SLUSEY7B December   2022  – December 2024 TPSM82816

PRODMIX  

  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 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 Precise Enable (EN)
      2. 7.3.2 Output Discharge
      3. 7.3.3 COMP/FSET
      4. 7.3.4 MODE/SYNC
      5. 7.3.5 Spread Spectrum Clocking (SSC)
      6. 7.3.6 Undervoltage Lockout (UVLO)
      7. 7.3.7 Power-Good Output (PG)
      8. 7.3.8 Thermal Shutdown
    4. 7.4 Device Functional Modes
      1. 7.4.1 Pulse Width Modulation (PWM) Operation
      2. 7.4.2 Power Save Mode Operation (PSM)
      3. 7.4.3 100% Duty-Cycle Operation
      4. 7.4.4 Current Limit and Short-Circuit Protection
      5. 7.4.5 Soft Start / Tracking (SS/TR)
  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 Setting the Output Voltage
        2. 8.2.2.2 Feedforward Capacitor
        3. 8.2.2.3 Input Capacitor
        4. 8.2.2.4 Output Capacitor
      3. 8.2.3 Application Curves
    3. 8.3 System Examples
      1. 8.3.1 Voltage Tracking
      2. 8.3.2 Synchronizing to an External Clock
    4. 8.4 Power Supply Recommendations
    5. 8.5 Layout
      1. 8.5.1 Layout Guidelines
      2. 8.5.2 Layout Example
        1. 8.5.2.1 Thermal Consideration
  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

7.4.5 Soft Start / Tracking (SS/TR)

The soft-start circuitry controls the output voltage slope during start-up. This action avoids excessive inrush current and makes sure of a controlled output voltage rise time. This action also prevents unwanted voltage drops from high impedance power sources or batteries. When EN is set high, the device starts switching after a delay of about 270 μs. Then VOUT rises with a slope controlled by an external capacitor connected to the SS/TR pin.

A capacitor connected from SS/TR to GND is charged with 10 µA by an internal current source during soft start until it reaches the reference voltage of 0.6 V. After reaching 0.6 V, the SS/TR pin voltage is clamped internally while the SS/TR pin voltage keeps rising to a maximum of about 3.3 V. The capacitance required to set a certain ramp-time (tramp) is:

Equation 8. C S S n F = 10 μ A   ×   t r a m p m s 0.6   V

Leaving the SS/TR pin disconnected provides the fastest start-up ramp of 150 µs typically. If the device is set to shutdown (EN = GND), undervoltage lockout, or thermal shutdown, an internal resistor of about 1.1 kΩ pulls the SS/TR pin to GND to make sure of a proper low level. Returning from those states causes a new start-up sequence.

A voltage applied at the SS/TR pin can also be used to track a controller voltage. The output voltage follows this voltage in both directions up and down in forced PWM mode. In PSM mode, the output voltage decreases based on the load current. An external voltage applied on SS/TR is internally clamped to the feedback voltage (0.6 V). TI recommends to set the final value of the external voltage on SS/TR to be slightly above 0.6 V to make sure the device operates with the internal reference voltage when the power-up sequencing is finished. See Voltage Tracking.