SNVSCO0 January   2024 LM63635C-Q1

PRODUCTION DATA  

  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
    6. 6.6 Timing Characteristics
    7. 6.7 Switching Characteristics
    8. 6.8 System Characteristics
    9. 6.9 Typical Characteristics
  8. 7 Detailed Description
    1. 7.1 Overview
    2. 7.2 Functional Block Diagram
    3. 7.3 Feature Description
      1. 7.3.1 Output Voltage Selection
      2. 7.3.2 Switching Frequency Selection
        1. 7.3.2.1 Spread Spectrum Option
      3. 7.3.3 Enable and Start-Up
      4. 7.3.4 RESET Flag Output
      5. 7.3.5 Undervoltage Lockout, Thermal Shutdown, and Output Discharge
    4. 7.4 Device Functional Modes
      1. 7.4.1 Overview
      2. 7.4.2 Light Load Operation
      3. 7.4.3 Dropout Operation
      4. 7.4.4 Minimum On-Time Operation
      5. 7.4.5 Current Limit and Short-Circuit Operation
  9. 8 Application and Implementation
    1. 8.1 Application Information
    2. 8.2 Typical Application
      1. 8.2.1 Design Requirements
      2. 8.2.2 Detailed Design Procedure
        1. 8.2.2.1 Choosing the Switching Frequency
        2. 8.2.2.2 Inductor Selection
        3. 8.2.2.3 Output Capacitor Selection
        4. 8.2.2.4 Input Capacitor Selection
        5. 8.2.2.5 CBOOT
        6. 8.2.2.6 VCC
        7. 8.2.2.7 External UVLO
        8. 8.2.2.8 Maximum Ambient Temperature
      3. 8.2.3 Application Curves
      4. 8.2.4 EMI Performance Curves
    3. 8.3 Best Design Practices
    4. 8.4 Power Supply Recommendations
    5. 8.5 Layout
      1. 8.5.1 Layout Guidelines
        1. 8.5.1.1 Ground and Thermal Considerations
      2. 8.5.2 Layout Example
  10. 9 Device and Documentation Support
    1. 9.1 Device Support
      1. 9.1.1 Device Nomenclature
    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 Glossary
    7. 9.7 Electrostatic Discharge Caution
  11. 10Revision History
  12. 11Mechanical, Packaging, and Orderable Information

Package Options

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

8.2.2.8 Maximum Ambient Temperature

As with any power conversion device, the LM63635C-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 LM63635C-Q1 must be limited to 150°C. This limit establishes a limit on the maximum device power dissipation and, therefore, the load current. Equation 10 shows the relationships between the important parameters. Seeing that larger ambient temperatures (TA) and larger values of RθJA reduce the maximum available output current is easy. 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 Section 6.4 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 10. I O U T m a x = T J - T A R θ J A × η 1 - η × 1 V O U T

where

  • η = efficiency

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

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

The WSON12 package uses a die attach paddle, or "thermal pad" (DAP), to provide a place to solder down to the PCB heat-sinking copper. This usage 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 8-3. The copper area given in the graph is for each layer. The top and bottom layers are 2oz copper each, while the inner layers are 1 oz. Figure 8-4 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-49FBC9E8-6ACD-4742-949F-56275BB4BF09-low.gif Figure 8-3 Typical RθJA vs Copper Area for the WSON Package
GUID-717F9430-F3D6-45E6-A477-0DAA4E6BEA9D-low.gif Figure 8-4 Maximum Output Current versus Ambient Temperature VIN = 12V, VOUT = 5V, ƒSW = 400kHz, RθJA = 22°C/W

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