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

2.1 Switching Frequency

With the given design parameters, selecting the proper switching frequency is an key step for any design. 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 to strict EMC requirements. The required frequency set resistor for the internal oscillator is given in Equation 1.

Equation 1. RT=2.21×1010fsw-955=2.21×1010440KHz-955=49.2kΩ
Figure 2-1 Frequency vs RT

The example application is selected to have a switching frequency of 440 kHz, and the standard value of 49.9kΩ with 1% tolerance is chosen for RT. 440 kHz is selected as it provides a well balanced solutions between power density and efficiency and avoids the AM band (530 kHz to 1.8 MHz) where conducted emissions are greatly limited.

The internal oscillator of the device can be synchronized to an external clock as described in the data sheet. The LM5123 has a maximum duty cycle limit that is frequency dependent, which is also characterized in the data sheet.