SNVA866A February   2019  – January 2023 LM5155 , LM5155-Q1 , LM51551 , LM51551-Q1

 

  1.   How to design an Isolated Flyback using LM5155
  2.   Trademarks
  3. 1Introduction
  4. 2Example Application
  5. 3Calculations and Component Selection
    1. 3.1 Switching Frequency
    2. 3.2 Transformer Selection
      1. 3.2.1 Maximum Duty Cycle and Turns Ratio Selection
      2. 3.2.2 Primary Winding Inductance Selection
    3. 3.3 Current Sense Resistor Calculations
      1. 3.3.1 Current Sense Resistor and Slope Compensation Resistor Selection
      2. 3.3.2 Current Sense Resistor Filter Selection
    4. 3.4 MOSFET Selection
    5. 3.5 Diode Selection
    6. 3.6 Output Capacitor Selection
    7. 3.7 Input Capacitor Selection
    8. 3.8 UVLO Resistor Selection
    9. 3.9 Control Loop Compensation
      1. 3.9.1 Feedback Resistor Selection
      2. 3.9.2 RPULLUP Selection
      3. 3.9.3 Optocoupler Selection
      4. 3.9.4 RLED Selection
      5. 3.9.5 Crossover Frequency Selection
      6. 3.9.6 Determine Required RCOMP
      7. 3.9.7 Determine Required CCOMP
  6. 4Component Selection Summary
  7. 5Small Signal Frequency Analysis
    1. 5.1 Flyback Regulator Modulator Modeling
    2. 5.2 Compensation Modeling
  8. 6Revision History

3.9.6 Determine Required RCOMP

The RCOMP value directly affects the crossover frequency of the control loop. The higher the crossover frequency, the faster the control loop reacts to transient conditions. Decreasing the RCOMP resistance value lowers the crossover frequency but helps ensure the control loop remains stable over the specified supply voltage range. Knowing the desired loop crossover frequency, 6 kHz, RCOMP is calculate using Equation 30.

Equation 30. RCOMP=NSNP∙2∙π∙CLOAD∙RS∙fCROSS∙RLEDGCOMP∙KOPTO_max∙(1-D)=0.51∙2∙π∙540μF∙20mΩ∙6kHz∙1kΩ0.142∙2∙(1-0.357)=1.15kΩ

RCOMP is selected to be 1 kΩ.