SNVS726F July   2011  – March 2018 LM25118

PRODUCTION DATA.  

  1. 1 Features
  2. 2 Applications
  3. 3 Description
    1.     Device Images
      1.      Simplified Schematic
      2.      Efficiency vs VIN and IOUT, VOUT = 12 V
  4. 4 Revision History
  5. 5 Pin Configuration and Functions
    1.     Pin Functions
  6. 6 Specifications
    1. 6.1 Absolute Maximum Ratings
    2. 6.2 ESD Ratings
    3. 6.3 Recommended Operating Conditions
    4. 6.4 Thermal Information
    5. 6.5 Electrical Characteristics
    6. 6.6 Typical Characteristics
  7. 7 Detailed Description
    1. 7.1 Overview
    2. 7.2 Functional Block Diagram
    3. 7.3 Feature Description
      1. 7.3.1 UVLO
      2. 7.3.2 Oscillator and Sync Capability
      3. 7.3.3 Error Amplifier and PWM Comparator
      4. 7.3.4 Ramp Generator
      5. 7.3.5 Current Limit
      6. 7.3.6 Maximum Duty Cycle
      7. 7.3.7 Soft Start
      8. 7.3.8 HO Output
      9. 7.3.9 Thermal Protection
    4. 7.4 Device Functional Modes
      1. 7.4.1 Buck Mode Operation: VIN > VOUT
      2. 7.4.2 Buck-Boost Mode Operation: VIN ≊ VOUT
      3. 7.4.3 High Voltage Start-Up Regulator
      4. 7.4.4 Enable
  8. 8 Application and Implementation
    1. 8.1 Application Information
    2. 8.2 Typical Application
      1. 8.2.1 Design Requirements
      2. 8.2.2 Detailed Design Procedure
        1. 8.2.2.1  Custom Design With WEBENCH® Tools
        2. 8.2.2.2  R7 = RT
        3. 8.2.2.3  Inductor Selection – L1
        4. 8.2.2.4  R13 = RSENSE
        5. 8.2.2.5  C15 = CRAMP
        6. 8.2.2.6  Inductor Current Limit Calculation
        7. 8.2.2.7  C9 - C12 = Output Capacitors
        8. 8.2.2.8  D1
        9. 8.2.2.9  D4
        10. 8.2.2.10 C1 – C5 = Input Capacitors
        11. 8.2.2.11 C20
        12. 8.2.2.12 C8
        13. 8.2.2.13 C16 = CSS
        14. 8.2.2.14 R8, R9
        15. 8.2.2.15 R1, R3, C21
        16. 8.2.2.16 R2
        17. 8.2.2.17 Snubber
        18. 8.2.2.18 Error Amplifier Configuration
          1. 8.2.2.18.1 R4, C18, C17
      3. 8.2.3 Application Curves
  9. 9 Power Supply Recommendations
    1. 9.1 Bias Power Dissipation Reduction
    2. 9.2 Thermal Considerations
  10. 10Layout
    1. 10.1 Layout Guidelines
    2. 10.2 Layout Example
  11. 11Device and Documentation Support
    1. 11.1 Device Support
      1. 11.1.1 Development Support
        1. 11.1.1.1 Custom Design With WEBENCH® Tools
    2. 11.2 Receiving Notification of Documentation Updates
    3. 11.3 Community Resources
    4. 11.4 Trademarks
    5. 11.5 Electrostatic Discharge Caution
    6. 11.6 Glossary
  12. 12Mechanical, Packaging, and Orderable Information

Package Options

Mechanical Data (Package|Pins)
Thermal pad, mechanical data (Package|Pins)
Orderable Information

8.2.2.3 Inductor Selection – L1

The inductor value is determined based upon the operating frequency, load current, ripple current, and the input and output voltages. See Figure 20 for details.

To keep the circuit in continuous conduction mode (CCM), the maximum ripple current IRIPPLE should be less than twice the minimum load current. For the specified minimum load of 0.6 A, the maximum ripple current is 1.2 Ap-p. Also, the minimum value of L must be calculated both for a buck and buck-boost configurations. The final value of inductance will generally be a compromise between the two modes. It is desirable to have a larger value inductor for buck mode, but the saturation current rating for the inductor must be large for buck-boost mode, resulting in a physically large inductor. Additionally, large value inductors present buck-boost mode loop compensation challenges which will be discussed in Error Amplifier Configuration . For the design example, the inductor values in both modes are calculated as:

Equation 10. LM25118 30165130a.gif
Equation 11. LM25118 30165130b.gif

where

  • VOUT is the output voltage
  • VIN(MAX) is the maximum input voltage
  • f is the switching frequency
  • IRIPPLE is the selected inductor peak to peak ripple current (1.2 A selected for this example)
  • VIN(MIN) is the minimum input voltage

The resulting inductor values are:

Equation 12. L1 = 23.8 µH, Buck Mode
Equation 13. L1 = 9.8 µH, Buck-Boost mode

A 10-µH inductor was selected which is a compromise between these values, while favoring the buck-boost mode. As illustrated in the compensation section, the inductor value should be as low as possible to move the buck-boost right-half-plane zero to a higher frequency. The ripple current is then rechecked with the selected inductor value using Equation 10 and Equation 11.

Equation 14. IRIPPLE(BUCK) = 2.86 A
Equation 15. IRIPPLE(BUCK-BOOST) = 1.17 A

Because the inductor selected is lower than calculated for the Buck mode, the minimum load current for CCM in buck mode is 1.42 A at maximum VIN.

With a 10-µH inductor, the worst case peak inductor currents can be estimated for each case, assuming a 20% inductor value tolerance and 80% efficiency of the converter.

Equation 16. LM25118 30165131a.gif
Equation 17. LM25118 30165131b.gif

where

  • η is efficiency
  • LTOL is the inductor tolerance

For this example, Equation 16 and Equation 17 yield:

Equation 18. I1(PEAK) = 5.33 A
Equation 19. I2(PEAK) = 13.4 A

An acceptable current limit setting would be 6.7 A for buck mode because the LM25118 automatically doubles the current limit threshold in buck-boost mode. The selected inductor must have a saturation current rating at least as high as the buck-boost mode cycle-by-cycle current limit threshold, in this case at least 13.5 A. A 10-µH, 15-A inductor was chosen for this application.