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

3.8 UVLO Resistor Selection

The external under voltage lockout (UVLO) resistors set the minimum operating supply voltage of the regulator. Two levels must be specified; the voltage the LM5155 starts operation (VSUPPLY(ON)) and the voltage the LM5155 enters stand-by mode (VSUPPLY(OFF)). In this example, VSUPPLY(ON) voltage is 17 V and the VSUPPLY(OFF) is 16 V. Using Equation 24, the top UVLO resistor (RUVLOT) is calculated.

Equation 24. GUID-41A2F70D-774B-4F23-973A-ACBF1571571B-low.gif

RUVLOT is selected to be 100kΩ. RUVLOB is calculated using Equation 25.

Equation 25. GUID-45C5D4F5-C764-4280-B418-66124A4943E1-low.gif

RUVLOB is selected to be 9.67kΩ.