SLVSB98A March   2012  – July 2015 TPS2379

PRODUCTION DATA.  

  1. 1 Features
  2. 2 Applications
  3. 3 Description
  4. 4 Revision History
  5. 5 Pin Configuration and 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 Typical Characteristics
  7. 7 Parameter Measurement Information
  8. 8 Detailed Description
    1. 8.1 Overview
    2. 8.2 Functional Block Diagram
    3. 8.3 Feature Description
      1. 8.3.1 CDB Converter Disable Bar Pin Interface
      2. 8.3.2 CLS Classification
      3. 8.3.3 DEN Detection and Enable
      4. 8.3.4 GATE Auxiliary Gate Driver
        1. 8.3.4.1 External Boost Circuit (Q1, Q2, and RBLST) Considerations
      5. 8.3.5 Internal Pass MOSFET
      6. 8.3.6 T2P Type-2 PSE Indicator
      7. 8.3.7 VDD Supply Voltage
      8. 8.3.8 VSS
      9. 8.3.9 PowerPAD
    4. 8.4 Device Functional Modes
      1. 8.4.1 PoE Overview
        1. 8.4.1.1  Threshold Voltages
        2. 8.4.1.2  PoE Start-Up Sequence
        3. 8.4.1.3  Detection
        4. 8.4.1.4  Hardware Classification
        5. 8.4.1.5  Inrush and Start-up
        6. 8.4.1.6  Maintain Power Signature
        7. 8.4.1.7  Start-up and Operation
        8. 8.4.1.8  PD Hotswap Operation
        9. 8.4.1.9  CDB and T2P
        10. 8.4.1.10 Auxiliary Pass MOSFET Control
        11. 8.4.1.11 Using DEN to Disable PoE
  9. 9 Application and Implementation
    1. 9.1 Application Information
    2. 9.2 Typical Application
      1. 9.2.1 Design Requirements
      2. 9.2.2 Detailed Design Procedures
        1. 9.2.2.1 Input Bridges and Schottky Diodes
        2. 9.2.2.2 Protection, D1
        3. 9.2.2.3 Capacitor, C1
        4. 9.2.2.4 Detection Resistor, RDEN
        5. 9.2.2.5 Classification Resistor, RCLS
        6. 9.2.2.6 External Boost Circuit
      3. 9.2.3 Application Curves
  10. 10Power Supply Recommendations
  11. 11Layout
    1. 11.1 Layout Guidelines
      1. 11.1.1 EMI Containment
    2. 11.2 Layout Example
    3. 11.3 Thermal Considerations and OTSD
    4. 11.4 ESD
  12. 12Device and Documentation Support
    1. 12.1 Documentation Support
      1. 12.1.1 Related Documentation
    2. 12.2 Community Resources
    3. 12.3 Trademarks
    4. 12.4 Electrostatic Discharge Caution
    5. 12.5 Glossary
  13. 13Mechanical, Packaging, and Orderable Information

Package Options

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

9 Application and Implementation

NOTE

Information in the following applications sections is not part of the TI component specification, and TI does not warrant its accuracy or completeness. TI’s customers are responsible for determining suitability of components for their purposes. Customers should validate and test their design implementation to confirm system functionality.

9.1 Application Information

The TPS2379 has the flexibility to be implemented in IEEE802.3at PDs, Universal Power Over Ethernet (UPOE) PDs, or high power non-standard PDs. Therefore, it can be used in a wide range applications such as video and VoIP telephones, multiband access points, security cameras, and pico-base stations.

9.2 Typical Application

TPS2379 typ_app_lvsb98.gifFigure 26. Typical Application Circuit

9.2.1 Design Requirements

Table 3. TPS2379EVM Electrical and Performance Specifications

PARAMETER TEST CONDITIONS MIN TYP MAX UNIT
POWER INTERFACE
Input voltage Applied to the power pins of connectors J1 or J3 0 57 V
Operating voltage After start-up 30 57 V
Input UVLO Rising input voltage at device terminals 40 V
Falling input voltage 30.5
Detection voltage At device terminals 1.4 10.1 V
Classification voltage At device terminals 11.9 23 V
Detection signature 24.9 kΩ
Classification current Class 4 38 42 mA
Inrush current limit 100 180 mA
Operating current limit Internal plus external 2260 mA

9.2.2 Detailed Design Procedures

Given in Equation 5, RBLST can be calulated using Equation 4

Equation 4. TPS2379 equation_02_slvsb99.gif

FMMT493TC can be used for Q2.

9.2.2.1 Input Bridges and Schottky Diodes

Using Schottky diodes instead of PN junction diodes for the PoE input bridges will reduce 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. Take care 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.

9.2.2.2 Protection, D1

A TVS, D1, across the rectified PoE voltage per Figure 26 must be used. A SMAJ58A, or equivalent, is recommended for general indoor applications. Adequate capacitive filtering or a TVS must limit input transient voltage to within the absolute maximum ratings. Outdoor transient levels or special applications require additional protection.

9.2.2.3 Capacitor, C1

The IEEE 802.3at standard specifies an input bypass capacitor (from VDD to VSS) of 0.05 μF to 0.12 μF. Typically a 0.1 μF, 100 V, 10% ceramic capacitor is used.

9.2.2.4 Detection Resistor, RDEN

The IEEE 802.3at standard specifies a detection signature resistance, RDEN between 23.75 kΩ and 26.25 kΩ, or 25 kΩ ± 5%. A resistor of 24.9 kΩ ± 1% is recommended for RDEN.

9.2.2.5 Classification Resistor, RCLS

Select RCLS according to Table 1. For Class 4, choose RCLS = 63.4 Ω.

9.2.2.6 External Boost Circuit

For a PD application requiring current limit of 2.26 A at the PD input (approximately 96 W) and using Q1 FET BUK7275-100A

Equation 5. TPS2379 I2.gif

9.2.3 Application Curves

TPS2379 powerup_lvsb98.gifFigure 27. Power Up and Start
TPS2379 startup_lvsb99.gifFigure 28. Start-up