JAJS472C August   2010  – April 2018 TPS54320

PRODUCTION DATA.  

  1. 1 特長
  2. 2 アプリケーション
  3. 3 概要
    1.     Device Images
      1.      概略回路図
      2.      効率と負荷電流との関係
  4. 4 改訂履歴
  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 Typical Characteristics
  7. 7 Detailed Description
    1. 7.1 Overview
    2. 7.2 Functional Block Diagram
    3. 7.3 Feature Description
      1. 7.3.1  Fixed Frequency PWM Control
      2. 7.3.2  Continuous Current Mode Operation (CCM)
      3. 7.3.3  VIN and Power VIN Pins (VIN and PVIN)
      4. 7.3.4  Voltage Reference
      5. 7.3.5  Adjusting the Output Voltage
      6. 7.3.6  Safe Start-up into Prebiased Outputs
      7. 7.3.7  Error Amplifier
      8. 7.3.8  Slope Compensation
      9. 7.3.9  Enable and Adjusting UVLO
      10. 7.3.10 Slow Start (SS/TR)
      11. 7.3.11 Power Good (PWRGD)
      12. 7.3.12 Bootstrap Voltage (BOOT) and Low Dropout Operation
      13. 7.3.13 Sequencing (SS/TR)
      14. 7.3.14 Output Overvoltage Protection (OVP)
      15. 7.3.15 Overcurrent Protection
        1. 7.3.15.1 High-Side MOSFET Overcurrent Protection
        2. 7.3.15.2 Low-Side MOSFET Overcurrent Protection
      16. 7.3.16 Thermal Shutdown
      17. 7.3.17 Small Signal Model for Loop Response
      18. 7.3.18 Simple Small Signal Model for Peak Current Mode Control
      19. 7.3.19 Small Signal Model for Frequency Compensation
    4. 7.4 Device Functional Modes
      1. 7.4.1 Adjustable Switching Frequency and Synchronization (RT/CLK)
      2. 7.4.2 Adjustable Switching Frequency (RT Mode)
      3. 7.4.3 Synchronization (CLK Mode)
  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  Custom Design With WEBENCH® Tools
        2. 8.2.2.2  Operating Frequency
        3. 8.2.2.3  Output Inductor Selection
        4. 8.2.2.4  Output Capacitor Selection
        5. 8.2.2.5  Input Capacitor Selection
        6. 8.2.2.6  Slow-Start Capacitor Selection
        7. 8.2.2.7  Bootstrap Capacitor Selection
        8. 8.2.2.8  UVLO Set Point
        9. 8.2.2.9  Output Voltage Feedback Resistor Selection
          1. 8.2.2.9.1 Minimum Output Voltage
        10. 8.2.2.10 Compensation Component Selection
      3. 8.2.3 Application Curves
  9. 9 Power Supply Recommendations
  10. 10Layout
    1. 10.1 Layout Guidelines
    2. 10.2 Layout Example
  11. 11デバイスおよびドキュメントのサポート
    1. 11.1 デバイス・サポート
      1. 11.1.1 WEBENCH®ツールによるカスタム設計
    2. 11.2 ドキュメントのサポート
    3. 11.3 商標
    4. 11.4 静電気放電に関する注意事項
    5. 11.5 Glossary
  12. 12メカニカル、パッケージ、および注文情報

8.2.2.3 Output Inductor Selection

To calculate the value of the output inductor, Equation 18 is used. KIND is a coefficient that represents the amount of inductor ripple current relative to the maximum output current. The inductor ripple current is filtered by the output capacitor. Therefore, choosing a high inductor ripple current impacts the selection of the output capacitor because the output capacitor must have a ripple current rating equal to or greater than the inductor ripple current. Usually, the inductor ripple value is at the discretion of the designer; however, KIND is normally from 0.2 to 0.4 for the majority of applications.

Equation 18. TPS54320 eq17_L1_lvs982.gif

For this design example using KIND = 0.3, the inductor value is calculated to be 6.2 µH. The nearest standard value of 6.8 µH was chosen. For the output filter inductor, it is important that the RMS current and saturation current ratings not be exceeded. The inductor ripple current, RMS current, and peak inductor current can be found from Equation 19, Equation 20, and Equation 21.

Equation 19. TPS54320 eq17_irip_lvs949.gif
Equation 20. TPS54320 eq19_ilrms_lvs982.gif
Equation 21. TPS54320 eq19_ilpeak_lvs949.gif

For this design, the inductor ripple current is 815 mA, the RMS inductor current is 3.01 A, and the peak inductor current is 3.41 A. A 6.8-µH TDK VLP8040 series inductor was chosen for its small size and low DCR. It has a saturation current rating of 3.6 A and a RMS current rating of 4 A.

The current flowing through the inductor is the inductor ripple current plus the output current. During power up, faults or transient load conditions, the inductor current can increase above the calculated peak inductor current level calculated above. In transient conditions, the inductor current can increase up to the switch current limit of the device. For this reason, the most conservative approach is to specify an inductor with a saturation current rating equal to or greater than the switch current limit rather than the peak inductor current.