SLVSB97E July   2012  – January 2018 TPS23751 , TPS23752

PRODUCTION DATA.  

  1. 1 Features
  2. 2 Applications
  3. 3 Description
    1.     Device Images
      1. 3.1 Typical Application Circuit
  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 ESD Ratings: Surge
    4. 6.4 Recommended Operating Conditions
    5. 6.5 Thermal Information
    6. 6.6 Electric Characteristics - Controller Section
    7. 6.7 Electrical Characteristics - Sleep Mode (TPS23752 Only)
    8. 6.8 Electrical Characteristics - PoE Interface Section
    9. 6.9 Typical Characteristics
  7. 7 Detailed Description
    1. 7.1 Overview
    2. 7.2 Functional Block Diagrams
    3. 7.3 Feature Description
      1. 7.3.1 Pin Description
    4. 7.4 Device Functional Modes
      1. 7.4.1 PoE Overview
        1. 7.4.1.1 Threshold Voltages
        2. 7.4.1.2 PoE Startup Sequence
        3. 7.4.1.3 Detection
        4. 7.4.1.4 Hardware Classification
        5. 7.4.1.5 Inrush and Startup
        6. 7.4.1.6 Maintain Power Signature
        7. 7.4.1.7 Startup and Converter Operation
        8. 7.4.1.8 PD Hotswap Operation
      2. 7.4.2 Sleep Mode Operation (TPS23752 only)
        1. 7.4.2.1  Converter Controller Features
        2. 7.4.2.2  PWM and VFO Operation; CTL, SRT, and SRD Pin Relationships to Output Load Current
        3. 7.4.2.3  Bootstrap Topology
        4. 7.4.2.4  Current Slope Compensation and Current Limit
        5. 7.4.2.5  RT
        6. 7.4.2.6  T2P, Startup and Power Management
        7. 7.4.2.7  Thermal Shutdown
        8. 7.4.2.8  Adapter ORing
        9. 7.4.2.9  Using DEN to Disable PoE
        10. 7.4.2.10 ORing Challenges
  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  Input Bridges and Schottky Diodes
        2. 8.2.2.2  Protection, D1
        3. 8.2.2.3  Capacitor, C1
        4. 8.2.2.4  Detection Resistor, RDEN
        5. 8.2.2.5  Classification Resistor, RCLS
        6. 8.2.2.6  APD Pin Divider Network, RAPD1, RAPD2
        7. 8.2.2.7  Setting the PWM-VFO Threshold using the SRT pin
        8. 8.2.2.8  Setting Frequency (RT)
        9. 8.2.2.9  Current Slope Compensation
        10. 8.2.2.10 Voltage Feed-Forward Compensation
        11. 8.2.2.11 Estimating Bias Supply Requirements and Cvc
        12. 8.2.2.12 Switching Transformer Considerations and RVC
        13. 8.2.2.13 T2P Pin Interface
        14. 8.2.2.14 Softstart
        15. 8.2.2.15 Special Switching MOSFET Considerations
        16. 8.2.2.16 ESD
        17. 8.2.2.17 Thermal Considerations and OTSD
      3. 8.2.3 Application Curves
  9. 9 Power Supply Recommendations
  10. 10Layout
    1. 10.1 Layout Guidelines
    2. 10.2 Layout Example
  11. 11Device and Documentation Support
    1. 11.1 Documentation Support
      1. 11.1.1 Related Documentation
      2. 11.1.2 Related Links
    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

8.2.2.1 Input Bridges and Schottky Diodes

Using Schottky diodes instead of PN junction diodes for the PoE input bridges reduces the power dissipation in these devices by about 30%. There are, however, some things to consider when using them.

The IEEE standard specifies a maximum backfeed voltage of 2.8 V. A 100-kΩ resistor is placed between the unpowered pairs and the voltage is measured across the resistor. Schottky diodes often have a higher reverse leakage current than PN diodes, making this a harder requirement to meet. To compensate, use conservative design for diode operating temperature, select lower-leakage devices where possible, and match leakage and temperatures by using packaged bridges.

Schottky diode leakage currents and lower dynamic resistances can impact the detection signature. Setting reasonable expectations for the temperature range over which the detection signature is accurate is the simplest solution. Increasing RDEN slightly may also help meet the requirement.

Schottky diodes have proven less robust to the stresses of ESD transients than PN junction diodes. After exposure to ESD, Schottky diodes may become shorted or leak. Care must be taken to provide adequate protection in line with the exposure levels. This protection may be as simple as ferrite beads and capacitors.

As a general recommendation, use 1 A or 2 A, 100-V rated discrete or bridge diodes for the input rectifiers.