SNVA941A June   2020  – November 2022 LM5156 , LM5156-Q1 , LM51561 , LM51561-Q1 , LM51561H , LM51561H-Q1 , LM5156H , LM5156H-Q1

 

  1.   How to Design a Boost Converter Using the LM5156
  2. 1LM5156 Design Example
  3. 2Example Application
  4. 3Calculations and Component Selection
    1. 3.1  Switching Frequency
    2. 3.2  Inductor Calculation
    3. 3.3  Current Sense Resistor Calculation
      1. 3.3.1 Current Sense Resistor and Slope Compensation Resistor Selection
      2. 3.3.2 Current Sense Resistor Filter Calculation
    4. 3.4  Inductor Selection
    5. 3.5  Diode Selection
    6. 3.6  MOSFET Selection
    7. 3.7  Output Capacitor Selection
    8. 3.8  Input Capacitor Selection
    9. 3.9  UVLO Resistor Selection
    10. 3.10 Soft-Start Capacitor Selection.
    11. 3.11 Feedback Resistor Selection
    12. 3.12 Control Loop Compensation
      1. 3.12.1 Select the Loop Crossover Frequency (fCROSS)
      2. 3.12.2 Determine Required RCOMP
      3. 3.12.3 Determine Required CCOMP
      4. 3.12.4 Determine Required CHF
    13. 3.13 Efficiency Estimation
  5. 4Component Selection Summary
    1.     25
  6. 5Small-Signal Frequency Analysis
    1. 5.1 Boost Regulator Modulator Modeling
    2. 5.2 Compensation Modeling
    3. 5.3 Open-Loop Modeling
  7. 6Revision History

3.2 Inductor Calculation

Three main parameters are considered when selecting the inductance value: inductor current ripple ratio (RR), falling slope of the inductor current and the right-half plane zero frequency (ωZ_RHP) of the control loop. Finding a balance between these three parameters helps simplify the rest of the design process.

  • The inductor current ripple ratio is selected to balance the copper loss and core loss of the inductor. As the relative ripple current increases; the core loss increases and the copper loss decreases
  • The falling slope of the inductor current should be small enough to prevent sub-harmonic oscillation. A relatively larger inductance value results in a smaller falling slope of the inductor current. This increases the impact internal slope compensation provided by the LM5156.
  • The right-half plane zero should be placed at relatively high frequencies, allowing a higher crossover frequency of the control loop. As the relative inductance value decrease the right-half plane zero frequency increases.

A maximum ripple ratio between 30% and 70% results in a good balance between the power loss of the inductor, the down slope of the inductor current and the right-half plane zero frequency. The maximum ripple ratio of the inductor current is set to 60%. In continuos conduction mode (CCM) operation, the maximum ripple ratio occurs at a duty cycle of 33% (DmaxΔIL = 0.33). In the case that the application specification does not result in a duty cycle of 33% the maximum supply voltage is used to calculate the maximum ripple ratio. Use Equation 2 to calculate the supply voltage that results in a duty cycle of 33% (D = 0.33).

Equation 2. GUID-A84633BE-1CF1-4E2A-8577-132967A62756-low.gif

where

  • DmaxΔIL is the duty cycle where the maximum inductor ripple current occurs

Knowing VSUPPLY_max_ΔIL the desired ripple ratio and the switching frequency, use Equation 3 to calculate the inductor value.

Equation 3. GUID-2736F34F-E289-4A1F-A7FA-F1C09D825DA2-low.gif

where

  • D is the duty cycle where the maximum inductor ripple current occurs
  • RR is the ripple ratio of inductor ripple current to average supply current

A standard value of 2.2 µH is selected for the value of LM. The maximum peak inductor current occurs when the supply voltage is at the minimum value, VSUPPLY_min, and the maximum load current, ILOAD_max. The peak inductor current is calculated using Equation 4. This is the sum of the average input current and one-half the inductor ripple current.

Equation 4. GUID-062B8B17-9902-4F8D-8923-D4BF10F80A6B-low.gif

where

  • η is the estimated efficiency at the minimum supply voltage and maximum load current

The peak inductor current is used to properly size the current sense resistor, RS.