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

3.6 Input Capacitor Selection

The input capacitors smooth the supply ripple voltage during operation. The input capacitor sees fairly low ripple currents due to the input inductor; like in a boost converter, the input current waveform is continuous and triangular. For this design, the input voltage ripple is designed to be less than 250 mV when the supply voltage is at the minimum value. Equation 16 is used to estimate the required input capacitor based on the supply ripple voltage specification.

Equation 16. CIN_min= POUT_totalVSUPPLY_min  ×(1-D)ΔVSUPPLY × fSW = 12W6V  ×(1-0.67)250mV × 2.1MHz=1.26 µF

Where ΔVSUPPLY is the maximum ripple that is allowed.

The input capacitor is selected to be > 40 µF. Ceramic capacitors are added to help lower the ESR of the input capacitor bank.