SNLA490 March   2025 ISOM8110-Q1 , ISOM8111-Q1 , ISOM8112-Q1 , ISOM8113-Q1 , ISOM8115-Q1 , ISOM8116-Q1 , ISOM8117-Q1 , ISOM8118-Q1

 

  1.   1
  2.   Summary
  3. 1Relay Contact Sticking in OBCs
    1. 1.1 Relays Used in OBCs
    2. 1.2 Causes and Hazards of Relay Contact Sticking
  4. 2About Opto-emulators
    1. 2.1 Introduction to ISOM811x-Q1 and Key Parameters Like CTR
    2. 2.2 Performance Comparison with Optocoupler and Single-Channel Digital Isolator
  5. 3AC Relay Weld Detection in V2L Circuit for OBCs Based on ISOM8118-Q1
    1. 3.1 What Is V2L for OBCs
    2. 3.2 Design Reference for AC Relay Weld Detection Circuit in V2L
    3. 3.3 Design Examples and Selection Guidelines Based on Typical Circuits
      1. 3.3.1 Select Voltage Judgment Threshold VTH and IFmin
      2. 3.3.2 Select Current-limiting Resistor R1
      3. 3.3.3 Select the pull-up resistor R2
  6. 4Summary
  7. 5References

3.3.3 Select the pull-up resistor R2

The ISOM8118-Q1 has transistor output characteristics similar to a conventional transistor and can operate in active, saturation, reverse, and cutoff regions. To ensure that a low level is correctly output after the input drops below the threshold at the minimum CTR, the R2 resistance must satisfy:

Equation 5. R 2 < V D D 2 - V C E ( S A T ) I F m i n × C T R M I N

where VCE(SAT) = 0.3V and CTRMIN = 295%, so that R2<1.02kΩ is calculated. In the meantime, the R2 resistance should not be too small to avoid over-current at the ISOM8118-Q1 output, so the R2 resistance should also satisfy:

Equation 6. R 2 > V D D 2 - V C E ( S A T ) I C M A X

The calculation yields R2>60Ω. With these considerations in mind, choosing R2 to be 1kΩ ensures that the circuit works properly while effectively avoiding any risk of over-current