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

3.6 Input Capacitor Selection

The input capacitors smooth the supply ripple voltage during operation. For this design, the input voltage ripple is designed to be less than 50 mV when the supply voltage is at the minimum value. Equation 17 is used to estimate the required input capacitor based on the supply ripple voltage specification.

Equation 17. C I N _ m i n =   P O U T _ t o t a l V S U P P L Y _ m i n     × ( 1 - D ) Δ V S U P P L Y   ×   f S W   =   8.5 W 8 V     × ( 1 - 0.51 ) 250 m V   ×   250 k H z = 8.33 µ F

The input capacitor is selected to be 100 µF. Ceramic capacitors are added to help lower the ESR of the input capacitor bank.