SNVAA43 March   2023 LM5157 , LM5157-Q1 , LM51571-Q1 , LM5158 , LM5158-Q1 , LM51581 , LM51581-Q1

 

  1.   Abstract
  2.   Trademarks
  3. 1Introduction
  4. 2Application Parameters
  5. 3Calculations and Component Selection
    1. 3.1 Switching Frequency
    2. 3.2 Coupled Inductor Selection
      1. 3.2.1 Coupling Capacitor (AC Cap)
    3. 3.3 Diode Selection
    4. 3.4 MOSFET Selection
    5. 3.5 Output Capacitor Selection
    6. 3.6 Input Capacitor Selection
    7. 3.7 Output Voltage Setting
  6. 4Simplified SEPIC Schematic
  7. 5References

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 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 14.

Equation 14. fRHPZ=(1-DMAX)2×VOUT2π×D×L×Iout

For this design example, the load transient specification indicates that the load voltage must not overshoot or undershoot more than 100 mV during a load transient from 50% load current (0.5 A) to 100% load current (1 A) occurs. Equation 15 is used to calculate the estimated load capacitance to achieve the specified load transient load voltage ripple requirements.

Equation 15. COUT≥∆ILOAD2π×fBW×∆VLOAD

where

  • ΔILOAD is the difference in the load current conditions (1 A - 0.5 A)
  • ΔVLOAD is the specified overshoot voltage specification and undershoot voltage specification

In this design CLOAD is selected to be 150 µF.