SLVSEJ2A February   2018  – April 2018 TPSM84424

PRODUCTION DATA.  

  1. 1 Features
  2. 2 Applications
  3. 3 Description
    1.     Device Images
      1.      Simplified Schematic
      2.      Transient Response
  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 Switching Characteristics
    7. 6.7 Typical Characteristics (VIN = 12 V)
    8. 6.8 Typical Characteristics (VIN = 5 V)
  7. 7 Detailed Description
    1. 7.1 Overview
    2. 7.2 Functional Block Diagram
    3. 7.3 Feature Description
      1. 7.3.1  Adjusting the Output Voltage
      2. 7.3.2  Switching Frequency (RT)
      3. 7.3.3  Synchronization (CLK)
      4. 7.3.4  Output On/Off Enable (EN)
      5. 7.3.5  Input Capacitor Selection
      6. 7.3.6  Output Capacitor Selection
      7. 7.3.7  TurboTrans (TT)
        1. 7.3.7.1 Low-ESR Output Capacitors
        2. 7.3.7.2 Transient Response
          1. 7.3.7.2.1 Transient Waveforms (VIN = 12 V)
      8. 7.3.8  Undervoltage Lockout (UVLO)
      9. 7.3.9  Soft Start (SS/TR)
      10. 7.3.10 Sequencing (SS/TR)
      11. 7.3.11 Power Good (PGOOD)
      12. 7.3.12 Safe Start-up into Pre-Biased Outputs
      13. 7.3.13 Overcurrent Protection
      14. 7.3.14 Thermal Shutdown
    4. 7.4 Device Functional Modes
      1. 7.4.1 Active Mode
      2. 7.4.2 Shutdown Mode
  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 Output Voltage Setpoint
        3. 8.2.2.3 Setting the Switching Frequency
        4. 8.2.2.4 Input Capacitors
        5. 8.2.2.5 Output Capacitors
        6. 8.2.2.6 TurboTrans Resistor
        7. 8.2.2.7 Application Waveforms
  9. 9 Power Supply Recommendations
  10. 10Layout
    1. 10.1 Layout Guidelines
    2. 10.2 Layout Examples
    3. 10.3 EMI
      1. 10.3.1 EMI Plots
    4. 10.4 Package Specifications
  11. 11Device and Documentation Support
    1. 11.1 Device Support
      1. 11.1.1 Third-Party Products Disclaimer
    2. 11.2 Development Support
      1. 11.2.1 Custom Design With WEBENCH® Tools
    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
    1. 12.1 Tape and Reel Information

Package Options

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

7.3.9 Soft Start (SS/TR)

Leaving SS/TR pin open enables the internal soft-start time interval of approximately 1.25 ms. Adding additional capacitance between the SS pin and AGND increases the soft-start time. Increasing the soft-start time reduces inrush current seen by the input source and reduces the current seen by the device when charging the output capacitors. To avoid the activation of current limit and ensure proper start-up, the SS capacitor may need to be increased when operating near the maximum output capacitance limit.

See Table 10 for several SS capacitor values and timing interval or use Equation 4 to calculate the value.

Equation 4. TPSM84424 SSequation.gif

Table 10. Soft-Start Capacitor Values and Soft-Start Time

CSS (nF) open 10 15 22 47
SS Time (ms) 1.25 2.4 3 3.8 6.8

During soft-start, the output voltage increases from its starting voltage and rises into regulation. The device is allowed to skip pulses as needed whenever the application conditions exceed the minimum on-time of the device. This behavior is a function of input voltage, output voltage, switching frequency, and load current. During the initial rise of the output voltage, adding an additional non-ceramic output capacitor in parallel with the required ceramic capacitance will improve the output voltage ramp-up.

NOTE

When testing soft start performance with an electronic load, the output voltage noise can be exaggerated due to the control loop of the load. Testing with a pure resistive load is a better way to quantify the device performance.