SNVSCW3 November   2024 LM644A2-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
    1. 5.1 Wettable Flanks
  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 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  Input Voltage Range (VIN)
      2. 7.3.2  Enable EN Pin and Use as VIN UVLO
      3. 7.3.3  Output Voltage Selection and Soft Start
      4. 7.3.4  SYNC Allows Clock Synchronization and Mode Selection
      5. 7.3.5  Clock Locking
      6. 7.3.6  Adjustable Switching Frequency
      7. 7.3.7  Power-Good Output Voltage Monitoring
      8. 7.3.8  Internal LDO, VCC UVLO, and BIAS Input
      9. 7.3.9  Bootstrap Voltage and VCBOOT-UVLO (CB1 and CB2 Pin)
      10. 7.3.10 CONFIG Device Configuration Pin
      11. 7.3.11 Spread Spectrum
      12. 7.3.12 Soft Start and Recovery From Dropout
      13. 7.3.13 Overcurrent and Short-Circuit Protection
      14. 7.3.14 Hiccup
      15. 7.3.15 Thermal Shutdown
    4. 7.4 Device Functional Modes
      1. 7.4.1 Shutdown Mode
      2. 7.4.2 Standby Mode
      3. 7.4.3 Active Mode
        1. 7.4.3.1 Peak Current Mode Operation
        2. 7.4.3.2 Auto Mode Operation
          1. 7.4.3.2.1 Diode Emulation
        3. 7.4.3.3 FPWM Mode Operation
        4. 7.4.3.4 Minimum On-time (High Input Voltage) Operation
        5. 7.4.3.5 Dropout
        6. 7.4.3.6 Recovery from Dropout
        7. 7.4.3.7 Other Fault Modes
  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  Setting the Output Voltage
        3. 8.2.2.3  Inductor Selection
        4. 8.2.2.4  Output Capacitor Selection
        5. 8.2.2.5  Input Capacitor Selection
        6. 8.2.2.6  BOOT Capacitor
        7. 8.2.2.7  VCC
        8. 8.2.2.8  CFF and RFF Selection
        9. 8.2.2.9  SYNCHRONIZATION AND MODE
        10. 8.2.2.10 External UVLO
        11. 8.2.2.11 Typical Thermal Performance
      3. 8.2.3 Application Curves
    3. 8.3 Power Supply Recommendations
    4. 8.4 Layout
      1. 8.4.1 Layout Guidelines
        1. 8.4.1.1 Ground and Thermal Considerations
      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

Package Options

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

8.2.2.11 Typical Thermal Performance

The dual thermal pads of the LM644A2-Q1 allow the part to be cooled through the PCB and with a top side heatsink to extend the temperature range of the device. However, as with any power conversion device, the LM644A2-Q1 dissipates internal power while operating, so careful design of the thermal environment is important. The effect of this power dissipation is to raise the internal temperature of the converter above ambient temperature. The internal die temperature (TJ) is a function of the following:

  • Ambient temperature
  • Power loss
  • Effective thermal resistance, RθJA of the device
  • PCB layout
The maximum internal die temperature for the LM644A2-Q1 must be limited to 150°C. This limit establishes a limit on the maximum device power dissipation and, therefore, the load current. The following equation shows the relationships between the important parameters. Larger ambient temperatures (TA) and larger values of RθJA reduce the maximum available output current. For low ambient temperature designs the converter efficiency can be estimated by using the curves provided in the Application Curves section. If the desired operating conditions cannot be found in one of the curves, then the junction temperature can be roughly estimated using the EVM thermal performance as a starting point. 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 note, the JEDEC value of RθJA given in the electrical characteristics table is not always valid for design purposes and must not be used to estimate the thermal performance of the device in a real application. Additionally, adding a heatsink to the top of the package will create a parallel thermal path through RθJC(top) and lower RθJA accordingly. The values reported in the electrical characteristics table were measured under a specific set of conditions that are rarely obtained in an actual application.

Equation 9. IOUT_MAX=TJ-TARθJA×η(1-η)×1VOUT

where

  • η = efficiency
  • TA = ambient temperature
  • TJ = junction temperature
  • RθJA = the effective thermal resistance of the IC junction to the air, mainly through the PCB

The effective RθJA is a critical parameter and depends on many factors (just to mention a few of the most critical parameters:

  • Power dissipation
  • Air temperature
  • Airflow
  • PCB area
  • Copper heat-sink area
  • Number of thermal vias under or near the package
  • Adjacent component placement
A typical curve of maximum output current versus ambient temperature is shown in Figure 8-3 and Figure 8-4 for a good thermal layout. This data is calculated using the RθJA of the EVM without a heatsink and adding the calculated effect of a HSB43-454515P heatsink under either natural convection or airflow conditions. 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.

LM644A2-Q1 Typical Output Current vs Ambient Temperature, VIN = 12V FSW = 400kHz, Dual Output
Figure 8-3 Typical Output Current vs Ambient Temperature, VIN = 12V FSW = 400kHz, Dual Output
LM644A2-Q1 Typical Output Current versus Ambient Temperature, VIN = 12V FSW = 1MHz, Dual OutputFigure 8-5 Typical Output Current versus Ambient Temperature, VIN = 12V FSW = 1MHz, Dual Output
LM644A2-Q1 Typical Output Current vs Ambient Temperature, FSW = 400kHz, Single OutputFigure 8-7 Typical Output Current vs Ambient Temperature, FSW = 400kHz, Single Output
LM644A2-Q1 Typical Output Current vs Ambient Temperature, VIN = 24V FSW = 400kHz, Dual OutputFigure 8-4 Typical Output Current vs Ambient Temperature, VIN = 24V FSW = 400kHz, Dual Output
LM644A2-Q1 Typical Output Current vs Ambient Temperature, VIN = 24V FSW = 1MHz, Dual OutputFigure 8-6 Typical Output Current vs Ambient Temperature, VIN = 24V FSW = 1MHz, Dual Output
LM644A2-Q1 Typical Output Current versus Ambient Temperature, FSW = 1MHz, Single OutputFigure 8-8 Typical Output Current versus Ambient Temperature, FSW = 1MHz, Single Output

Use the following resources as a guide to excellent thermal PCB design and estimating RθJA for a given application environment: