SNVSCT0 July   2025 LMR71915

ADVANCE INFORMATION  

  1.   1
  2. 1 Features
  3. 2 Applications
  4. 3 Description
  5. 4 Device Comparison Table
  6. 5 Pin Configuration and Functions
  7. 6 Specifications
    1. 6.1 Absolute Maximum Ratings
    2. 6.2 ESD Ratings
    3. 6.3 Recommended Operating Conditions
    4. 6.4 Thermal Information
    5. 6.5 Electrical Characteristics
  8. 7 Detailed Description
    1. 7.1 Overview
    2. 7.2 Functional Block Diagram
    3. 7.3 Feature Description
      1. 7.3.1  Control Architecture
      2. 7.3.2  Internal VCC Regulator and Bootstrap Capacitor
      3. 7.3.3  Internal Soft Start
      4. 7.3.4  On-Time Generator
      5. 7.3.5  Current Limit
      6. 7.3.6  N-Channel Buck Switch and Driver
      7. 7.3.7  Synchronous Rectifier
      8. 7.3.8  Enable, Undervoltage Lockout (EN/UVLO)
      9. 7.3.9  Power Good (PGOOD)
      10. 7.3.10 Thermal Protection
    4. 7.4 Device Functional Modes
      1. 7.4.1 Shutdown Mode
      2. 7.4.2 Active Mode
      3. 7.4.3 Sleep Mode
  9. 8 Application and Implementation
    1. 8.1 Application Information
    2. 8.2 Typical Fly-Buck™ Converter Application
      1. 8.2.1 Design Requirements
      2. 8.2.2 Detailed Design Procedure
        1. 8.2.2.1 Switching Frequency (RT)
        2. 8.2.2.2 Transformer Selection
        3. 8.2.2.3 Secondary Output Diode
        4. 8.2.2.4 CBST Selection
        5. 8.2.2.5 Minimum Secondary Output Load
    3. 8.3 Power Supply Recommendations
    4. 8.4 Layout
      1. 8.4.1 Layout Guidelines
        1. 8.4.1.1 Compact PCB Layout for EMI Reduction
        2. 8.4.1.2 Feedback Resistors
      2. 8.4.2 Layout Example
  10. 9 Device and Documentation Support
    1. 9.1 Device Support
      1. 9.1.1 Third-Party Products Disclaimer
    2. 9.2 Documentation Support
      1. 9.2.1 Related Documentation
    3. 9.3 Receiving Notification of Documentation Updates
    4. 9.4 Support Resources
    5. 9.5 Trademarks
    6. 9.6 Electrostatic Discharge Caution
    7. 9.7 Glossary
  11. 10Revision History
  12. 11Mechanical, Packaging, and Orderable Information
    1. 11.1 Tape and Reel Information

8.2 Typical Fly-Buck™ Converter Application

The LMR719xxF is designed for Fly-Buck converter applications by operating in FPWM mode. Figure 8-1 shows the schematic for a 12V output Fly-Buck converter regulator with a 12V auxiliary output, capable of delivering 625mA from each output, used as an example application for the LMR719xxF. Note that the secondary output ground can be floating with respect to the input supply ground. See Table 8-1 for a description of Fly-Buck converter terminology used in this example.

LMR71915 Example Fly-Buck™ Converter Application Circuit Figure 8-1 Example Fly-Buck™ Converter Application Circuit
Table 8-1 Fly-Buck™ Converter Terminology
TERM DESCRIPTION
VOUT1 Primary output voltage, as for a buck regulator. This output is tightly regulated by the LMR719xx.
VOUT2 Secondary output voltage from couple inductor secondary winding. This voltage is not tightly regulated, but depends on parasitic voltage drops on the primary and secondary sides.
IOUT1 Primary output current, as for a buck regulator
IOUT2 Secondary output current from coupled inductor secondary winding
Note:

In this data sheet, the effective value of capacitance is defined as the actual capacitance under D.C. bias and temperature, not the rated or nameplate values. Use high-quality, low ESR, ceramic capacitors with an X5R or better dielectric throughout. All high value ceramic capacitors have a large voltage coefficient in addition to normal tolerances and temperature effects. Under D.C. bias, the capacitance drops considerably. Large case sizes and higher voltage ratings are better in this regard. To help mitigate these effects, multiple capacitors can be used in parallel to bring the minimum effective capacitance up to the required value. This use can also ease the RMS current requirements on a single capacitor. A careful study of bias and temperature variation of any capacitor bank must be made to make sure that the minimum value of effective capacitance is provided.