SLVS885I October   2008  – December 2017 TPS23754 , TPS23754-1 , TPS23756


  1. Features
  2. Applications
  3. Description
    1.     Device Images
      1.      High-Efficiency Converter Using TPS23754
  4. Revision History
  5. Pin Configuration and Functions
    1.     Pin Functions
  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 Electrical Characteristics: PoE and Control
    7. 6.7 Switching Characteristics
    8. 6.8 Typical Characteristics
  7. Detailed Description
    1. 7.1 Overview
    2. 7.2 Functional Block Diagram
    3. 7.3 Feature Description
      1. 7.3.1  APD
      2. 7.3.2  BLNK
      3. 7.3.3  CLS
      4. 7.3.4  Current Sense (CS)
      5. 7.3.5  Control (CTL)
      6. 7.3.6  Detection and Enable (DEN)
      7. 7.3.7  DT
      8. 7.3.8  Frequency and Synchronization (FRS)
      9. 7.3.9  GATE
      10. 7.3.10 GAT2
      11. 7.3.11 PPD
      12. 7.3.12 RTN, ARTN, COM
      13. 7.3.13 T2P
      14. 7.3.14 VB
      15. 7.3.15 VC
      16. 7.3.16 VDD
      17. 7.3.17 VDD1
      18. 7.3.18 VSS
      19. 7.3.19 PowerPAD
    4. 7.4 Device Functional Modes
      1. 7.4.1 PoE Overview
        1.  Threshold Voltages
        2.  PoE Start-Up Sequence
        3.  Detection
        4.  Hardware Classification
        5.  Inrush and Start-Up
        6.  Maintain Power Signature
        7.  Start-Up and Converter Operation
        8.  PD Hotswap Operation
        9.  Converter Controller Features
        10. Bootstrap Topology
        11. Current Slope Compensation and Current Limit
        12. Blanking – RBLNK
        13. Dead Time
        14. FRS and Synchronization
        15. T2P, Start-Up, and Power Management
        16. Thermal Shutdown
        17. Adapter ORing
        18. PPD ORing Features
        19. Using DEN to Disable PoE
        20. ORing Challenges
  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.  Input Bridges and Schottky Diodes
        2.  Protection, D1
        3.  Capacitor, C1
        4.  Detection Resistor, RDEN
        5.  Classification Resistor, RCLS
        6.  Dead Time Resistor, RDT
        7.  Switching Transformer Considerations and RVC
        8.  Special Switching MOSFET Considerations
        9.  Thermal Considerations and OTSD
        10. APD Pin Divider Network, RAPD1, RAPD2
        11. PPD Pin Divider Network, RPPD1, RPPD2
        12. Setting Frequency (RFRS) and Synchronization
        13. Current Slope Compensation
        14. Blanking Period, RBLNK
        15. Estimating Bias Supply Requirements and CVC
        16. T2P Pin Interface
        17. Advanced ORing Techniques
        18. Soft Start
        19. Frequency Dithering for Conducted Emissions Control
      3. 8.2.3 Application Curves
  9. Power Supply Recommendations
  10. 10Layout
    1. 10.1 Layout Guidelines
    2. 10.2 Layout Example
    3. 10.3 ESD
  11. 11Device and Documentation Support
    1. 11.1 Documentation Support
      1. 11.1.1 Related Documentation
    2. 11.2 Community Resources
    3. 11.3 Trademarks
    4. 11.4 Electrostatic Discharge Caution
    5. 11.5 Glossary
  12. 12Mechanical, Packaging, and Orderable Information

Package Options

Refer to the PDF data sheet for device specific package drawings

Mechanical Data (Package|Pins)
  • PWP|20
Thermal pad, mechanical data (Package|Pins)
Orderable Information

Bootstrap Topology

The internal start-up current source and control logic implement a bootstrap-type start-up as discussed in Start-Up and Converter Operation. The start-up current source charges CVC from VDD1 when the converter is disabled (either by the PD control or the VC control) to store enough energy to start the converter. Steady-state operating power must come from a converter (bias winding) output or other source. Loading on VC and VB must be minimal while CVC charges, otherwise the converter may never start. The optocoupler will not load VB when the converter is off for most situations; however take care in ORing topologies where the output is powered when PoE is off.

The converter will shut off when VC falls below its lower UVLO. This can happen when power is removed from the PD, or during a fault on a converter output rail. When one output is shorted, all the output voltages fall including the one that powers VC. The control circuit discharges VC until it hits the lower UVLO and turns off. A restart will initiate, as described in Start-Up and Converter Operation, if the converter turns off and there is sufficient VDD1 voltage. This type of operation is sometimes referred to as hiccup mode, which provides robust output short protection by providing time-average heating reduction of the output rectifier.

The bootstrap control logic disables most of the converter controller circuits except the VB regulator and internal reference. Both GATE and GAT2 (assuming GAT2 is enabled) will be low when the converter is disabled. FRS, BLNK, and DT will be at ARTN while the VC UVLO disables the converter. While the converter runs, FRS, BLNK, and DT will be about 1.25 V.

The start-up current source transitions to a resistance as (VVDD1 – VVC) falls less than 7 V, but will start the converter from adapters within tST. The lower test voltage for tST was chosen based on an assumed adapter tolerance, but is not meant to imply a hard cutoff exists. start-up takes longer and eventually will not occur as VDD1 decreases below the test voltage. The bootstrap source provides reliable start-up from widely varying input voltages, and eliminates the continual power loss of external resistors. The start-up current source will not charge above the maximum recommended VVC if the converter is disabled and there is sufficient VDD1 to charge higher.