SNVA994A February   2022  – March 2023 LM5157 , LM5157-Q1 , LM51571-Q1 , LM5158 , LM5158-Q1 , LM51581 , LM51581-Q1

 

  1.   1
  2.   Trademarks
  3. 1Introduction
  4. 2Example Application
  5. 3Calculations and Component Selection
    1. 3.1 Switching Frequency
    2. 3.2 Transformer Selection
      1. 3.2.1 Maximum Duty Cycle and Turns Ratio Selection
      2. 3.2.2 Primary Winding Inductance Selection
    3. 3.3 Slope Compensation Check
    4. 3.4 Diode Selection
    5. 3.5 Output Capacitor Selection
    6. 3.6 Input Capacitor Selection
    7. 3.7 UVLO Resistor Selection
    8. 3.8 Control Loop Compensation
      1. 3.8.1 Crossover Frequency (fcross) Selection
      2. 3.8.2 RCOMP Selection
      3. 3.8.3 CCOMP Selection
      4. 3.8.4 CHF Selection
  6. 4Component Selection Summary
    1. 4.1 Application Circuit
    2. 4.2 Bill of Materials
  7. 5Small Signal Frequency Analysis
    1. 5.1 Flyback Regulator Modulator Modeling
    2. 5.2 Compensation Modeling
  8. 6Revision History

3.1 Switching Frequency

Selecting the switching frequency is the first step in the design process. Higher switching frequencies yields a smaller total solution size. However, the small size comes at the cost of increased switching losses, decreasing the efficiency of the converter. 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 that have strict EMC requirements. Equation 1 is used to set the frequency of the internal oscillator of the LM5157x/LM5158x. The example application is selected to have a switching frequency of 250 kHz.

Equation 1. RT=2.21 × 1010fSW-955= 2.21 × 1010250kHz-955=87.45kΩ

A standard value of 86.6 kΩ is chosen for RT.

Note that the internal oscillator of the LM5157x/LM5158x can be synchronized to an external clock as described in the data sheet. The LM5157x/LM5158x has a maximum duty cycle limit that is frequency dependent. See the LM5157x/LM5158x data sheet for details on the maximum duty cycle limit.