SNVSAX8 April   2018 LM3478Q-Q1

PRODUCTION DATA.  

  1. 1 Features
  2. 2 Applications
  3. 3 Description
    1.     Device Images
      1.      Typical High Efficiency Step-Up (Boost) Converter
  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 - LM3478Q-Q1
    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 Overvoltage Protection
      2. 7.3.2 Slope Compensation Ramp
      3. 7.3.3 Frequency Adjust/Shutdown
      4. 7.3.4 Short-Circuit Protection
    4. 7.4 Device Functional Modes
  8. 8 Application and Implementation
    1. 8.1 Application Information
    2. 8.2 Typical Applications
      1. 8.2.1 Typical High Efficiency Step-Up (Boost) Converter
        1. 8.2.1.1 Design Requirements
        2. 8.2.1.2 Detailed Design Procedure
          1. 8.2.1.2.1  Custom Design with WEBENCH Tools
          2. 8.2.1.2.2  Power Inductor Selection
          3. 8.2.1.2.3  Programming the Output Voltage
          4. 8.2.1.2.4  Setting the Current Limit
          5. 8.2.1.2.5  Current Limit with External Slope Compensation
          6. 8.2.1.2.6  Power Diode Selection
          7. 8.2.1.2.7  Power MOSFET Selection
          8. 8.2.1.2.8  Input Capacitor Selection
          9. 8.2.1.2.9  Output Capacitor Selection
          10. 8.2.1.2.10 Compensation
        3. 8.2.1.3 Application Curves
      2. 8.2.2 Typical SEPIC Converter
        1. 8.2.2.1 Design Requirements
        2. 8.2.2.2 Detailed Design Procedure
          1. 8.2.2.2.1 Power MOSFET Selection
          2. 8.2.2.2.2 Power Diode Selection
          3. 8.2.2.2.3 Selection of Inductors L1 and L2
          4. 8.2.2.2.4 Sense Resistor Selection
          5. 8.2.2.2.5 Sepic Capacitor Selection
          6. 8.2.2.2.6 Input Capacitor Selection
          7. 8.2.2.2.7 Output Capacitor Selection
        3. 8.2.2.3 Application Curves
  9. 9 Power Supply Recommendations
  10. 10Layout
    1. 10.1 Layout Guidelines
    2. 10.2 Layout Example
  11. 11Device and Documentation Support
    1. 11.1 Custom Design with WEBENCH Tools
    2. 11.2 Receiving Notification of Documentation Updates
    3. 11.3 Documentation Support
      1. 11.3.1 Related Documentation
    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

8.2.1.2.2 Power Inductor Selection

The inductor is one of the two energy storage elements in a boost converter. Figure 29 shows how the inductor current varies during a switching cycle. The current through an inductor is quantified using Equation 10, which shows the relationship of L, IL and VL.

Equation 10. LM3478Q-Q1 10135525.png

The important quantities in determining a proper inductance value are IL (the average inductor current) and ΔIL (the inductor current ripple). If ΔIL is larger than IL, the inductor current will drop to zero for a portion of the cycle and the converter will operate in the DCM. All the analysis in this datasheet assumes operation in the CCM. To operate in the CCM, the following condition must be met by using Equation 11.

Equation 11. LM3478Q-Q1 10135528.png

Choose the minimum IOUT to determine the minimum inductance value. A common choice is to set ΔIL to 30% of IL. Choosing an appropriate core size for the inductor involves calculating the average and peak currents expected through the inductor. Use Equation 12, Equation 13, and Equation 14 to the peak inductor current in a boost converter.

Equation 12. ILPEAK = Average IL(max) + ΔIL(max)
Equation 13. Average IL(max) = Iout / (1-D)
Equation 14. ΔIL(max) = D x Vin / (2 x fs x L)

An inductor size with ratings higher than these values has to be selected. If the inductor is not properly rated, saturation will occur and may cause the circuit to malfunction.

The LM3478Q-Q1 can be set to switch at very high frequencies. When the switching frequency is high, the converter can be operated with very small inductor values. The LM3478Q-Q1 senses the peak current through the switch which is the same as the peak inductor current as calculated in the previous equation.

LM3478Q-Q1 10135524.gifFigure 29. Inductor Current and Diode Current