ZHCSB15B May   2013  – October 2023 TPS54531

PRODUCTION DATA  

  1.   1
  2. 1 特性
  3. 2 应用
  4. 3 说明
  5. 4 Revision History
  6. 5 Pin Configuration and Functions
  7. 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
  8. 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  Voltage Reference (Vref)
      3. 7.3.3  Bootstrap Voltage (BOOT)
      4. 7.3.4  Enable and Adjustable Input Undervoltage Lockout (VIN UVLO)
      5. 7.3.5  Programmable Slow Start Using SS Pin
      6. 7.3.6  Error Amplifier
      7. 7.3.7  Slope Compensation
      8. 7.3.8  Current-Mode Compensation Design
      9. 7.3.9  Overcurrent Protection and Frequency Shift
      10. 7.3.10 Overvoltage Transient Protection
      11. 7.3.11 Thermal Shutdown
    4. 7.4 Device Functional Modes
      1. 7.4.1 Eco-mode
      2. 7.4.2 Operation With VIN < 3.5 V
      3. 7.4.3 Operation With EN Control
  9. 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  Switching Frequency
        3. 8.2.2.3  Output Voltage Set Point
        4. 8.2.2.4  Undervoltage Lockout Set Point
        5. 8.2.2.5  Input Capacitors
        6. 8.2.2.6  Output Filter Components
          1. 8.2.2.6.1 Inductor Selection
          2. 8.2.2.6.2 Capacitor Selection
        7. 8.2.2.7  Compensation Components
        8. 8.2.2.8  Bootstrap Capacitor
        9. 8.2.2.9  Catch Diode
        10. 8.2.2.10 Slow-Start Capacitor
        11. 8.2.2.11 Output Voltage Limitations
        12. 8.2.2.12 Power Dissipation Estimate
      3. 8.2.3 Application Curves
    3. 8.3 Power Supply Recommendations
    4. 8.4 Layout
      1. 8.4.1 Layout Guidelines
      2. 8.4.2 Layout Example
      3. 8.4.3 Electromagnetic Interference (EMI) Considerations
  10. 9 Device and Documentation Support
    1. 9.1 Device Support
      1. 9.1.1 Development Support
        1. 9.1.1.1 Custom Design With WEBENCH® Tools
    2. 9.2 接收文档更新通知
    3. 9.3 支持资源
    4. 9.4 Trademarks
    5. 9.5 静电放电警告
    6. 9.6 术语表
  11. 10Mechanical, Packaging, and Orderable Information

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8.2.2.6.2 Capacitor Selection

Selecting the value of the output capacitor is based on three primary considerations. The output capacitor determines the modulator pole, the output voltage ripple, and how the regulator responds to a large change in load current. The output capacitance must be selected based on the more stringent of these three criteria.

The desired response to a large change in the load current is the first criteria. The output capacitor must supply the load with current when the regulator can not. This situation occurs if desired hold-up times occur for the regulator where the output capacitor must hold the output voltage above a certain level for a specified amount of time after the input power is removed. The regulator is also temporarily not able to supply sufficient output current if a large, fast increase occurs in the current needs of the load, such as a transition from no load to full load. The regulator usually requires two or more clock cycles for the control loop to respond to the change in load current and output voltage and adjust the duty cycle to react to the change. The output capacitor must be sized to supply the extra current to the load until the control loop responds to the load change. The output capacitance must be large enough to supply the difference in current for 2 clock cycles while only allowing a tolerable amount of drop in the output voltage. Use Equation 12 to calculate minimum output capacitance (CO) required in this case.

Equation 12. C O > 2 × ∆ I O U T F S W × ∆ V O U T

where

  • ΔIOUT is the change in output current
  • FSW is the switching frequency of the regulator
  • ΔVOUT is the allowable change in the output voltage

For this example, the transient load response is specified as a 5% change in VOUT for a load step of 2.5 A. For this example, ΔIOUT = 2.5 A and ΔVOUT = 0.05 x 5 = 0.25 V. Using these values results in a minimum capacitance of 35 μF. This value does not consider the ESR of the output capacitor in the output voltage change. For ceramic capacitors, the ESR is usually small enough to ignore in this calculation.

Use Equation 13 to calculate the minimum output capacitance needed to meet the output voltage ripple specification. In this case, the maximum output voltage ripple is 30 mV. Under this requirement Equation 13, yields 14 µF.

Equation 13. C O > 1 8 × F S W × 1 V O U T R I P P L E I R I P P L E

where

  • FSW is the switching frequency
  • VOUTRIPPLE is the maximum allowable output voltage ripple
  • IRIPPLE is the inductor ripple current

Use Equation 14 to calculate the maximum ESR an output capacitor can have to meet the output-voltage ripple specification. Equation 14 indicates the ESR must be less than 15.6 mΩ. In this case, the ESR of the ceramic capacitor is much smaller than 15.6 mΩ.

Equation 14. R E S R < V O U T R I P P L E I R I P P L E

Additional capacitance deratings for aging, temperature, and DC bias must be considered which increases this minimum value. For this example, two 47-μF 10-V X5R ceramic capacitors with 3 mΩ of ESR are used. Capacitors generally have limits to the amount of ripple current they can handle without failing or producing excess heat. An output capacitor that can support the inductor ripple current must be specified. Some capacitor data sheets specify the RMS (root mean square) value of the maximum ripple current. Use Equation 15 to calculate the RMS ripple current that the output capacitor must support. For this application, Equation 15 yields 554 mA.

Equation 15. ICOUTRMS=112×VOUT×VINMAX-VOUTVINMAX×LOUT×FSW×NC