SNVSBS7A December   2021  – September 2022 LM5168 , LM5169

PRODUCTION DATA  

  1. 1 Features
  2. 2 Applications
  3. 3 Description
  4. 4 Revision History
  5. 5 Device Comparison Table
  6. 6 Pin Configuration and Functions
  7. 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
    6. 7.6 Typical Characteristics
  8. 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. 9 Application and Implementation
    1. 9.1 Application Information
    2. 9.2 Typical Fly-Buck™ Converter 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  Setting Output Voltage
        6. 9.2.2.6  Input Capacitor
        7. 9.2.2.7  Type-3 Ripple Network
        8. 9.2.2.8  CBST Selection
        9. 9.2.2.9  Minimum Secondary Output Load
        10. 9.2.2.10 Example Design Summary
      3. 9.2.3 Application Curves
    3. 9.3 Typical Buck Application
      1. 9.3.1 Design Requirements
      2. 9.3.2 Detailed Design Procedure
        1. 9.3.2.1 Switching Frequency (RT)
        2. 9.3.2.2 Buck Inductor Selection
        3. 9.3.2.3 Setting the Output Voltage
        4. 9.3.2.4 Type-3 Ripple Network
        5. 9.3.2.5 Output Capacitor Selection
        6. 9.3.2.6 Input Capacitor Considerations
        7. 9.3.2.7 CBST Selection
        8. 9.3.2.8 Example Design Summary
      3. 9.3.3 Application Curves
    4. 9.4 Power Supply Recommendations
    5. 9.5 Layout
      1. 9.5.1 Thermal Considerations
      2. 9.5.2 Typical EMI Results
      3. 9.5.3 Layout Guidelines
        1. 9.5.3.1 Compact PCB Layout for EMI Reduction
        2. 9.5.3.2 Feedback Resistors
      4. 9.5.4 Layout Example
  10. 10Device and Documentation Support
    1. 10.1 Device Support
      1. 10.1.1 Third-Party Products Disclaimer
    2. 10.2 Documentation Support
      1. 10.2.1 Related Documentation
    3. 10.3 Receiving Notification of Documentation Updates
    4. 10.4 Support Resources
    5. 10.5 Trademarks
    6. 10.6 Electrostatic Discharge Caution
    7. 10.7 Glossary
  11. 11Mechanical, Packaging, and Orderable Information

Package Options

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

9.2.2.2 Transformer Selection

For this fly-buck converter application, a coupled inductor (sometimes called a transformer) is required. The first step is to decide upon the turns ratio. In a fly-buck converter, the secondary output voltage is slightly less than the reflected primary output voltage scaled by the turns ratio. Equation 8 can be used to calculate the turns ratio for a given VOUT1 and VOUT2. The nearest integer ratio must 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, VOUT2 is equal to VOUT1, therefore, use a 1:1 turns ratio.

Equation 8.

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 9 gives the primary current in a fly-buck converter and Equation 10 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 9.
Equation 10.

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 of the device. Equation 11 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 12 and Equation 13, respectively.

Equation 11.

where

  • ICL = device current limit = IHS_PK(OC)
Equation 12.
Equation 13.