SNVSB35C May   2018  – November 2024

PRODUCTION DATA  

  1.   1
  2. 1 Features
  3. 2 Applications
  4. 3 Description
  5. 4 Pin Configuration and Functions
  6. 5 Specifications
    1. 5.1 Absolute Maximum Ratings
    2. 5.2 ESD Ratings
    3. 5.3 Recommended Operating Conditions
    4. 5.4 Thermal Information
    5. 5.5 Electrical Characteristics Per Buck
    6. 5.6 Typical Characteristics
  7. 6 Detailed Description
    1. 6.1 Overview
    2. 6.2 Functional Block Diagram
    3. 6.3 Feature Description
      1. 6.3.1 Soft Start
      2. 6.3.2 Power Good
      3. 6.3.3 Precision Enable
    4. 6.4 Device Functional Modes
      1. 6.4.1 Output Overvoltage Protection
      2. 6.4.2 Undervoltage Lockout
      3. 6.4.3 Current Limit
      4. 6.4.4 Thermal Shutdown
  8. 7 Application and Implementation
    1. 7.1 Application Information
      1. 7.1.1 Programming Output Voltage
      2. 7.1.2 VINC Filtering Components
      3. 7.1.3 Using Precision Enable and Power Good
      4. 7.1.4 Overcurrent Protection for HTSSOP-20 Package
      5. 7.1.5 Current Limit and Short-Circuit Protection for WQFN-16 Package
    2. 7.2 Typical Applications
      1. 7.2.1 2.2-MHz, 0.8-V Typical High-Efficiency Application Circuit
        1. 7.2.1.1 Design Requirements
        2. 7.2.1.2 Detailed Design Procedure
          1. 7.2.1.2.1 Custom Design With WEBENCH® Tools
          2. 7.2.1.2.2 Inductor Selection
          3. 7.2.1.2.3 Input Capacitor Selection
          4. 7.2.1.2.4 Output Capacitor
          5. 7.2.1.2.5 Calculating Efficiency and Junction Temperature
        3. 7.2.1.3 Application Curves
      2. 7.2.2 2.2-MHz, 1.8-V Typical High-Efficiency Application Circuit
        1. 7.2.2.1 Design Requirements
        2. 7.2.2.2 Detailed Design Procedure
        3. 7.2.2.3 Application Curves
      3. 7.2.3 LM26420-Q12.2-MHz, 2.5-V Typical High-Efficiency Application Circuit
        1. 7.2.3.1 Design Requirements
        2. 7.2.3.2 Detailed Design Procedure
        3. 7.2.3.3 Application Curves
    3. 7.3 Power Supply Recommendations
      1. 7.3.1 Power Supply Recommendations - HTSSOP-20 Package
      2. 7.3.2 Power Supply Recommendations - WQFN-16 Package
    4. 7.4 Layout
      1. 7.4.1 Layout Guidelines
      2. 7.4.2 Layout Example
      3. 7.4.3 Thermal Considerations
        1. 7.4.3.1 Method 1: Silicon Junction Temperature Determination
        2. 7.4.3.2 Thermal Shutdown Temperature Determination
  9. 8 Device and Documentation Support
    1. 8.1 Device Support
      1. 8.1.1 Third-Party Products Disclaimer
      2. 8.1.2 Custom Design With WEBENCH® Tools
    2. 8.2 Documentation Support
      1. 8.2.1 Related Documentation
    3. 8.3 Receiving Notification of Documentation Updates
    4. 8.4 Support Resources
    5. 8.5 Trademarks
    6. 8.6 Electrostatic Discharge Caution
    7. 8.7 Glossary
  10. 9 Revision History
  11. 10Mechanical, Packaging, and Orderable Information
7.2.1.2.2 Inductor Selection

The duty cycle (D) can be approximated as the ratio of output voltage (VOUT) to input voltage (VIN):

Equation 7. LM26420-Q1

The voltage drop across the internal NMOS (SW_BOT) and PMOS (SW_TOP) must be included to calculate a more accurate duty cycle. Calculate D by using the following formulas:

Equation 8. LM26420-Q1

VSW_TOP and VSW_BOT can be approximated by:

Equation 9. VSW_TOP = IOUT × RDSON_TOP
Equation 10. VSW_BOT = IOUT × RDSON_BOT

The inductor value determines the output ripple voltage. Smaller inductor values decrease the size of the inductor, but increase the output ripple voltage. An increase in the inductor value decreases the output ripple current.

Make sure that the minimum current limit (2.4 A) is not exceeded, so the peak current in the inductor must be calculated. The peak current (ILPK) in the inductor is calculated by:

Equation 11. ILPK = IOUT + ΔiL
LM26420-Q1 Inductor CurrentFigure 7-8 Inductor Current
Equation 12. LM26420-Q1

In general,

Equation 13. ΔiL = 0.1 × (IOUT) → 0.2 × (IOUT)

If ΔiL = 20% of 2 A, the peak current in the inductor is 2.4 A. The minimum specified current limit over all operating conditions is 2.4 A. Either reduce ΔiL, or make the engineering judgment that zero margin is safe enough. The typical current limit is 3.3 A.

The LM26420-Q1 operates at frequencies allowing the use of ceramic output capacitors without compromising transient response. Ceramic capacitors allow higher inductor ripple without significantly increasing output ripple voltage. See the Section 7.2.1.2.4 section for more details on calculating output voltage ripple. Now that the ripple current is determined, the inductance is calculated by:

Equation 14. LM26420-Q1

where

Equation 15. LM26420-Q1

When selecting an inductor, make sure that the inductor is capable of supporting the peak output current without saturating. Inductor saturation results in a sudden reduction in inductance and prevents the regulator from operating correctly. The peak current of the inductor is used to specify the maximum output current of the inductor and saturation is not a concern due to the exceptionally small delay of the internal current limit signal. Ferrite based inductors are preferred to minimize core losses when operating with the frequencies used by the LM26420-Q1. This presents little restriction because the variety of ferrite-based inductors is huge. Lastly, inductors with lower series resistance (RDCR) provides better operating efficiency. For recommended inductors, see Table 7-2.