SLVS974F September   2009  – May 2020 TPS54218

PRODUCTION DATA.  

  1. 1 Features
  2. 2 Applications
  3. 3 Description
    1.     Device Images
      1.      Simplified Schematic
      2.      Efficiency versus Output Current
  4. 4 Revision History
  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  Soft-Start Pin
      9. 7.3.9  Sequencing
      10. 7.3.10 Constant Switching Frequency and Timing Resistor (RT/CLK Pin)
      11. 7.3.11 Overcurrent Protection
      12. 7.3.12 Frequency Shift
      13. 7.3.13 Reverse Overcurrent Protection
      14. 7.3.14 Synchronize Using the RT/CLK Pin
      15. 7.3.15 Power Good (PWRGD Pin)
      16. 7.3.16 Overvoltage Transient Protection
      17. 7.3.17 Thermal Shutdown
    4. 7.4 Device Functional Modes
      1. 7.4.1 Small Signal Model for Loop Response
      2. 7.4.2 Simple Small Signal Model for Peak Current Mode Control
      3. 7.4.3 Small Signal Model for Frequency Compensation
  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  Step One: Select the Switching Frequency
        2. 8.2.2.2  Step Two: Select the Output Inductor
        3. 8.2.2.3  Step Three: Choose the Output Capacitor
        4. 8.2.2.4  Step Four: Select the Input Capacitor
        5. 8.2.2.5  Step Five: Minimum Load DC COMP Voltage
        6. 8.2.2.6  Step Six: Choose the Soft-Start Capacitor
        7. 8.2.2.7  Step Seven: Select the Bootstrap Capacitor
        8. 8.2.2.8  Step Eight: Undervoltage Lockout Threshold
        9. 8.2.2.9  Step Nine: Select Output Voltage and Feedback Resistors
          1. 8.2.2.9.1 Output Voltage Limitations
        10. 8.2.2.10 Step 10: Select Loop Compensation Components
        11. 8.2.2.11 Power Dissipation Estimate
      3. 8.2.3 Application Curves
  9. 9 Power Supply Recommendations
  10. 10Layout
    1. 10.1 Layout Guidelines
    2. 10.2 Layout Example
  11. 11Device and Documentation Support
    1. 11.1 Device Support
      1. 11.1.1 Development Support
        1. 11.1.1.1 Custom Design With WEBENCH® Tools
    2. 11.2 Receiving Notification of Documentation Updates
    3. 11.3 Support Resources
    4. 11.4 Trademarks
    5. 11.5 Electrostatic Discharge Caution
    6. 11.6 Glossary
  12. 12Mechanical, Packaging, and Orderable Information

8.2.2.9.1 Output Voltage Limitations

Due to the internal design of the TPS54218, there are limitations to the minimum and maximum achievable output voltages. The output voltage can never be lower than the internal voltage reference of 0.8 V. Above 0.8 V, the output voltage can be limited by the minimum controllable on-time. The minimum output voltage in this case is given by Equation 35. There is also a maximum achievable output voltage which is limited by the minimum off-time. The maximum output voltage is given by Equation 36. These equations represent the results when the power MOSFETs are matched. Refer to the Understanding Output Voltage Limitations of DC/DC Buck Converters Technical Brief for more information.

Equation 35. TPS54218 eq_vout_min.gif

where

  • VOUT(min) is the minimum achievable output voltage
  • tON(min) is the minimum controllable on-time (110 nsec typical)
  • fSW(max) is the maximum switching frequency including tolerance
  • VIN(max) is the maximum input voltage
  • IOUT(min) is the minimum load current
  • RLS(min) is the minimum low-side MOSFET on-resistance (30 mΩ typical)
  • RDCR is the series resistance of output inductor
Equation 36. TPS54218 eq_vout_max.gif

where

  • VOUT(max) is the maximum achievable output voltage
  • tOFF(max) is the maximum, minimum controllable off-time (60 ns typical)
  • fSW(max) is the maximum switching frequency including tolerance
  • VIN(min) is the minimum input voltage
  • IOUT(max) is the maximum load current
  • RHS(max) is the maximum high-side MOSFET on-resistance (70 mΩ max)
  • RDCR is the series resistance of output inductor