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

7.3.6 Adjustable Switching Frequency

The frequency is set using a resistor on the RT pin. A resistor to AGND is used to set the adjustable operating frequency. See the following table for resistor values. Note that if a resistor value falls outside of the recommended range, this fall can cause the device to stop switching. Do not apply a pulsed signal to this pin to force synchronization. If synchronization is needed, see the SYNC pin.

Equation 4. RTkΩ=16.4FSWMHz-0.633

For example, for fSW = 400kHz, RT = (16.4 / 0.4) – 0.633 = 40.37, so a 40.2kΩ resistor is selected as the closest choice.

Table 7-1 Typical RT values
RT (kΩ) Frequency (kHz)
6.81 2206
7.15 2106
15.4 1005
31.6 497.4
39.2 402
158 101
LM644A2-Q1 Setting Clock FrequencyFigure 7-6 Setting Clock Frequency