Wheatstone
bridge. Depending on the direction of the magnetic field, the bridge causes the output comparator to switch following the input logic signal.
Since the current pulses are narrow, about 2.5 ns, the power consumption is independent of mark-to-space ratio and solely dependent on
frequency. This has obvious advantages over optocouplers, which have power consumption heavily dependent on frequency and time.
Table 2. Typical Dynamic Supply Currents.
Data Rate (Mbps)
1
10
20
40
I_DD1
100 μA
1 mA
2 mA
4 mA
I_DD2
100 μA
1 mA
2 mA
4 mA
Power Supply Decoupling
Both V_DD1 and V_DD2 must be bypassed with 47 nF ceramic capacitors. These should be placed as close as possible to V_DD pins for proper
operation. Additionally, V_DD2 should be bypassed with a 10 µF tantalum capacitor.
DC Correctness
The IL3585 incorporates a patented refresh circuit to maintain the correct output state with respect to data input. At power up, the bus outputs
will follow the Function Table shown on Page 1. The DE input should be held low during power-up to eliminate false drive data pulses from the
bus. An external power supply monitor to minimize glitches caused by slow power-up and power-down transients is not required.
Application Information
Receiver Features
The receiver includes a “fail-safe if open” function that guarantees a high level output if the receiver inputs are unconnected (floating). The
receiver output “R” has tri-state capability via the active low RE input.
Driver Features
The RS-422 driver is differential output and delivers at least 1.5 V across a 54 Ω load. Drivers feature low propagation delay skew to maximize
bit width and minimize EMI. Drivers have tri-state capability via the active-high DE input.
Receiver Data Rate, Cables and Terminations
The IL3522 is intended for networks up to 4,000 feet (1,200 m), but the maximum data rate decreases as cable length increases. Twisted pair
cable should be used in all networks since they tend to pick up noise and other electromagnetically induced voltages as common mode signals,
which are effectively rejected by the differential receivers.
Fail-Safe Operation
“Fail-safe operation” is defined here as the forcing of a logic high state on the “R” output in response to an open-circuit condition between the
“A” and “B” lines of the bus, or when no drivers are active on the bus.
Proper biasing can ensure fail-safe operation, that is a known state when there are no active drivers on the bus. IL3000-Series Isolated
Transceivers include internal pull-up and pull-down resistors of approximately 30 kΩ in the receiver section (R_FS-INT; see figure below). These
internal resistors are designed to ensure failsafe operation but only if there are no termination resistors. The entire V_DD will appear between inputs
“A” and “B” if there is no loading and no termination resistors, and there will be more than the required 200 mV with up to four RS-422 worst-
case Unit Loads of 12 kΩ. Many designs operating below 1 Mbps or less than 1,000 feet are unterminated. Termination resistors may not be
necessary for very low data rates and very short cable runs because reflections have time to settle before data sampling, which occurs at the
middle of the bit interval.
In busses with low-impedance termination resistors, however, the differential voltage across the conductor pair will be close to zero with no
active drivers. In this case the state of the bus is indeterminate, and the idle bus will be susceptible to noise. For example, with 120 Ω termination
resistors (R_T) on each end of the cable, and four Unit Loads (12 kΩ each), without external fail-safe biasing resistors the internal pull-up and pull-
down resistors will produce a voltage between inputs “A” and “B” of only about 5 mV. This is not nearly enough to ensure a known state.
External fail-safe biasing resistors (R_FS-EXT) at one end of the bus can ensure fail-safe operation with a terminated bus. Resistors should be
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