SLLS897E March   2008  – June 2015 ISO1176

PRODUCTION DATA.  

  1. Features
  2. Applications
  3. Description
  4. Revision History
  5. Description (continued)
  6. Pin Configuration and Functions
  7. Specifications
    1. 7.1  Absolute Maximum Ratings
    2. 7.2  ESD Ratings
    3. 7.3  Recommended Operating Conditions
    4. 7.4  Thermal Information
    5. 7.5  Electrical Characteristics: ISODE-Pin
    6. 7.6  Supply Current
    7. 7.7  Electrical Characteristics: Driver
    8. 7.8  Electrical Characteristics: Receiver
    9. 7.9  Power Dissipation Characteristics
    10. 7.10 Switching Characteristics: Driver
    11. 7.11 Switching Characteristics: Receiver
    12. 7.12 Typical Characteristics
  8. Parameter Measurement Information
  9. Detailed Description
    1. 9.1 Overview
    2. 9.2 Functional Block Diagram
    3. 9.3 Feature Description
      1. 9.3.1 Insulation and Safety-Related Package Characteristics
      2. 9.3.2 DIN V VDE V 0884-10 Insulation Characteristics
      3. 9.3.3 IEC 60664-1 Ratings Table
      4. 9.3.4 Safety Limiting Values
      5. 9.3.5 Regulatory Information
    4. 9.4 Device Functional Modes
  10. 10Application and Implementation
    1. 10.1 Application Information
    2. 10.2 Typical Application
      1. 10.2.1 Design Requirements
      2. 10.2.2 Detailed Design Procedure
        1. 10.2.2.1 Transient Voltages
        2. 10.2.2.2 ISO1176 “Sticky Bit” Issue (Under Certain Conditions)
      3. 10.2.3 Application Curve
  11. 11Power Supply Recommendations
  12. 12Layout
    1. 12.1 Layout Guidelines
    2. 12.2 Layout Example
  13. 13Device and Documentation Support
    1. 13.1 Documentation Support
      1. 13.1.1 Related Documentation
    2. 13.2 Community Resources
    3. 13.3 Trademarks
    4. 13.4 Electrostatic Discharge Caution
    5. 13.5 Glossary
  14. 14Mechanical, Packaging, and Orderable Information

Package Options

Mechanical Data (Package|Pins)
Thermal pad, mechanical data (Package|Pins)
Orderable Information

10 Application and Implementation

NOTE

Information in the following applications sections is not part of the TI component specification, and TI does not warrant its accuracy or completeness. TI’s customers are responsible for determining suitability of components for their purposes. Customers should validate and test their design implementation to confirm system functionality.

10.1 Application Information

The ISO1176 device consists of a RS-485 transceiver, commonly used for asynchronous data transmissions. For half-duplex transmission, only one pair is shared for both transmission and reception of data. To eliminate line reflections, each cable end is terminated with a termination resistor, R(T), whose value matches the characteristic impedance, Z0, of the cable. This method, known as parallel termination, allows for higher data rates over longer cable length.

10.2 Typical Application

ISO1176 trans_app_llse11.gifFigure 31. Half-Duplex Transceiver Configurations

10.2.1 Design Requirements

RS-485 is a robust electrical standard suitable for long-distance networking that may be used in a wide range of applications with varying requirements, such as distance, data rate, and number of nodes.

Table 3. Design Parameters

PARAMETER VALUE
Pullup and Pulldown Resistors 1 kΩ to 10 kΩ
Decoupling Capacitors 100 nF

10.2.2 Detailed Design Procedure

Isolating of a circuit insulates it from other circuits and earth, so that noise voltage develops across the insulation rather than circuit components. The most common noise threat to data-line circuits is voltage surges or electrical fast transients that occur after installation. The transient ratings of the ISO1176 standard are sufficient for all but the most severe installations. However, some equipment manufacturers use ESD generators to test equipment transient susceptibility. This practice can exceed insulation ratings. ESD generators simulate static discharges that may occur during device or equipment handling with low-energy but high-voltage transients.

ISO1176 appschem_lls897.gifFigure 32. Device Model for Static Discharge Testing

Figure 32 models the ISO1176 bus IO connected to a noise generator. CIN and RIN is the device, and any other stray or added capacitance or resistance across the A or B pin to GND2. CISO and RISO is the capacitance and resistance between GND1 and GND2 of the ISO1176, plus those of any other insulation (transformer, and so forth). Stray inductance is assumed to be negligible.

10.2.2.1 Transient Voltages

From this model, the voltage at the isolated bus return is

Equation 1. ISO1176 q1_lls897.gif

and is always less than 16 V from VN. If the ISO1176 is tested as a stand-alone device,

  • RIN= 6 x 104 Ω,
  • CIN= 16 x 10–12 F,
  • RISO= 109 Ω and
  • CISO= 10–12 F.

Notice from Figure 32 that the resistor ratio determines the voltage ratio at low frequencies, and that the inverse capacitance ratio determines the voltage ration at high frequencies. In the stand-alone case and for low frequencies,

Equation 2. ISO1176 q2_lls897.gif

or essentially all of the noise appears across the barrier.

At high frequencies,

Equation 3. ISO1176 q3_lls897.gif

and 94% of VN appears across the barrier. As long as RISO is greater than RIN and CISO is less than CIN, most of the transient noise appears across the isolation barrier, as it should.

Using ESD generators to test equipment transient susceptibility, or considering product claims of ESD ratings greater than the barrier transient ratings of an isolated interface is not recommended. ESD is best managed through recessing or covering connector pins in a conductive connector shell, and by proper installer training.

10.2.2.2 ISO1176 “Sticky Bit” Issue (Under Certain Conditions)

Summary: In applications with sufficient differential noise on the bus, the output of the ISO1176 receiver may “stick” at an incorrect state for up to 30 µs.

Description: The ISO1176 isolated Profibus (RS-485) transceiver is rated for signaling up to 40 Mbps on twisted-pair bus lines. The receiver thresholds comply with RS-485 and Profibus specifications; an input differential voltage VID = VA – VB > 200 mV causes a logic High on the R output, and VID < –200 mV causes a logic Low on the R output. To assure a known receiver output when the bus is shorted or idle, the upper threshold is set below zero, such that VID = 0 mV causes a logic High on the R output. The data sheet specifies a typical upper threshold (VIT+) of –80 mV and a typical lower threshold (VIT-) of –120 mV.

At a signaling rate of 40 Mbps, each valid data bit has a duration of 25 ns. At typical Profibus signaling rates of 12 Mbps or lower, each valid data bit has a duration of 83 ns or more. The ISO1176 correctly sets the R output for each of these valid data bits.

In applications with a high degree of differential noise on the bus lines, it is possible to get short periods when an invalid bus voltage triggers a change in state of the internal receiver circuits. An issue with the digital isolation channel in the ISO1176 may cause the invalid receiver state to “stick” rather than immediately transition back to the correct state. The receiver output will always transition to the correct state, but may stick in the incorrect state for up to 30 µs. This can cause a temporary loss of data.

Figure 33 shows two cases which could result in temporary loss of data.

ISO1176 temp_data_loss_lls897.gifFigure 33. "Sticky Bit" Issue Waveforms

10.2.3 Application Curve

At maximum working voltage, ISO1176 isolation barrier has more than 28 years of life.

ISO1176 life_dr_lls755.gifFigure 34. Time-Dependent Dielectric Breakdown Test Results