TIDUDI9A January   2018  – May 2025 ISOM8610

 

  1.   1
  2.   Description
  3.   Resources
  4.   Features
  5.   Applications
  6.   6
  7. 1System Description
    1. 1.1 Key System Specifications
  8. 2System Overview
    1. 2.1 Block Diagram
    2. 2.2 Highlighted Products
      1. 2.2.1 ISO121x
      2. 2.2.2 SN74LV165A
      3. 2.2.3 SN74LVC1GU04
      4. 2.2.4 TVS3300
      5. 2.2.5 ISOM8600
    3. 2.3 System Design Theory
      1. 2.3.1 Digital Input Stage
      2. 2.3.2 Broken Wire Detection
        1. 2.3.2.1 Case 1: Wire Intact and Input State '1'
        2. 2.3.2.2 Case 2: Wire Intact and Input State '0'
        3. 2.3.2.3 Case 3: Broken Wire
      3. 2.3.3 Readout of Digital Outputs
  9. 3Hardware, Software, Testing Requirements, and Test Results
    1. 3.1 Required Hardware and Software
      1. 3.1.1 Hardware
      2. 3.1.2 Software
    2. 3.2 Testing and Results
      1. 3.2.1 Test Setup
      2. 3.2.2 Test Results
        1. 3.2.2.1 Group-Channel Configuration
        2. 3.2.2.2 Single-Channel Configuration
      3. 3.2.3 Conclusion
  10. 4Design Files
    1. 4.1 Schematics
    2. 4.2 Bill of Materials
    3. 4.3 PCB Layout Recommendations
      1. 4.3.1 Layout Prints
    4. 4.4 Altium Project
    5. 4.5 Gerber Files
    6. 4.6 Assembly Drawings
  11. 5Software Files
  12. 6Related Documentation
    1. 6.1 Trademarks
  13. 7About the Author
    1. 7.1 Acknowledgments
  14. 8Revision History

3.2.2.1 Group-Channel Configuration

The mechanical switch is turned off. The common pullup resistor is 100 kΩ. Then, the broken wire detection is executed. Figure 3-3 shows a scope shot where channel 2 of the scope is connected to the control signal of the optical switch (BrWi) and channel 1 is connected to the OUT pin of one ISO121x (ChOut). Figure 3-4 shows a scope shot where channel 2 of the scope is connected to the input capacitor CIN of one ISO121x channel (V_cap) and channel 1 is connected to the OUT pin of one ISO121x (ChOut).

TIDA-01509 Group Broken Wire Detection, Rpullup = 800kΩ/Channel - ChOut (2), BrWi (1) Figure 3-3 Group Broken Wire Detection, Rpullup = 800kΩ/Channel - ChOut (2), BrWi (1)
TIDA-01509 Group Broken Wire Detection, Rpullup = 800 kΩ/Channel - V_cap (2), BrWi (1) Figure 3-4 Group Broken Wire Detection, Rpullup = 800 kΩ/Channel - V_cap (2), BrWi (1)

Figure 3-3 shows that as soon as the ground connection is enabled again, the ISO121x gives out a pulse. The device takes 24 ms until CIN is charged. Figure 3-4 shows that CIN is charged to a maximum value of 22.6 V. The 24 V is not reached because of the voltage drop across the pullup resistor, which equals to a current of 1.4 V / 100 kΩ = 14 µA.

Note that this current is not used by the ISO121x, but is leakage current caused by the different protection devices and leakage through the input capacitor. Also, the voltage drop across the pullup resistor is lower if the pullup resistor itself is lower. This result is also shown in the single-channel setup.

Figure 3-5 shows a zoom into Figure 3-4.

TIDA-01509 Zoom Into (Figue 3-4: Group Broken Wire Detection, Rpullup = 800 kΩ/Channel - V_cap (2), BrWi (1)) - V_cap (1), ChOut (2) Figure 3-5 Zoom Into (Figue 3-4: Group Broken Wire Detection, Rpullup = 800 kΩ/Channel - V_cap (2), BrWi (1)) - V_cap (1), ChOut (2)

After the ISO1211 is enabled again, CIN is discharged and the ISO1211 starts operating. It takes around 14 µs until first, there is a solid GND connection for the ISO1211 established, and second, enough current (2.2 mA, typical, taken from ISO121x data sheet) is flowing through RSENSE while at the same time the voltage at the SENSE pin is still above the logic high level, so above 13.65 V - 1.2 V (hysteresis) = 12.45 V. Now, the output of the ISO1211 switches high. As soon as the energy stored in CIN is not anymore high enough to supply 2.2 mA and keep the voltage at the SENSE pin in the high level range, the output of the ISO1211 switches low.

For this configuration, the resulting pulse lasts around 50.5 µs.