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

8.2.2.10 Step 10: Select Loop Compensation Components

There are several industry techniques used to compensate DC/DC regulators. The method presented here is easy to calculate and yields high phase margins. For most conditions, the regulator has a phase margin between 60 and 90 degrees. The method presented here ignores the effects of the slope compensation that is internal to the TPS54218. Because the slope compensation is ignored, the actual crossover frequency is usually lower than the crossover frequency used in the calculations.

To get started, the modulator pole, fP(mod), and the esr zero, fZ1, must be calculated using Equation 37 and Equation 38. For COUT, derating the capacitor is not needed as the 1.8-V output is a small percentage of the 10-V capacitor rating. If the output is a high percentage of the capacitor rating, use the capacitor manufacturer information to derate the capacitor value. Use Equation 39 and Equation 40 to estimate a starting point for the crossover frequency, fC. For the example design, fP(mod) is 4.02 kHz and fZ1 is 1206 kHz. Equation 39 is the geometric mean of the modulator pole and the esr zero and Equation 40 is the mean of modulator pole and the switching frequency. Equation 39 yields 69.6 kHz and Equation 40 gives 44.8 kHz. Use the lower value of Equation 39 or Equation 40 as the maximum crossover frequency. For this example, fc is 45 kHz. Next, the compensation components are calculated. A resistor in series with a capacitor is used to create a compensating zero. A capacitor in parallel to these two components forms the compensating pole (if needed).

Equation 37. TPS54218 q_fpmod_slvs946.gif
Equation 38. TPS54218 q_fzmod_slvs946.gif
Equation 39. TPS54218 q_fc_1_slvs94.gif
Equation 40. TPS54218 q_fc_2_slvs94.gif

The compensation design takes the following steps:

  1. Set up the anticipated cross-over frequency. Use Equation 41 to calculate the resistor value of the compensation network. In this example, the anticipated cross-over frequency, fC, is 45 kHz. The power stage gain (gM(ps)) is 13 A/V and the error amplifier gain (gM(ea)) is 225 µA/V.
    2. Equation 41. TPS54218 q_r3_slvs946.gif
  2. Place compensation zero at the pole formed by the load resistor and the output capacitor. The capacitor of the compensation network can be calculated from Equation 42.
    3. Equation 42. TPS54218 eq27_c4_lvs975.gif
  3. An additional pole can be added to attenuate high frequency noise. In this application, it is not necessary to add it.

From the procedures above, start with a 14.3-kΩ resistor and a 4130-pF capacitor. After prototyping and bode plot measurement, the optimized compensation network selected for this design includes a 9.53-kΩ resistor and a 3900-pFΩ capacitor.