SLUSEA4D June   2021  – August 2022 TPS62932 , TPS62933 , TPS62933F , TPS62933O , TPS62933P

PRODUCTION DATA  

  1. 1 Features
  2. 2 Applications
  3. 3 Description
  4. 4 Revision History
  5. 5 Description (continued)
  6. 6 Device Comparison Table
  7. 7 Pin Configuration and Functions
  8. 8 Specifications
    1. 8.1 Absolute Maximum Ratings
    2. 8.2 ESD Ratings
    3. 8.3 Recommended Operating Conditions
    4. 8.4 Thermal Information
    5. 8.5 Electrical Characteristics
    6. 8.6 Typical Characteristics
  9. 9 Detailed Description
    1. 9.1 Overview
    2. 9.2 Functional Block Diagram
    3. 9.3 Feature Description
      1. 9.3.1  Fixed Frequency Peak Current Mode
      2. 9.3.2  Pulse Frequency Modulation
      3. 9.3.3  Voltage Reference
      4. 9.3.4  Output Voltage Setting
      5. 9.3.5  Switching Frequency Selection
      6. 9.3.6  Enable and Adjusting Undervoltage Lockout
      7. 9.3.7  External Soft Start and Prebiased Soft Start
      8. 9.3.8  Power Good
      9. 9.3.9  Minimum On Time, Minimum Off Time, and Frequency Foldback
      10. 9.3.10 Frequency Spread Spectrum
      11. 9.3.11 Overvoltage Protection
      12. 9.3.12 Overcurrent and Undervoltage Protection
      13. 9.3.13 Thermal Shutdown
    4. 9.4 Device Functional Modes
      1. 9.4.1 Modes Overview
      2. 9.4.2 Heavy Load Operation
      3. 9.4.3 Light Load Operation
      4. 9.4.4 Out of Audio Operation
      5. 9.4.5 Forced Continuous Conduction Operation
      6. 9.4.6 Dropout Operation
      7. 9.4.7 Minimum On-Time Operation
      8. 9.4.8 Shutdown Mode
  10. 10Application and Implementation
    1. 10.1 Application Information
    2. 10.2 Typical Application
      1. 10.2.1 Design Requirements
      2. 10.2.2 Detailed Design Procedure
        1. 10.2.2.1  Custom Design With WEBENCH® Tools
        2. 10.2.2.2  Output Voltage Resistors Selection
        3. 10.2.2.3  Choosing Switching Frequency
        4. 10.2.2.4  Soft-Start Capacitor Selection
        5. 10.2.2.5  Bootstrap Capacitor Selection
        6. 10.2.2.6  Undervoltage Lockout Setpoint
        7. 10.2.2.7  Output Inductor Selection
        8. 10.2.2.8  Output Capacitor Selection
        9. 10.2.2.9  Input Capacitor Selection
        10. 10.2.2.10 Feedforward Capacitor CFF Selection
        11. 10.2.2.11 Maximum Ambient Temperature
      3. 10.2.3 Application Curves
    3. 10.3 What to Do and What Not to Do
  11. 11Power Supply Recommendations
  12. 12Layout
    1. 12.1 Layout Guidelines
    2. 12.2 Layout Example
  13. 13Device and Documentation Support
    1. 13.1 Device Support
      1. 13.1.1 Third-Party Products Disclaimer
      2. 13.1.2 Development Support
        1. 13.1.2.1 Custom Design With WEBENCH® Tools
    2. 13.2 Receiving Notification of Documentation Updates
    3. 13.3 Support Resources
    4. 13.4 Trademarks
    5. 13.5 Electrostatic Discharge Caution
    6. 13.6 Glossary
  14. 14Mechanical, Packaging, and Orderable Information

Package Options

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

10.2.2.7 Output Inductor Selection

The most critical parameters for the inductor are the inductance, saturation current, and the RMS current. The inductance is based on the desired peak-to-peak ripple current, ΔiL, which can be calculated by Equation 15.

Equation 15. GUID-20210219-CA0I-LQLB-MP0N-MTMX9GSXJ26R-low.gif

Usually, define K coefficient represents the amount of inductor ripple current relative to the maximum output current of the device, a reasonable value of K is 20% to 60%. Experience shows that the best value of K is 40%. Since the ripple current increases with the input voltage, the maximum input voltage is always used to calculate the minimum inductance L. Use Equation 16 to calculate the minimum value of the output inductor.

Equation 16. GUID-20210125-CA0I-RZ7C-B3TD-PQCFWNTKLTPG-low.gif

where

  • K is the ripple ratio of the inductor current (ΔIL / IOUT_MAX).

In general, it is preferable to choose lower inductance in switching power supplies, because it usually corresponds to faster transient response, smaller DCR, and reduced size for more compact designs. Too low of an inductance can generate too large of an inductor current ripple such that overcurrent protection at the full load can be falsely triggered. The device also generates more inductor core loss since the current ripple is larger. Larger inductor current ripple also implies larger output voltage ripple with the same output capacitors.

After inductance L is determined, the maximum inductor peak current and RMS current can be calculated by Equation 17 and Equation 18.
Equation 17. GUID-20210219-CA0I-CVW1-CXX1-2K6JB8ML6SVM-low.gif
Equation 18. GUID-20210219-CA0I-TW6R-5JRZ-BLVXBHCZV1QS-low.gif

Ideally, the saturation current rating of the inductor is at least as large as the high-side switch current limit, IHS_LIMIT (see Section 8.5). This ensures that the inductor does not saturate even during a short circuit on the output. When the inductor core material saturates, the inductance falls to a very low value, causing the inductor current to rise very rapidly. Although the valley current limit, ILS_LIMIT, is designed to reduce the risk of current runaway, a saturated inductor can cause the current to rise to high values very rapidly, this can lead to component damage, so do not allow the inductor to saturate. In any case, the inductor saturation current must not be less than the maximum peak inductor current at full load.

For this design example, choose the following values:

  • K = 0.4
  • VIN_MAX = 30 V
  • fSW = 500 kHz
  • IOUT_MAX = 3 A

The inductor value is calculated to be 6.94 μH. Choose the nearest standard value of 6.8 μH, which gives a new K value of 0.408. The maximum IHS_LIMIT is 5.8 A, the calculated peak current is 3.61 A, and the calculated RMS current is 3.02 A. The chosen inductor is a Würth Elektronik, 74439346068, 6.8 μH, which has a saturation current rating of 10 A and a RMS current rating of 6.5 A.

The maximum inductance is limited by the minimum current ripple required for the peak current mode control to perform correctly. To avoid subharmonic oscillation, as a rule-of-thumb, the minimum inductor ripple current must be no less than approximately 10% of the device maximum rated current (3 A) under nominal conditions.