SNVA994A February   2022  – March 2023 LM5157 , LM5157-Q1 , LM51571-Q1 , LM5158 , LM5158-Q1 , LM51581 , LM51581-Q1

 

  1.   1
  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 Slope Compensation Check
    4. 3.4 Diode Selection
    5. 3.5 Output Capacitor Selection
    6. 3.6 Input Capacitor Selection
    7. 3.7 UVLO Resistor Selection
    8. 3.8 Control Loop Compensation
      1. 3.8.1 Crossover Frequency (fcross) Selection
      2. 3.8.2 RCOMP Selection
      3. 3.8.3 CCOMP Selection
      4. 3.8.4 CHF Selection
  6. 4Component Selection Summary
    1. 4.1 Application Circuit
    2. 4.2 Bill of Materials
  7. 5Small Signal Frequency Analysis
    1. 5.1 Flyback Regulator Modulator Modeling
    2. 5.2 Compensation Modeling
  8. 6Revision History

3.7 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 LM5157/58 starts operation (VSUPPLY(ON)) and the voltage the LM5157/58 enters stand-by mode (VSUPPLY(OFF)). In this example, VSUPPLY(ON) voltage is 7.5 V and the VSUPPLY(OFF) is 7 V. Using Equation 18, the top UVLO resistor (RUVLOT) is calculated.

Equation 18. R U V L O T =   0.967   ×   V S U P P L Y ( O N ) -   V S U P P L Y ( O F F ) 5 µ A   =   0.967   ×   7.5 V -   7 V 5 µ A = 50.5 k Ω

RUVLOT is selected to be 49.9 kΩ. RUVLOB is calculated using Equation 19.

Equation 19. R U V L O B =   1.5 V   ×   R U V L O T V S U P P L Y ( O N ) - 1.5 V   =   1.5 V ×   49.9 k Ω 7.5 V -   1.5 V = 12.48 k Ω

RUVLOB is selected to be 12.4 kΩ.