SNVSBK9C November   2019  – September 2020 LM63635-Q1


  1. Features
  2. Applications
  3. Description
  4. Revision History
  5. Device Comparison Table
  6. Pin Configuration and Functions
    1.     Pin 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
    6. 7.6 Timing Characteristics
    7. 7.7 Switching Characteristics
    8. 7.8 System Characteristics
    9. 7.9 Typical Characteristics
  8. Detailed Description
    1. 8.1 Overview
    2. 8.2 Functional Block Diagram
    3. 8.3 Feature Description
      1. 8.3.1 Sync/Mode Selection
      2. 8.3.2 Output Voltage Selection
      3. 8.3.3 Switching Frequency Selection
        1. Spread Spectrum Option
      4. 8.3.4 Enable and Start-up
      5. 8.3.5 RESET Flag Output
      6. 8.3.6 Undervoltage Lockout and Thermal Shutdown and Output Discharge
    4. 8.4 Device Functional Modes
      1. 8.4.1 Overview
      2. 8.4.2 Light Load Operation
        1. Sync/FPWM Operation
      3. 8.4.3 Dropout Operation
      4. 8.4.4 Minimum On-time Operation
      5. 8.4.5 Current Limit and Short-Circuit Operation
  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. Choosing the Switching Frequency
        2. Setting the Output Voltage
          1. CFF Selection
        3. Inductor Selection
        4. Output Capacitor Selection
        5. Input Capacitor Selection
        6. CBOOT
        7. VCC
        8. External UVLO
        9. Maximum Ambient Temperature
      3. 9.2.3 Full Feature Design Example
      4. 9.2.4 Application Curves
      5. 9.2.5 EMI Performance Curves
    3. 9.3 What to Do and What Not to Do
  10. 10Power Supply Recommendations
  11. 11Layout
    1. 11.1 Layout Guidelines
      1. 11.1.1 Ground and Thermal Considerations
    2. 11.2 Layout Example
  12. 12Device and Documentation Support
    1. 12.1 Documentation Support
      1. 12.1.1 Related Documentation
    2. 12.2 Receiving Notification of Documentation Updates
    3. 12.3 Support Resources
    4. 12.4 Trademarks
    5. 12.5 Glossary
    6. 12.6 Electrostatic Discharge Caution
  13. 13Mechanical, Packaging, and Orderable Information

Package Options

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

Maximum Ambient Temperature

As with any power conversion device, the LM63635-Q1 dissipates internal power while operating. The effect of this power dissipation is to raise the internal temperature of the converter above ambient. The internal die temperature (TJ) is a function of the ambient temperature, the power loss, and the effective thermal resistance, RθJA, of the device, and PCB combination. The maximum internal die temperature for the LM63635-Q1 must be limited to 150°C. This establishes a limit on the maximum device power dissipation and, therefore, the load current. Equation 13 shows the relationships between the important parameters. It is easy to see that larger ambient temperatures (TA) and larger values of RθJA reduce the maximum available output current. The converter efficiency can be estimated by using the curves provided in this data sheet. Note that these curves include the power loss in the inductor. If the desired operating conditions cannot be found in one of the curves, then interpolation can be used to estimate the efficiency. Alternatively, the EVM can be adjusted to match the desired application requirements and the efficiency can be measured directly. The correct value of RθJA is more difficult to estimate. As stated in the Semiconductor and IC Package Thermal Metrics Application Report, the value of RθJA given in the Thermal Information table is not valid for design purposes and must not be used to estimate the thermal performance of the application. The values reported in that table were measured under a specific set of conditions that are rarely obtained in an actual application. The data given for RθJC(bott) and ΨJT can be useful when determining thermal performance. See the Semiconductor and IC Package Thermal Metrics Application Report for more information and the resources given at the end of this section.

Equation 13. GUID-F61A2C5D-0A01-45B4-82A1-5051FF2999D7-low.gif


  • η = efficiency

The effective RθJA is a critical parameter and depends on many factors such as the following:

  • Power dissipation
  • Air temperature/flow
  • PCB area
  • Copper heat-sink area
  • Number of thermal vias under the package
  • Adjacent component placement

The HTSSOP and the DRR0012 package use a die attach paddle, or "thermal pad" (DAP), to provide a place to solder down to the PCB heat-sinking copper. This provides a good heat conduction path from the regulator junction to the heat sink and must be properly soldered to the PCB heat sink copper. Typical examples of RθJA versus copper board area can be found in Figure 9-4 and Figure 9-5. The copper area given in the graph is for each layer. The top and bottom layers are 2-oz. copper each, while the inner layers are 1 oz. Figure 9-6 shows a typical curve of maximum output current versus ambient temperature. This data was taken with a device and PCB combination, giving an RθJA of approximately 22°C/W. Remember that the data given in these graphs are for illustration purposes only, and the actual performance in any given application depends on all of the previously mentioned factors.

GUID-B25A3621-B894-4B35-A0EA-AC31F3D4C506-low.gif Figure 9-4 Typical RθJA versus Copper Area for the HTSSOP Package
GUID-AFB9CA13-08B4-4DBD-8D1F-7EB05C418831-low.gif Figure 9-5 Typical RθJA vs Copper Area for the WSON Package
GUID-8FFF3B22-595E-4FAE-AD15-586E758079F2-low.gif Figure 9-6 Maximum Output Current versus Ambient Temperature VIN = 12 V, VOUT = 5 V, ƒSW = 400 kHz, RθJA = 22°C/W

The following resources can be used as a guide to optimal thermal PCB design and estimating RθJA for a given application environment: