SNVA941A June   2020  – November 2022 LM5156 , LM5156-Q1 , LM51561 , LM51561-Q1 , LM51561H , LM51561H-Q1 , LM5156H , LM5156H-Q1

 

  1.   How to Design a Boost Converter Using the LM5156
  2. 1LM5156 Design Example
  3. 2Example Application
  4. 3Calculations and Component Selection
    1. 3.1  Switching Frequency
    2. 3.2  Inductor Calculation
    3. 3.3  Current Sense Resistor Calculation
      1. 3.3.1 Current Sense Resistor and Slope Compensation Resistor Selection
      2. 3.3.2 Current Sense Resistor Filter Calculation
    4. 3.4  Inductor Selection
    5. 3.5  Diode Selection
    6. 3.6  MOSFET Selection
    7. 3.7  Output Capacitor Selection
    8. 3.8  Input Capacitor Selection
    9. 3.9  UVLO Resistor Selection
    10. 3.10 Soft-Start Capacitor Selection.
    11. 3.11 Feedback Resistor Selection
    12. 3.12 Control Loop Compensation
      1. 3.12.1 Select the Loop Crossover Frequency (fCROSS)
      2. 3.12.2 Determine Required RCOMP
      3. 3.12.3 Determine Required CCOMP
      4. 3.12.4 Determine Required CHF
    13. 3.13 Efficiency Estimation
  5. 4Component Selection Summary
    1.     25
  6. 5Small-Signal Frequency Analysis
    1. 5.1 Boost Regulator Modulator Modeling
    2. 5.2 Compensation Modeling
    3. 5.3 Open-Loop Modeling
  7. 6Revision History

3.1 Switching Frequency

Selecting the proper switching frequency is the first step in the design process. Higher switching frequencies yield a smaller total solution size. However, the small size comes at the cost of increased switching losses, decreasing the total efficiency regulator. Higher efficiency is achieved by selecting a relatively lower switching frequency but requires physically larger components. Harmonics of the switching frequency should be considered in designs with strict EMC requirements. Equation 1 is used to set the frequency of the oscillator in the LM5156 device. The example application is selected to have a switching frequency of 440 kHz.

Equation 1. GUID-71E043CE-2A2E-43EB-8E26-2A6A70216247-low.gif

A standard value of 49.9 kΩ is chosen for RT.

The internal oscillator of the LM5156 can be synchronized to an external clock as described in the data sheet. The LM5156 has a maximum duty cycle limit that is frequency dependent. See the LM5156 data sheet for details on step-up ratio limitations.