SLVSBF3B June   2012  – May 2019 TPS54678

PRODUCTION DATA.  

  1. 1 Features
  2. 2 Applications
  3. 3 Description
    1.     Device Images
      1.      Simplified Schematic
      2.      Efficiency vs Output Current
  4. 4 Revision History
  5. 5 Pin Configuration and Functions
    1.     Pin Functions
  6. 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
  7. 7 Detailed Description
    1. 7.1 Overview
    2. 7.2 Functional Block Diagram
    3. 7.3 Feature Description
      1. 7.3.1  Fixed Frequency PWM Control
      2. 7.3.2  Slope Compensation and Output Current
      3. 7.3.3  Bootstrap Voltage (Boot) and Low Dropout Operation
      4. 7.3.4  Error Amplifier
      5. 7.3.5  Voltage Reference
      6. 7.3.6  Adjusting the Output Voltage
      7. 7.3.7  Enable and Adjusting Undervoltage Lockout
      8. 7.3.8  Soft-Start Pin
      9. 7.3.9  Sequencing
      10. 7.3.10 Constant Switching Frequency and Timing Resistor (RT/CLK Pin)
      11. 7.3.11 Overcurrent Protection
        1. 7.3.11.1 High-Side Overcurrent Protection
        2. 7.3.11.2 Low-Side Overcurrent Protection
      12. 7.3.12 Safe Start-Up into Prebiased Outputs
      13. 7.3.13 Synchronize Using the RT/CLK Pin
      14. 7.3.14 Power Good (PWRGD Pin)
      15. 7.3.15 Overvoltage Transient Protection
      16. 7.3.16 Thermal Shutdown
    4. 7.4 Device Functional Modes
      1. 7.4.1 Small Signal Model for Loop Response
      2. 7.4.2 Simple Small Signal Model for Peak Current Mode Control
      3. 7.4.3 Small Signal Model for Frequency Compensation
  8. 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 Custom Design With WEBENCH® Tools
        2. 8.2.2.2 Step One: Select the Switching Frequency
        3. 8.2.2.3 Step Two: Select the Output Inductor
        4. 8.2.2.4 Step Three: Choose the Output Capacitor
        5. 8.2.2.5 Step Four: Select the Input Capacitor
        6. 8.2.2.6 Step Five: Choose the Soft-Start Capacitor
        7. 8.2.2.7 Step Six: Select the Bootstrap Capacitor
        8. 8.2.2.8 Step Eight: Select Output Voltage and Feedback Resistors
          1. 8.2.2.8.1 Output Voltage Limitations
        9. 8.2.2.9 Step Nine: Select Loop Compensation Components
      3. 8.2.3 Application Curves
        1. 8.2.3.1 Additional Information About Application Curves
          1. 8.2.3.1.1 Efficiency
          2. 8.2.3.1.2 Voltage Ripple Measurements
          3. 8.2.3.1.3 Start-Up and Shutdown Waveforms
          4. 8.2.3.1.4 Hiccup Mode Current Limit
  9. 9 Power Supply Recommendations
  10. 10Layout
    1. 10.1 Layout Guidelines
    2. 10.2 Layout Example
    3. 10.3 Power Dissipation Estimate
  11. 11Device and Documentation Support
    1. 11.1 Device Support
      1. 11.1.1 Third-Party Products Disclaimer
      2. 11.1.2 Development Support
        1. 11.1.2.1 Custom Design With WEBENCH® Tools
    2. 11.2 Documentation Support
      1. 11.2.1 Related Documentation
    3. 11.3 Receiving Notification of Documentation Updates
    4. 11.4 Community Resources
    5. 11.5 Trademarks
    6. 11.6 Electrostatic Discharge Caution
    7. 11.7 Glossary
  12. 12Mechanical, Packaging, and Orderable Information

Package Options

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

8.2.2.6 Step Five: Choose the Soft-Start Capacitor

The slow-start capacitor determines the minimum amount of time it takes for the output voltage to reach the nominal programmed value during power up. This is useful if a load requires a controlled voltage slew rate. This is also used if the output capacitance is very large and would require large amounts of current to quickly charge the capacitor to the output voltage level. The large currents necessary to charge the capacitor may cause the TPS54678 to trip OCP, or excessive current draw from the input power supply may cause the input voltage rail to sag. Limiting the output voltage slew rate mitigates both of these issues.

The slow-start capacitor value can be calculated using Equation 25. For the example circuit, the slow-start time is not critical because the output capacitor value is 5 × 47 μF which does not require much current to charge to
1.2 V. The example circuit has the slow-start time set to an arbitrary value of 3.33 ms, which requires a 10-nF capacitor.

Equation 25. TPS54678 Eq25_SLVSBF3.gif