SLVSER6B May   2020  – November 2020 TPS23730

PRODUCTION DATA  

  1. 1 Features
  2. 2 Applications
  3. 3 Description
  4. 4 Revision History
  5. 5 Device Comparison Table
  6. 6 Pin Configuration and Functions
  7. 7 Specifications
    1. 7.1 Absolute Maximum Ratings
    2. 7.2 ESD Ratings
    3. 7.3 Recommended Operating Conditions
    4. 7.4 Thermal Information
    5. 7.5 Electrical Characteristics: DC-DC Controller Section
    6. 7.6 Electrical Characteristics PoE
    7.     14
    8. 7.7 Typical Characteristics
  8. 8 Detailed Description
    1. 8.1 Overview
    2. 8.2 Functional Block Diagram
    3. 8.3 Feature Description
      1. 8.3.1  CLSA, CLSB Classification
      2. 8.3.2  DEN Detection and Enable
      3. 8.3.3  APD Auxiliary Power Detect
      4. 8.3.4  PPD Power Detect
      5. 8.3.5  Internal Pass MOSFET
      6. 8.3.6  TPH, TPL and BT PSE Type Indicators
      7. 8.3.7  DC-DC Controller Features
        1. 8.3.7.1 VCC, VB, VBG and Advanced PWM Startup
        2.       28
        3. 8.3.7.2 CS, Current Slope Compensation and Blanking
        4. 8.3.7.3 COMP, FB, EA_DIS, CP, PSRS and Opto-less Feedback
        5. 8.3.7.4 FRS Frequency Setting and Synchronization
        6. 8.3.7.5 DTHR and Frequency Dithering for Spread Spectrum Applications
        7. 8.3.7.6 SST and Soft-Start of the Switcher
        8. 8.3.7.7 SST, I_STP, LINEUV and Soft-Stop of the Switcher
      8. 8.3.8  Switching FET Driver - GATE, GTA2, DT
      9. 8.3.9  EMPS and Automatic MPS
      10. 8.3.10 VDD Supply Voltage
      11. 8.3.11 RTN, AGND, GND
      12. 8.3.12 VSS
      13. 8.3.13 Exposed Thermal pads - PAD_G and PAD_S
    4. 8.4 Device Functional Modes
      1. 8.4.1  PoE Overview
      2. 8.4.2  Threshold Voltages
      3. 8.4.3  PoE Start-Up Sequence
      4. 8.4.4  Detection
      5. 8.4.5  Hardware Classification
      6. 8.4.6  Maintain Power Signature (MPS)
      7. 8.4.7  Advanced Start-Up and Converter Operation
      8. 8.4.8  Line Undervoltage Protection and Converter Operation
      9. 8.4.9  PD Self-Protection
      10. 8.4.10 Thermal Shutdown - DC-DC Controller
      11. 8.4.11 Adapter ORing
  9. 9 Application and Implementation
    1. 9.1 Application Information
    2. 9.2 Typical Application
      1. 9.2.1 Design Requirements
        1. 9.2.1.1 Detailed Design Procedure
          1. 9.2.1.1.1  Input Bridges and Schottky Diodes
          2. 9.2.1.1.2  Input TVS Protection
          3. 9.2.1.1.3  Input Bypass Capacitor
          4. 9.2.1.1.4  Detection Resistor, RDEN
          5. 9.2.1.1.5  Classification Resistor, RCLSA and RCLSB.
          6. 9.2.1.1.6  Dead Time Resistor, RDT
          7. 9.2.1.1.7  APD Pin Divider Network, RAPD1, RAPD2
          8. 9.2.1.1.8  PPD Pin Divider Network, RPPD1, RPPD2
          9. 9.2.1.1.9  Setting Frequency (RFRS) and Synchronization
          10. 9.2.1.1.10 Bias Supply Requirements and CVCC
          11. 9.2.1.1.11 TPH, TPL, and BT Interface
          12. 9.2.1.1.12 Secondary Soft Start
          13. 9.2.1.1.13 Frequency Dithering for Conducted Emissions Control
  10. 10Power Supply Recommendations
  11. 11Layout
    1. 11.1 Layout Guidelines
    2. 11.2 Layout Example
    3. 11.3 EMI Containment
    4. 11.4 Thermal Considerations and OTSD
    5. 11.5 ESD
  12. 12Device and Documentation Support
    1. 12.1 Documentation Support
      1. 12.1.1 Related Documentation
    2. 12.2 Support Resources
    3. 12.3 Trademarks
    4. 12.4 Electrostatic Discharge Caution
    5. 12.5 Glossary
  13. 13Mechanical, Packaging, and Orderable Information

8.3.7.6 SST and Soft-Start of the Switcher

Converters require a soft-start to prevent output overshoot on startup. In PoE applications, the PD also needs soft-start to limit its input current at turn on below the limit allocated by the power source equipment (PSE).

For flyback applications using primary-side control, the TPS23730 provides closed loop controlled soft-start, which applies a slowly rising ramp voltage to a second control input of the error amplifier. The lower of the reference input and soft-start ramp controls the error amplifier, allowing the output voltage to rise in a smooth monotonic fashion.

In all other applications where secondary-side regulation is used, the TPS23730 provides a current-loop soft-start, which controls the switching MOSFET peak current by applying a slowly rising ramp voltage to a second PWM control input. The lower of COMP-derived current demand and soft-start ramp controls the PWM comparator. Note that in this case there is usually a (slower) secondary-side soft-start implemented with the typical TL431 or TLV431 error amplifier to complement the action of the primary-side soft-start.

The soft-start period of the TPS23730 is adjustable with a capacitor between SST and RTN. During soft-start, CSST (Figure 9-1) is being charged from less than 0.2 V to 2.45 V by a ~10-µA current source. Once VSST has exceeded approximately 2.1 V (VSTUOF), the VCC startup is also turned off.

The actual control range of the primary-side closed-loop soft-start capacitor voltage is between 0.25 V and 2 V nominally. Therefore, the soft-start capacitor value must be based on this control range and the required soft-start period (tSS) according to:

Equation 5. GUID-C7F20DA4-007E-48EB-9D18-CF956EDBE0D0-low.gif

The actual control range of the current-loop soft-start capacitor voltage is between 0.6 V and 1.2 V nominally. Therefore, the soft-start capacitor value must be based on this control range and the required soft-start period (tSS) according to:

Equation 6. GUID-E9F65530-CB12-4EF3-807A-2417AB66D5EC-low.gif

Note that the VCC startup turns off only when 2.1 V is reached, allowing additional time for the secondary-side soft-start to complete. For more details regarding the secondary-side soft start, refer to Application Information.