SNVA940 November   2021 LM5157 , LM5157-Q1 , LM51571-Q1 , LM5158 , LM5158-Q1 , LM51581 , LM51581-Q1

 

  1.   Trademarks
  2. 1LM5157 Boost Design Example
  3. 2Calculations and Component Selection
    1. 2.1  Switching Frequency
    2. 2.2  Inductor Calculation
    3. 2.3  Slope Compensation Check
    4. 2.4  Inductor Selection
    5. 2.5  Diode Selection
    6. 2.6  Output Capacitor Selection
    7. 2.7  Input Capacitor Selection
    8. 2.8  UVLO Resistor Selection
    9. 2.9  Soft-Start Capacitor Selection
    10. 2.10 Feedback Resistor Selection
    11. 2.11 Control Loop Compensation
      1. 2.11.1 Crossover Frequency (fcross) Selection
      2. 2.11.2 RCOMP Selection
      3. 2.11.3 CCOMP Selection
      4. 2.11.4 CHF Selection
    12. 2.12 Power Loss and Efficiency Estimation
  4. 3Implementation Results
  5. 4Small Signal Frequency Analysis
    1. 4.1 Boost Regulator Modulator Modeling
    2. 4.2 Compensation Modeling
    3. 4.3 Open Loop Modeling

2.6 Output Capacitor Selection

The output capacitor determines the output voltage ripple and load transient performance. In this example, the output capacitor is appropriately sized based on the required output voltage ripple. With the required VLOAD ripple of 100mV, Equation 12 is used to calculate the minimum output capacitance.

Equation 12. C OUT _ min = I LOAD ∙ D f sw ∙ ∆ V LOAD = 1.6 A ∙ 0.5 2.1 MHz ∙ 100 mV = 3.8 μF

The output capacitor must be rated to handle the AC current. The maximum output ripple current is estimated using Equation 13.

Equation 13. I RMS _ COUT = 1 - D ∙ [ I LOAD 2 ∙ D 1 - D 2 + ∆ i L 2 3 ] = 1 - 0.5 ∙ [ 1.6 A 2 ∙ 0.5 1 - 0.5 2 + 0.48 A 2 3 ] = 1.6 A  

Note that similar as in the inductor calculation, the above condition is selected to be the worst-case scenario with the highest capacitance requirement and highest RMS current between the full load case and the derated half load case. For this design, a total output capacitance of 28µF is selected, which becomes around 22µF considering the capacitance drop under 12 V DC bias. The capacitor bank ESR (RESR) is estimated to be around 0.22mΩ.