SNVA991 October   2022 LM5123-Q1

 

  1.   How to Design a Boost Converter Using LM5123
  2.   Trademarks
  3. 1Design Example
  4. 2Calculations and Component Selection
    1. 2.1  Switching Frequency
    2. 2.2  Initial Inductor Calculation
    3. 2.3  Current Sense Resistor Selection
    4. 2.4  Inductor Selection
    5. 2.5  Output Capacitor Selection
    6. 2.6  Input Capacitor Selection
    7. 2.7  Feedback Resistor Selection
    8. 2.8  UVLO Resistor Selection
    9. 2.9  Soft-Start Capacitor Selection
    10. 2.10 Control Loop Compensation
      1. 2.10.1 Crossover Frequency (fcross) Selection
      2. 2.10.2 RCOMP Selection
      3. 2.10.3 CCOMP Selection
      4. 2.10.4 CHF Selection
    11. 2.11 MOSFET selection
  5. 3Implementation Results
  6. 4Small Signal Frequency Modeling
    1. 4.1 Boost Regulator Modulator Modeling
    2. 4.2 Compensation Modeling
    3. 4.3 Open Loop Modeling
  7. 5Resources

2.8 UVLO Resistor Selection

The external under voltage lockout (UVLO) resistors set the minimum operating voltage of the regulator. Two levels must be specified: the desired start-up voltage of the converter (VSUPPLY(ON)) and the desired turn-off voltage of the converter (VSUPPLY(OFF)). In this example, VSUPPLY(ON) voltage is 6.2 V and the VSUPPLY(OFF) is 5.2 V. Using Equation 18, the top UVLO resistor (RUVT) is calculated.

Equation 18. R U V T = 0.977 ∙ V S U P P L Y o n - V S U P P L Y o f f I U V L O - H Y S T =   0.967 ∙ 6.2 V - 5.2 V 10 µ A = 85.9   k Ω

where

  • IUVLO-HYST is the current sunk by the EN/UVLO pin in standby mode.

A standard value of 86.6 kΩ is selected for RUVT. Using Equation 19, the bottom UVLO resistor (RUVB) is calculated.

Equation 19. R U V B = 1.1 V ∙ R U V L O t V S U P P L Y o n - 1.1 V = 1.1 V ∙ 86.6   k Ω 6.2 V - 1.1 V = 18.68   k Ω

A standard value of 18.7 kΩ is selected for RUVB.