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

2.10.4 CHF Selection

The CHF capacitor sets the high frequency pole of the compensation network. The high frequency pole aids in attenuating high frequency noise due to the switching frequency and assuring enough gain margin. It is recommended to set the pole frequency between the minimum RHP zero (fRHPz) and one-half the switching frequency (fSW). The pole location is set at the geometric mean of the RHP zero and one-half the switching frequency using Equation 27.

Equation 27. fPea=fRHPz∙fsw2=19.5kHz∙440kHz2=65.5 kHz

The pole of the compensation network is selected to be approximately 65.5 kHz. CHF is calculated using Equation 28.

Equation 28. CHF=CCOMP2π∙CCOMP∙RCOMP∙fPea-1=6.8nF2π∙6.8nF∙54.9kΩ∙65.5 kHz-1=44.6 pF

CHF is estimated to be 44.6 pF. Selecting the nearest standard value, CHF is selected to be 47 pF.