SNVA940 November   2021 LM5157 , LM5157-Q1 , LM51571-Q1 , LM5158 , LM5158-Q1 , LM51581 , LM51581-Q1

 

  1.   Trademarks
  2. 1LM5157 Boost Design Example
  3. 2Calculations and Component Selection
    1. 2.1  Switching Frequency
    2. 2.2  Inductor Calculation
    3. 2.3  Slope Compensation Check
    4. 2.4  Inductor Selection
    5. 2.5  Diode Selection
    6. 2.6  Output Capacitor Selection
    7. 2.7  Input Capacitor Selection
    8. 2.8  UVLO Resistor Selection
    9. 2.9  Soft-Start Capacitor Selection
    10. 2.10 Feedback Resistor Selection
    11. 2.11 Control Loop Compensation
      1. 2.11.1 Crossover Frequency (fcross) Selection
      2. 2.11.2 RCOMP Selection
      3. 2.11.3 CCOMP Selection
      4. 2.11.4 CHF Selection
    12. 2.12 Power Loss and Efficiency Estimation
  4. 3Implementation Results
  5. 4Small Signal Frequency Analysis
    1. 4.1 Boost Regulator Modulator Modeling
    2. 4.2 Compensation Modeling
    3. 4.3 Open Loop Modeling

2.1 Switching Frequency

With the given design parameters, selecting the proper switching frequency is the first step in the design process. In general, a higher switching frequency yields a smaller solution size at the cost of a higher switching loss and lower efficiency. Therefore, the final selection of the switching frequency is a trade-off between the power density and efficiency based on the specific requirement of the application. Harmonics of the switching frequency should be considered in designs that have strict EMC requirements. Equation 1 is used to set the frequency of the oscillator in the LM5157x and LM5158x. The example application is selected to have a switching frequency of 2.1MHz.

Equation 1. R T = 2.21 × 10 10 f sw - 955 = 2.21 × 10 10 2.1 MHz - 955 = 9.57 kΩ

A standard value of 9.53kΩ with 1% tolerance is chosen for RT.

The internal oscillator of the LM5157x and LM5158x can be synchronized to an external clock as described in the data sheet. The LM5157x and LM5158x has a maximum duty cycle limit that is frequency dependent, which is also characterized in the data sheet.