SNVA940 November   2021 LM5157 , LM5157-Q1 , LM51571-Q1 , LM5158 , LM5158-Q1 , LM51581 , LM51581-Q1

 

  1.   Trademarks
  2. 1LM5157 Boost Design Example
  3. 2Calculations and Component Selection
    1. 2.1  Switching Frequency
    2. 2.2  Inductor Calculation
    3. 2.3  Slope Compensation Check
    4. 2.4  Inductor Selection
    5. 2.5  Diode Selection
    6. 2.6  Output Capacitor Selection
    7. 2.7  Input Capacitor Selection
    8. 2.8  UVLO Resistor Selection
    9. 2.9  Soft-Start Capacitor Selection
    10. 2.10 Feedback Resistor Selection
    11. 2.11 Control Loop Compensation
      1. 2.11.1 Crossover Frequency (fcross) Selection
      2. 2.11.2 RCOMP Selection
      3. 2.11.3 CCOMP Selection
      4. 2.11.4 CHF Selection
    12. 2.12 Power Loss and Efficiency Estimation
  4. 3Implementation Results
  5. 4Small Signal Frequency Analysis
    1. 4.1 Boost Regulator Modulator Modeling
    2. 4.2 Compensation Modeling
    3. 4.3 Open Loop Modeling

2.11.4 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 at the RHP zero(ωZ_RHP), which is usually smaller than the ESR zero, or between the RHP zero and half the switching frequency. In this design example, the high frequency pole is placed at the RHP zero. Equation 24 is used to calculate the value of CHF.

Equation 24. CHF=CCOMP∙LMCCOMP∙D'2∙RLOAD∙RCOMP-LM=10nF∙1.5μH10nF∙0.752∙7.5Ω∙2.62kΩ-1.5μH=138pF

CHF is chosen to be 100pF.