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

3.9.1 Feedback Resistor Selection

To implement the circuit shown in Figure 3-2, a voltage reference must first be selected. The TLV431 is selected due to the low reference voltage of 1.24 V. For higher load voltage designs is it acceptable to use the TL431, with a reference voltage of 2.5 V. The load voltage of this designs is 5 V, selecting a reference voltage of 1.24 V allows for more headroom to ensure the voltage reference is properly biased. The top feedback resistor, RFBT, is selected to be 30 kΩ in this design. Equation 26 is used to calculate the lower feedback resistor, RFBB.

Equation 26. GUID-EE25223E-ED84-4621-82AE-4D5A302B57BC-low.gif

RFBB is selected to be the standard value of 9.78kΩ.