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

Transformer Selection

For this fly-buck application, a coupled inductor (sometimes called a transformer) is required. The first step is to decide upon the turns ratio. In a fly-buck, the secondary output voltage is slightly less than the reflected primary output voltage scaled by the turns ratio. Equation 17 can be used to calculate the turns ratio for a given VOUT1 and VOUT2. The nearest integer ratio should be selected. VOUT2 will be slightly less than calculated due to the secondary diode drop and other parasitic voltage drops in the secondary. Also, keep in mind that the secondary voltage is not fed back to the controller, and is, therefore, not well regulated. For this example, it is required that VOUT2 is equal to VOUT1, therefore, use a 1:1 turns ratio.

Equation 17.

Next, the primary inductance must be calculated. This is the same as calculating the inductance for an ordinary buck regulator, and is based on the desired primary ripple current. Typically, a ripple current of between 20% and 40% of the primary current is used. Equation 18 gives the primary current in a fly-buck and Equation 19 gives the required primary inductance. Using an input voltage of 24 V and the other parameters in Table 9-2, the user arrives at a value of 38 μH. A standard value of 33 μH for this example is selected. Although the inductance can be selected based on the maximum input voltage and lower values of K, a somewhat smaller value of inductance is used in this example to save space on the PCB.

Equation 18.
Equation 19.

where

  • K = ripple current factor = 20% to 40%

Finally, the maximum currents in the transformer must be checked. A transformer with a saturation current equal to or greater than the device current limit must be selected. Also, the maximum primary current, and, therefore, the output current, is limited by the current limit if the device. Equation 20 can be used to calculate the maximum output current for a given inductance and application parameters.

The magnitude of the ripple current and peak current in the transformer are required to select the output capacitors. These are calculated using Equation 21 and Equation 22, respectively.

Equation 20.

where

  • ICL = device current limit = IHS_PK(OC)
Equation 21.
Equation 22.