JAJSBF8B June   2011  – April 2018 TPS54478

PRODUCTION DATA.  

  1. 1 特長
  2. 2 アプリケーション
  3. 3 概要
    1.     効率
  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  Slope Compensation and Output Current
      3. 7.3.3  Bootstrap Voltage (BOOT) and Low Dropout Operation
      4. 7.3.4  Error Amplifier
      5. 7.3.5  Voltage Reference
      6. 7.3.6  Adjusting the Output Voltage
      7. 7.3.7  Enable and Adjusting Undervoltage Lockout
      8. 7.3.8  Slow Start / Tracking Pin
      9. 7.3.9  Constant Switching Frequency and Timing Resistor (RT/CLK Pin)
      10. 7.3.10 Overcurrent Protection
      11. 7.3.11 START-UP into Prebiased Output
      12. 7.3.12 Synchronize Using the RT/CLK Pin
      13. 7.3.13 Power Good (PWRGD Pin)
      14. 7.3.14 Overvoltage Transient Protection
      15. 7.3.15 Thermal Shutdown
      16. 7.3.16 Small Signal Model for Loop Response
      17. 7.3.17 Simple Small Signal Model for Peak Current Mode Control
      18. 7.3.18 Small Signal Model for Frequency Compensation
    4. 7.4 Device Functional Modes
      1. 7.4.1 PWM Operation
      2. 7.4.2 Standby Operation
    5. 7.5 Programming
      1. 7.5.1 Sequencing
  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 Selecting the Switching Frequency
        2. 8.2.2.2 Output Inductor Selection
        3. 8.2.2.3 Output Capacitor
        4. 8.2.2.4 Input Capacitor
        5. 8.2.2.5 Slow Start Capacitor
        6. 8.2.2.6 Bootstrap Capacitor Selection
        7. 8.2.2.7 Output Voltage and Feedback Resistors Selection
        8. 8.2.2.8 Compensation
      3. 8.2.3 Application Curves
  9. 9 Power Supply Recommendations
  10. 10Layout
    1. 10.1 Layout Guidelines
    2. 10.2 Layout Example
    3. 10.3 Power Dissipation Estimate
  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 デベロッパー・ネットワークの製品に関する免責事項
      2. 11.1.2 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
      1. 11.2.1 関連資料
    3. 11.3 ドキュメントの更新通知を受け取る方法
    4. 11.4 コミュニティ・リソース
    5. 11.5 商標
    6. 11.6 静電気放電に関する注意事項
    7. 11.7 Glossary
  12. 12メカニカル、パッケージ、および注文情報

パッケージ・オプション

メカニカル・データ（パッケージ｜ピン）
サーマルパッド・メカニカル・データ
発注情報

8.2.2.2 Output Inductor Selection

The inductor selected works for the entire TPS54478 input voltage range. To calculate the value of the output inductor, use Equation 19. 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 high inductor ripple currents impacts the selection of the output capacitor since the output capacitor must have a ripple current rating equal to or greater than the inductor ripple current. In general, the inductor ripple value is at the discretion of the designer; however, KIND is normally from 0.1 to 0.3 for the majority of applications.

For this design example, use KIND = 0.3 and the inductor value is calculated to be 1.05 μH. For this design, a nearest standard value was chosen: 1.2 μH. For the output filter inductor, it is important that the RMS current and saturation current ratings not be exceeded. The RMS and peak inductor current can be found from Equation 21 and Equation 22.

For this design, the RMS inductor current is 4.01 A and the peak inductor current is 4.53 A. The chosen inductor is a Coilcraft XAL5030-122ME. It has a saturation current rating of 11.8 A (20% inductance loss) and a RMS current rating of 8.7 A (20 °C temperature rise). The series resistance is 6.78 mΩ typical.

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.

Equation 19. TPS54478 eq12_lo_lvs946.gif

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Equation 20. TPS54478 eq13_iripp_lvs946.gif

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Equation 21. TPS54478 eq14_ilrms_lvs946.gif

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Equation 22. TPS54478 eq15_ilpeak_lvs946.gif