SNVSBZ3 June   2021 LM5168-Q1

ADVANCE INFORMATION  

  1. Features
  2. Applications
  3. Description
  4. Revision History
  5. Device Comparison Table
  6. Pin Configuration and Functions
  7. Specifications
    1. 7.1 Absolute Maximum Ratings
    2. 7.2 ESD Ratings
    3. 7.3 Recommended Operating Conditions
    4. 7.4 Thermal Information
    5. 7.5 Electrical Characteristics
  8. Detailed Description
    1. 8.1 Overview
    2. 8.2 Functional Block Diagram
    3. 8.3 Feature Description
      1. 8.3.1  Control Architecture
      2. 8.3.2  Internal VCC Regulator and Bootstrap Capacitor
      3. 8.3.3  Internal Soft Start
      4. 8.3.4  On-Time Generator
      5. 8.3.5  Current Limit
      6. 8.3.6  N-Channel Buck Switch and Driver
      7. 8.3.7  Synchronous Rectifier
      8. 8.3.8  Enable/Undervoltage Lockout (EN/UVLO)
      9. 8.3.9  Power Good (PGOOD)
      10. 8.3.10 Thermal Protection
    4. 8.4 Device Functional Modes
      1. 8.4.1 Shutdown Mode
      2. 8.4.2 Active Mode
      3. 8.4.3 Sleep Mode
  9. Application and Implementation
    1. 9.1 Application Information
    2. 9.2 Typical Application
      1. 9.2.1 Design Requirements
      2. 9.2.2 Detailed Design Procedure
        1. 9.2.2.1  Switching Frequency (RT)
        2. 9.2.2.2  Transformer Selection
        3. 9.2.2.3  Output Capacitor Selection
        4. 9.2.2.4  Secondary Output Diode
        5. 9.2.2.5  Regulation Comparator
        6. 9.2.2.6  Input Capacitor
        7. 9.2.2.7  Type-3 Ripple Network
        8. 9.2.2.8  Minimum Secondary Output Load
        9. 9.2.2.9  Example Design Summary
        10. 9.2.2.10 Thermal Considerations
      3. 9.2.3 Application Curves
  10. 10Power Supply Recommendations
  11. 11Layout
    1. 11.1 Layout Guidelines
      1. 11.1.1 Compact PCB Layout for EMI Reduction
      2. 11.1.2 Feedback Resistors
    2. 11.2 Layout Example
  12. 12Device and Documentation Support
    1. 12.1 Device Support
      1. 12.1.1 Third-Party Products Disclaimer
    2. 12.2 Documentation Support
      1. 12.2.1 Related Documentation
    3. 12.3 Receiving Notification of Documentation Updates
    4. 12.4 Support Resources
    5. 12.5 Trademarks
    6. 12.6 Electrostatic Discharge Caution
    7. 12.7 Glossary
  13. 13Mechanical, Packaging, and Orderable Information

Package Options

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

Output Capacitor Selection

The primary output capacitor, COUT1, can be selected using either Equation 23 or Equation 24. For this design, an output voltage ripple of 5 mV and a load transient of 0.2 V is used. From this, a ripple current of 0.34 A at 60 V input, and a peak transformer current of 0.77 A at full load is calculated. The two output capacitor equations give values of 11 μF and 5 μF. Because of the large derating of ceramic capacitors, COUT1 = 1 × 22 μF is used. Keep in mind that the equations give the minimum capacitance value and in no case should the capacitance of COUT1 be less than 2.2 μF. More output capacitance can be used to improve load transient response. Also note that when using type III ripple injection, the actual ripple voltage appearing on the output can be kept small.

Equation 23.

where

  • IPK = peak transformer current from Equation 22
  • ΔVO = output voltage load transient
Equation 24.

where

  • ΔI = ripple current from Equation 21
  • Vripple = ripple voltage on primary output

COUT2 is selected using Equation 25. In this case, a ripple voltage on the secondary otuput of 20 mV is chosen. The minimum input voltage should be used in this equation. A value of 10 μF is calculated and 1 × 22 μF for COUT2 is selected. Again, the equation gives the minimum capacitance value and in no case should the capacitance of COUT2 be less than 2.2 μF.

Equation 25.

Both output capacitors should be X7R ceramics in a 1206 or 1210 case size, and rated for at least twice the output voltage.