SNVA991A October   2022  – October 2025 LM5123-Q1

 

  1.   1
  2.   How to Design a Boost Converter Using LM5123
  3.   Trademarks
  4. 1Design Example
  5. 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
  6. 3Implementation Results
  7. 4Small Signal Frequency Modeling
    1. 4.1 Boost Regulator Modulator Modeling
    2. 4.2 Compensation Modeling
    3. 4.3 Open Loop Modeling
  8. 5Resources
  9. 6Revision History

4.3 Open Loop Modeling

These equations model the open loop transfer function of the control loop.

Open Loop Transfer function Figure 4-3 Open Loop Transfer function
Table 4-3 Open Loop Modeling Equations
Open Loop Equations
Simplified Formula Comprehensive Formula
Open Loop Transfer Function T s = A M 1 + s ω Z e s r ∙ 1 - s ω Z r h p 1 + s ω P l f ∙ - A F B 1 + s ω Z e a s ∙ 1 + s ω P e a T s = A M 1 + s ω Z e s r ∙ 1 - s ω Z r h p 1 + s ω P l f ∙ 1 + s Q ∙ ω n + s 2 ω n ∙ - A F B 1 + s ω Z e a s ∙ 1 + s ω P e a
Estimated crossover frequency f C R O S S = V S U P P L Y ∙ g M e a ∙ R C O M P 2 π ∙ A C S ∙ K F B ∙ R C S ∙ C O U T ∙ V L O A D Use Bode Plot