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

7.3.18 Small Signal Model for Frequency Compensation

The TPS54478 uses a transconductance amplifier for the error amplifier and readily supports two of the commonly used frequency compensation circuits. The compensation circuits are shown in Figure 33. The Type 2 circuits are most likely implemented in high bandwidth power supply designs using low ESR output capacitors. In Type 2A, one additional high frequency pole is added to attenuate high frequency noise.

TPS54478 f_compen_lvsa83.gifFigure 33. Type-II of Frequency Compensation

The design guidelines for TPS54478 loop compensation are addressed in the Application Information section with more details. The approach is to run the Pspice model first to find the accurate response of the power stage with slope compensation effect. The compensation network is then designed based on the desired crossover frequency. The crossover frequency and phase margin are more closer to the measured results when the slope compensation effect is included.

For type-II compensation, the modulator pole, fpmod, and the esr zero, fz1 can be calculated using Equation 13 and Equation 14. Derating the output capacitor (COUT) is needed if the output voltage is a high percentage of the capacitor rating. Use the capacitor manufacturer information to derate the capacitor value. Use Equation 15 and Equation 16 to estimate a starting point for the crossover frequency, fc. Equation 15 is the geometric mean of the modulator pole and the esr zero and Equation 16 is the mean of modulator pole and the switching frequency. Use the lower value of Equation 15 or Equation 16 as the maximum crossover frequency.

Equation 13. TPS54478 comp_eq1_lvs946.gif

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Equation 14. TPS54478 comp_eq2_lvs946.gif

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Equation 15. TPS54478 comp_eq3_lvs946.gif

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Equation 16. TPS54478 comp_eq4_lvs946.gif

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The type-III compensation is recommended to achieve higher crossover frequency by introducing extra phase lift. By adding a small capacitor C3 in parallel with R1, one-pair of zero and pole is generated as given by Equation 17 and Equation 18. The Application Information section provides step-by-step design guidelines for Type-III compensation with the effect of slope compensation included.

TPS54478 freq_comp_lvsas2.gifFigure 34. Type-III of Frequency Compensation
Equation 17. TPS54478 eq_fz_lvsas2.gif
Equation 18. TPS54478 eq_fp_lvsas2.gif