SLVSDX2A September   2017  – December 2017 TPS23521

PRODUCTION DATA.  

  1. 1 Features
  2. 2 Applications
  3. 3 Description
    1.     Device Images
      1.      Simplified Schematic
  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
  7. 7 Parameter Measurement Information
    1. 7.1 Relationship between Sense Voltage, Gate Current, and Timer
  8. 8 Detailed Description
    1. 8.1 Overview
    2. 8.2 Functional Block Diagram
    3. 8.3 Feature Description
      1. 8.3.1 Current Limit
        1. 8.3.1.1 Programming the CL Switch-Over Threshold
        2. 8.3.1.2 Setting Up the PROG Pin
        3. 8.3.1.3 Programming CL1
        4. 8.3.1.4 Programming CL2
      2. 8.3.2 Soft Start Disconnect
      3. 8.3.3 Timer
      4. 8.3.4 Gate 2
    4. 8.4 Device Functional Modes
      1. 8.4.1 OFF State
      2. 8.4.2 Insertion Delay State
      3. 8.4.3 Start-up State
      4. 8.4.4 Normal Operation State
  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 Procedure
        1. 9.2.2.1 Selecting RSNS
        2. 9.2.2.2 Selecting Soft Start Setting: CSS and CSS,VEE
        3. 9.2.2.3 Selecting VDS Switch Over Threshold
        4. 9.2.2.4 Timer Selection
        5. 9.2.2.5 MOSFET Selection and SOA Checks
        6. 9.2.2.6 EMI Filter Consideration
        7. 9.2.2.7 Under Voltage and Over Voltage Settings
        8. 9.2.2.8 Choosing RVCC and CVCC
        9. 9.2.2.9 Power Good Interface to Downstream DC/DC
      3. 9.2.3 Application Curves
  10. 10Power Supply Recommendations
  11. 11Layout
    1. 11.1 Layout Guidelines
    2. 11.2 Layout Example
  12. 12Device and Documentation Support
    1. 12.1 Device Support
      1. 12.1.1 Third-Party Products Disclaimer
    2. 12.2 Documentation Support
      1. 12.2.1 Related Documentation
    3. 12.3 Receiving Notification of Documentation Updates
    4. 12.4 Community Resources
    5. 12.5 Trademarks
    6. 12.6 Electrostatic Discharge Caution
    7. 12.7 Glossary
  13. 13Mechanical, Packaging, and Orderable Information

Package Options

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

9.2.2.2 Selecting Soft Start Setting: CSS and CSS,VEE

First, compute the minimum inrush current where the timer will trip using equation below.

Equation 12. TPS23521 tps23521_equation2.gif

To avoid running the timer the inrush current needs to be sufficiently low. Target 0.5 A of inrush current to allow margin, and compute the target CSS using equation below. Note that a 10% total tolerance capacitance was assumed on Cout.

Equation 13. TPS23521 tps23521_equation3.gif

Chose CSS close to 58 nF. For this example 55 nF was used, which assumes a 33 nF and 22 nF cap in parallel. This results in an inrush current of 0.53 A at max COUT (1452 µF) and inrush current of 0.48 A at typical COUT (1320 µF). Also it is recommended to add a capacitor between the soft start pin and VEE (CSS,VEE) to improve immunity to input voltage noise during soft start. It's recommended to chose a capacitor that's 3x larger than CSS. In this case a 150 nF capacitor was chosen.

Finally the start-up time at maximum input voltage can be computed using the equation below:

Equation 14. TPS23521 tps23521_equation4.gif