SLVSBO1G July   2013  – June 2021 TPS54561

PRODUCTION DATA  

  1. 1 Features
  2. 2 Applications
  3. 3 Description
  4. 4 Revision History
  5. 5 Pin Configuration and 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 Timing Requirements
    7. 6.7 Switching Characteristics
    8. 6.8 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 Output Current
      3. 7.3.3  Pulse Skip Eco-mode
      4. 7.3.4  Low Dropout Operation and Bootstrap Voltage (BOOT)
      5. 7.3.5  Error Amplifier
      6. 7.3.6  Adjusting the Output Voltage
      7. 7.3.7  Enable and Adjusting Undervoltage Lockout
      8. 7.3.8  Soft Start/Tracking Pin (SS/TR)
      9. 7.3.9  Sequencing
      10. 7.3.10 Constant Switching Frequency and Timing Resistor (RT/CLK) Pin)
      11. 7.3.11 Maximum Switching Frequency
      12. 7.3.12 Synchronization to RT/CLK Pin
      13. 7.3.13 Accurate Current Limit Operation
      14. 7.3.14 Power Good (PWRGD Pin)
      15. 7.3.15 Overvoltage Protection
      16. 7.3.16 Thermal Shutdown
      17. 7.3.17 Small Signal Model for Loop Response
      18. 7.3.18 Simple Small Signal Model for Peak Current Mode Control
      19. 7.3.19 Small Signal Model for Frequency Compensation
    4. 7.4 Device Functional Modes
  8. 8 Application and Implementation
    1. 8.1 Application Information
    2. 8.2 Typical Applications
      1. 8.2.1 Buck Converter for 7-V to 60-V Input to 5-V at 5-A Output
        1. 8.2.1.1 Design Requirements
        2. 8.2.1.2 Detailed Design Procedure
          1. 8.2.1.2.1  Custom Design with WEBENCH® Tools
          2. 8.2.1.2.2  Selecting the Switching Frequency
          3. 8.2.1.2.3  Output Inductor Selection (LO)
          4. 8.2.1.2.4  Output Capacitor
          5. 8.2.1.2.5  Catch Diode
          6. 8.2.1.2.6  Input Capacitor
          7. 8.2.1.2.7  Slow Start Capacitor
          8. 8.2.1.2.8  Bootstrap Capacitor Selection
          9. 8.2.1.2.9  Undervoltage Lockout Set Point
          10. 8.2.1.2.10 Output Voltage and Feedback Resistors Selection
          11. 8.2.1.2.11 Compensation
          12. 8.2.1.2.12 Power Dissipation Estimate
          13. 8.2.1.2.13 Safe Operating Area
          14. 8.2.1.2.14 Discontinuous Conduction Mode and Eco-mode Boundary
        3. 8.2.1.3 Application Curves
      2. 8.2.2 Inverting Buck-Boost Topology for Positive Input to Negative Output
      3. 8.2.3 Split-Rail Topology for Positive Input to Negative and Positive Output
  9. 9 Power Supply Recommendations
  10. 10Layout
    1. 10.1 Layout Guidelines
    2. 10.2 Layout Example
    3. 10.3 Estimated Circuit Area
  11. 11Device and Documentation Support
    1. 11.1 Device Support
      1. 11.1.1 Development Support
        1. 11.1.1.1 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 Support 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

7.3.8 Soft Start/Tracking Pin (SS/TR)

The TPS54561 effectively uses the lower voltage of the internal voltage reference or the SS/TR pin voltage as the power-supply's reference voltage and regulates the output accordingly. A capacitor on the SS/TR pin to ground implements a soft start time. The TPS54561 has an internal pull-up current source of 1.7 μA that charges the external soft start capacitor. The calculations for the soft start time (10% to 90%) are shown in Equation 5. The voltage reference (VREF) is 0.8 V and the soft start current (ISS) is 1.7 μA. The soft start capacitor should remain lower than 0.47 μF and greater than 0.47 nF.

Equation 5. GUID-CCB6A4C7-118A-4695-A92F-98E66DEADAFB-low.gif

At power up, the TPS54561 will not start switching until the soft start pin is discharged to less than 54 mV to ensure a proper power up, see Figure 7-3.

Also, during normal operation, the TPS54561 will stop switching and the SS/TR must be discharged to 54 mV, when the VIN UVLO is exceeded, EN pin pulled below 1.2 V, or a thermal shutdown event occurs.

The FB voltage will follow the SS/TR pin voltage with a 42-mV offset up to 85% of the internal voltage reference. When the SS/TR voltage is greater than 85% on the internal reference voltage the offset increases as the effective system reference transitions from the SS/TR voltage to the internal voltage reference (see Figure 6-23). The SS/TR voltage will ramp linearly until clamped at 2.7 V typically as shown in Figure 7-3.

GUID-C30FF337-4625-407E-BDC3-0DC14F6C22DD-low.gifFigure 7-3 Operation of SS/TR Pin when Starting