SLVSAF1E October   2010  – May 2019 TPS54388-Q1


  1. Features
  2. Applications
  3. Description
    1.     Device Images
      1.      Simplified Schematic
      2.      Efficiency Curve
  4. Revision History
  5. Pin Configuration and Functions
    1.     Pin Functions
  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. 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
    4. 7.4 Device Functional Modes
      1. 7.4.1  Adjusting the Output Voltage
      2. 7.4.2  Enable Functionality and Adjusting Undervoltage Lockout
      3. 7.4.3  Slow-Start or Tracking Pin
      4. 7.4.4  Sequencing
      5. 7.4.5  Constant Switching Frequency and Timing Resistor (RT/CLK Pin)
      6. 7.4.6  Overcurrent Protection
      7. 7.4.7  Frequency Shift
      8. 7.4.8  Reverse Overcurrent Protection
      9. 7.4.9  Synchronize Using the RT/CLK Pin
      10. 7.4.10 Power Good (PWRGD Pin)
      11. 7.4.11 Overvoltage Transient Protection
      12. 7.4.12 Thermal Shutdown
      13. 7.4.13 Small-Signal Model for Loop Response
      14. 7.4.14 Simple Small-Signal Model for Peak-Current-Mode Control
      15. 7.4.15 Small-Signal Model for Frequency Compensation
  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.  Custom Design With WEBENCH® Tools
        2.  Selecting the Switching Frequency
        3.  Output Inductor Selection
        4.  Output Capacitor
        5.  Input Capacitor
        6.  Slow-Start Capacitor
        7.  Bootstrap Capacitor Selection
        8.  Output-Voltage and Feedback-Resistor Selection
        9.  Compensation
        10. Power-Dissipation Estimate
      3. 8.2.3 Application Curves
  9. Power Supply Recommendations
  10. 10Layout
    1. 10.1 Layout Guidelines
    2. 10.2 Layout Example
  11. 11Device and Documentation Support
    1. 11.1 Third-Party Products Disclaimer
    2. 11.2 Device Support
      1. 11.2.1 Development Support
        1. Custom Design With WEBENCH® Tools
    3. 11.3 Documentation Support
    4. 11.4 Receiving Notification of Documentation Updates
    5. 11.5 Community Resource
    6. 11.6 Trademarks
    7. 11.7 Electrostatic Discharge Caution
    8. 11.8 Glossary
  12. 12Mechanical, Packaging, and Orderable Information

Package Options

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

Output Capacitor

Three primary considerations must be considered for selecting the value of the output capacitor. The output capacitor determines the modulator pole, the output voltage ripple, and how the regulator responds to a large change in load current. Base the output-capacitance selection on the most-stringent of these three criteria.

The desired response to a large change in the load current is the first criterion. The output capacitor must supply the load with current when the regulator cannot. This situation would occur if there are desired hold-up times for the regulator where the output capacitor must hold the output voltage above a certain level for a specified amount of time after removal of the input power. The regulator is temporarily not able to supply sufficient output current if there is a large, fast increase in the current requirement of the load, such as transitioning from no load to a full load. The regulator usually requires two or more clock cycles for the control loop to see the change in load current and output voltage and then adjust the duty cycle to react to the change. The output capacitor must be large enough 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 two clock cycles while only allowing a tolerable amount of droop in the output voltage. Equation 26 shows the minimum output capacitance necessary to meet this requirement.

For this example, the specification for transient-load response is a 5% change in VO for a load step from 0 A (no load) to 1.5 A (50% load). For this example, ΔIO = 1.5 A – 0 A = 1.5 A and ΔVO = 0.05 × 1.8 V = 0.09 V. Using these numbers gives a minimum capacitance of 33 μF. This value does not take the ESR of the output capacitor into account in the output voltage change. For ceramic capacitors, the ESR is usually small enough to ignore in this calculation.

Equation 27 calculates 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 27 yields 2.3 µF.

Equation 26. TPS54388-Q1 eq16_co_SLVSAH5.gif


  • ΔIO is the change in output current
  • f(SW) is the regulator switching frequency
  • ΔVO is the allowable change in the output voltage
Equation 27. TPS54388-Q1 eq17_co_SLVSAH5.gif


  • f(SW) is the switching frequency
  • VO(ripple) is the maximum allowable output voltage ripple
  • I(ripple) is the inductor ripple current

Use Equation 28 to calculate the maximum ESR an output capacitor can have to meet the output-voltage ripple specification. Equation 28 indicates the ESR should be less than 55 mΩ. In this case, the ESR of the ceramic capacitor is much less than 55 mΩ.

Factoring in additional capacitance deratings for aging, temperature, and dc bias increases this minimum value. For this example, use two 22-μF, 10-V X5R ceramic capacitors with 3 mΩ of ESR.

Capacitors generally have limits to the amount of ripple current they can handle without failing or producing excess heat. Select an output capacitor that can support the inductor ripple current. Some capacitor data sheets specify the root-mean-square (rms) value of the maximum ripple current. Use Equation 29 to calculate the rms ripple current that the output capacitor must support. For this application, Equation 29 yields 333 mA.

Equation 28. TPS54388-Q1 eq18_resr_SLVSAH5.gif
Equation 29. TPS54388-Q1 eq19_icor_SLVSAH5.gif