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

3.8.3 CCOMP Selection

The RCOMP resistor and CCOMP capacitor set a low frequency zero of the compensation network, providing a phase boost. Placement of this zero frequency largely impacts the transient response of the control loop. A good strategy to help ensure adequate phase margin is to place the zero at geometric mean of the crossover frequency (fCROSS) and the low frequency pole of the modulator. Equation 24 places the low frequency zero of error amplifier a the geometric mean of fCROSS and low frequency pole of the plant (ωP_LF). Equation 24 is used to calculate the value of CCOMP.

Equation 24. CCOMP= CLOAD_total × VLOAD122 × π × RCOMP2 × fCROSS × POUT_total × (1+D) CCOMP= 300µF × 10V22 × π × 10kΩ2 × 5kHz ×8.5W ×(1+0.51)=27.2nF

CCOMP is selected to be 22 nF.