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

2.9 Soft-Start Capacitor Selection

The soft-start capacitor eliminates the overshoot of the load voltage and minimizes the inrush current during the start-up of the regulator when properly selected. The minimum recommended soft-start capacitor value is calculated using Equation 20.

Equation 20. CSSmin=ISS∙VLOAD∙COUTVTRK∙ILOAD=20µA∙35V∙900µF583mV∙5.71A= 189 nF

where

  • ISS is the current sourced from the SS pin
  • VLOAD is the maximum target load voltage
  • VTRK is the voltage on the TRK pin to set VLOAD
  • ILOAD is the maximum load current at VLOAD

The soft-start capacitor should be larger than 189 nF to eliminate overshoot on the load voltage during start-up.

In this design, for a given soft-start time of 7 ms at the minimum input voltage of 8 V, the soft-start capacitor can be calculated using Equation 21.

Equation 21. CSS=tSS∙ISSVTRK∙1-VSUPPLYminVLOADmax=7ms∙20µA583mV∙1-8V35V=313 nF

For this design a CSS value of 330nF is selected.