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

2.10.1 Crossover Frequency (fcross) Selection

The crossover frequency of the control loop is selected to be less than the RHP zero of the modulator small signal model to ensure stability and simplify the compensation component calculations. The control loop crossover frequency is selected to be 1/8 the RHP zero frequency. The crossover frequency can be increased based on the application requirements but it is not recommended to exceed 1/5 the RHP zero frequency for wide supply voltage ranges. Equation 22 calculates the crossover frequency at the worst case operating condition.

Equation 22. fCROSS=18∙VLOADILOAD∙D'22π∙LM=18∙35V5.71A∙8V35V22π∙2.6µH=2.45 kHz

where

  • D' is (1 -D) at the minimum supply voltage

The target crossover frequency at the worst case operating condition is selected to be 2.45kHz. Note this is the same value used to calculate the minimum output capacitance in Section 2.5.