SNVSB32B August   2018  – June 2021 LM5146-Q1

PRODUCTION DATA  

  1. 1 Features
  2. 2 Applications
  3. 3 Description
  4. 4 Revision History
  5. 5 Description (continued)
  6. 6 Pin Configuration and Functions
    1. 6.1 Wettable Flanks
  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 Switching Characteristics
    7. 7.7 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  Input Range (VIN)
      2. 8.3.2  Output Voltage Setpoint and Accuracy (FB)
      3. 8.3.3  High-Voltage Bias Supply Regulator (VCC)
      4. 8.3.4  Precision Enable (EN/UVLO)
      5. 8.3.5  Power Good Monitor (PGOOD)
      6. 8.3.6  Switching Frequency (RT, SYNCIN)
        1. 8.3.6.1 Frequency Adjust
        2. 8.3.6.2 Clock Synchronization
      7. 8.3.7  Configurable Soft Start (SS/TRK)
        1. 8.3.7.1 Tracking
      8. 8.3.8  Voltage-Mode Control (COMP)
      9. 8.3.9  Gate Drivers (LO, HO)
      10. 8.3.10 Current Sensing and Overcurrent Protection (ILIM)
      11. 8.3.11 OCP Duty Cycle Limiter
    4. 8.4 Device Functional Modes
      1. 8.4.1 Shutdown Mode
      2. 8.4.2 Standby Mode
      3. 8.4.3 Active Mode
      4. 8.4.4 Diode Emulation Mode
      5. 8.4.5 Thermal Shutdown
  9. 9 Application and Implementation
    1. 9.1 Application Information
      1. 9.1.1 Design and Implementation
      2. 9.1.2 Power Train Components
        1. 9.1.2.1 Inductor
        2. 9.1.2.2 Output Capacitors
        3. 9.1.2.3 Input Capacitors
        4. 9.1.2.4 Power MOSFETs
      3. 9.1.3 Control Loop Compensation
      4. 9.1.4 EMI Filter Design
    2. 9.2 Typical Applications
      1. 9.2.1 Design 1 – 12-A High-Efficiency Synchronous Buck DC/DC Regulator for Automotive Applications
        1. 9.2.1.1 Design Requirements
        2. 9.2.1.2 Detailed Design Procedure
        3. 9.2.1.3 Custom Design With WEBENCH® Tools
        4. 9.2.1.4 Custom Design With Excel Quickstart Tool
        5. 9.2.1.5 Application Curves
      2. 9.2.2 Design 2 – High Density, 12-V, 8-A Rail From 48-V Automotive Battery Power
        1. 9.2.2.1 Design Requirements
        2. 9.2.2.2 Detailed Design Procedure
        3. 9.2.2.3 Application Curves
  10. 10Power Supply Recommendations
  11. 11Layout
    1. 11.1 Layout Guidelines
      1. 11.1.1 Power Stage Layout
      2. 11.1.2 Gate Drive Layout
      3. 11.1.3 PWM Controller Layout
      4. 11.1.4 Thermal Design and Layout
      5. 11.1.5 Ground Plane Design
    2. 11.2 Layout Example
  12. 12Device and Documentation Support
    1. 12.1 Device Support
      1. 12.1.1 Third-Party Products Disclaimer
      2. 12.1.2 Development Support
        1. 12.1.2.1 Custom Design With WEBENCH® Tools
    2. 12.2 Documentation Support
      1. 12.2.1 Related Documentation
        1. 12.2.1.1 PCB Layout Resources
        2. 12.2.1.2 Thermal Design Resources
    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

11.1.4 Thermal Design and Layout

The useful operating temperature range of a PWM controller with integrated gate drivers and bias supply LDO regulator is greatly affected by:

  • Average gate drive current requirements of the power MOSFETs
  • Switching frequency
  • Operating input voltage (affecting bias regulator LDO voltage drop and hence its power dissipation)
  • Thermal characteristics of the package and operating environment

For a PWM controller to be useful over a particular temperature range, the package must allow for the efficient removal of the heat produced while keeping the junction temperature within rated limits. The LM5146-Q1 controller is available in a small 3.5-mm × 4.5-mm 20-pin VQFN (RGY) PowerPAD™ package to cover a range of application requirements. The thermal metrics of this package are summarized in . The application report Semiconductor and IC Package Thermal Metrics provides detailed information regarding the thermal information table.

The 20-pin VQFN package offers a means of removing heat from the semiconductor die through the exposed thermal pad at the base of the package. While the exposed pad of the package is not directly connected to any leads of the package, it is thermally connected to the substrate of the LM5146-Q1 device (ground). This allows a significant improvement in heat sinking, and it becomes imperative that the PCB is designed with thermal lands, thermal vias, and a ground plane to complete the heat removal subsystem. The exposed pad of the LM5146-Q1 is soldered to the ground-connected copper land on the PCB directly underneath the device package, reducing the thermal resistance to a very low value. Wide traces of the copper tying in the no-connect pins of the LM5146-Q1 (pins 9 and 16) and connection to this thermal land helps to dissipate heat.

Numerous vias with a 0.3-mm diameter connected from the thermal land to the internal and solder-side ground plane(s) are vital to help dissipation. In a multi-layer PCB design, a solid ground plane is typically placed on the PCB layer below the power components. Not only does this provide a plane for the power stage currents to flow but it also represents a thermally conductive path away from the heat generating devices.

The thermal characteristics of the MOSFETs also are significant. The drain pad of the high-side MOSFET is normally connected to a VIN plane for heat sinking. The drain pad of the low-side MOSFET is tied to the SW plane, but the SW plane area is purposely kept relatively small to mitigate EMI concerns.