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

2.10.3 CCOMP Selection

The RCOMP resistor and CCOMP capacitor set the zero frequency of the compensation network. The zero of the compensation network provides a phase boost to stabilize the control loop. To provide adequate phase margin the zero frequency is placed at the geometric mean of the crossover frequency (fCROSS) and the low frequency pole of the plant (fPlf). The low frequency pole of the modulator and the zero frequency of the compensation network are estimated using Equation 24and Equation 25, respectively

Equation 24. f P l f = I L O A D π ∙ C O U T ∙ V L O A D = 5.71 A π ∙ 900 µ F ∙ 35 V = 57 H z
Equation 25. f Z e a = f C R O S S ∙ f P l f = 2.45 k H z ∙ 57 H z = 373   H z

The the zero frequency of the compensation network estimated to be approximately 373 Hz. Knowing the target frequency, CCOMP is calculate using Equation 26.

Equation 26. C C O M P = 1 2 π ∙ f Z e a ∙ R C O M P = 1 2 π ∙ 2.45 k H z ∙ 57 H z ∙ 54.9 k Ω = 7.76 n F

CCOMP is calculated to be 7.7 nF. Selecting the nearest standard value CCOMP is selected to be 6.8 nF.