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

3.5 Output Capacitor Selection

The output capacitor is required to smooth the load voltage ripple and provides an energy source during load transients and provides energy to the load during the on-time of the MOSFET. A practical way to size the output capacitor is based on the required load transient specification. The load transient specification is related to the control loop crossover frequency. For this estimate it is expected that the control loop cross over frequency is set to 1/5th the RHPZ frequency, which is calculated using Equation 15.

Equation 15. fCROSS= fZ_RHP 5=Np2Ns12×VLOAD12POUT_totalDMAX'25×2×π×LM×DMAX=121.22×1028.5W1-0.5125×2×π×8µH×0.51=15.3kHz

For this design example, the load transient specification indicates that the load voltage on VLOAD1 should not overshoot or undershoot more than 100 mV during a load transient from 50% load current (125 mA) to 100% load current (250 mA) occurs. Equation 16 is used to calculate the estimated load capacitance to achieve the specified load transient load voltage ripple requirements.

Equation 16. C L O A D 1 _ m i n =   ∆ I L O A D 1 2 × π × f C R O S S × ∆ V L O A D 1 =   125   m A 2 × π × 15.3 k H z   × 100 m V = 13 μ F

where

  • ΔILOAD1 is the difference in the load current conditions (250 mA - 125 mA)
  • ΔVLOAD1 is the specified overshoot voltage specification and undershoot voltage specification

In this design CLOAD1 is selected to be 120 µF.