SNVSB35C May   2018  – November 2024

PRODUCTION DATA  

  1.   1
  2. 1 Features
  3. 2 Applications
  4. 3 Description
  5. 4 Pin Configuration and Functions
  6. 5 Specifications
    1. 5.1 Absolute Maximum Ratings
    2. 5.2 ESD Ratings
    3. 5.3 Recommended Operating Conditions
    4. 5.4 Thermal Information
    5. 5.5 Electrical Characteristics Per Buck
    6. 5.6 Typical Characteristics
  7. 6 Detailed Description
    1. 6.1 Overview
    2. 6.2 Functional Block Diagram
    3. 6.3 Feature Description
      1. 6.3.1 Soft Start
      2. 6.3.2 Power Good
      3. 6.3.3 Precision Enable
    4. 6.4 Device Functional Modes
      1. 6.4.1 Output Overvoltage Protection
      2. 6.4.2 Undervoltage Lockout
      3. 6.4.3 Current Limit
      4. 6.4.4 Thermal Shutdown
  8. 7 Application and Implementation
    1. 7.1 Application Information
      1. 7.1.1 Programming Output Voltage
      2. 7.1.2 VINC Filtering Components
      3. 7.1.3 Using Precision Enable and Power Good
      4. 7.1.4 Overcurrent Protection for HTSSOP-20 Package
      5. 7.1.5 Current Limit and Short-Circuit Protection for WQFN-16 Package
    2. 7.2 Typical Applications
      1. 7.2.1 2.2-MHz, 0.8-V Typical High-Efficiency Application Circuit
        1. 7.2.1.1 Design Requirements
        2. 7.2.1.2 Detailed Design Procedure
          1. 7.2.1.2.1 Custom Design With WEBENCH® Tools
          2. 7.2.1.2.2 Inductor Selection
          3. 7.2.1.2.3 Input Capacitor Selection
          4. 7.2.1.2.4 Output Capacitor
          5. 7.2.1.2.5 Calculating Efficiency and Junction Temperature
        3. 7.2.1.3 Application Curves
      2. 7.2.2 2.2-MHz, 1.8-V Typical High-Efficiency Application Circuit
        1. 7.2.2.1 Design Requirements
        2. 7.2.2.2 Detailed Design Procedure
        3. 7.2.2.3 Application Curves
      3. 7.2.3 LM26420-Q12.2-MHz, 2.5-V Typical High-Efficiency Application Circuit
        1. 7.2.3.1 Design Requirements
        2. 7.2.3.2 Detailed Design Procedure
        3. 7.2.3.3 Application Curves
    3. 7.3 Power Supply Recommendations
      1. 7.3.1 Power Supply Recommendations - HTSSOP-20 Package
      2. 7.3.2 Power Supply Recommendations - WQFN-16 Package
    4. 7.4 Layout
      1. 7.4.1 Layout Guidelines
      2. 7.4.2 Layout Example
      3. 7.4.3 Thermal Considerations
        1. 7.4.3.1 Method 1: Silicon Junction Temperature Determination
        2. 7.4.3.2 Thermal Shutdown Temperature Determination
  9. 8 Device and Documentation Support
    1. 8.1 Device Support
      1. 8.1.1 Third-Party Products Disclaimer
      2. 8.1.2 Custom Design With WEBENCH® Tools
    2. 8.2 Documentation Support
      1. 8.2.1 Related Documentation
    3. 8.3 Receiving Notification of Documentation Updates
    4. 8.4 Support Resources
    5. 8.5 Trademarks
    6. 8.6 Electrostatic Discharge Caution
    7. 8.7 Glossary
  10. 9 Revision History
  11. 10Mechanical, Packaging, and Orderable Information

7.4.3.2 Thermal Shutdown Temperature Determination

The second method, although more complicated, can give a very accurate silicon junction temperature.

The first step is to determine RθJA of the application. The LM26420-Q1 has over-temperature protection circuitry. When the silicon temperature reaches 165°C, the device stops switching. The protection circuitry has a hysteresis of about 15°C. After the silicon junction temperature has decreased to approximately 150°C, the device starts to switch again. Knowing this, the RθJA for any application can be characterized during the early stages of the design one can calculate the RθJA by placing the PCB circuit into a thermal chamber. Raise the ambient temperature in the given working application until the circuit enters thermal shutdown. If the SW pin is monitored, when the internal FETs stop switching is obvious, indicating a junction temperature of 165°C. Knowing the internal power dissipation from the above methods, the junction temperature, and the ambient temperature RθJA can be determined.

Equation 42. LM26420-Q1

After determined, the maximum ambient temperature allowed for a desired junction temperature can be found.

An example of calculating RθJA for an application using the LM26420-Q1 WQFN demonstration board is shown below.

The four layer PCB is constructed using FR4 with 1 oz copper traces. The copper ground plane is on the bottom layer. The ground plane is accessed by eight vias. The board measures 3 cm × 3 cm. It was placed in an oven with no forced airflow. The ambient temperature was raised to 152°C, and at that temperature, the device went into thermal shutdown.

From the previous example:

Equation 43. PINTERNAL = 304 mW
Equation 44. LM26420-Q1

If the junction temperature is to be kept below 125°C, then the ambient temperature can not go above 112°C.

Equation 45. TJ – (RθJA × PINTERNAL) = TA
Equation 46. 125°C – (42.8°C/W × 304 mW) = 112.0°C