SLVSEZ9 January   2019 TPS54540B

PRODUCTION DATA.  

  1. 1 Features
  2. 2 Applications
  3. 3 Description
    1.     Device Images
      1.      Simplified Schematic
      2.      Efficiency vs Load 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 Timing Requirements
    7. 6.7 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 Output Current
      3. 7.3.3  Pulse Skip Eco-mode
      4. 7.3.4  Low Dropout Operation and Bootstrap Voltage (BOOT)
      5. 7.3.5  Error Amplifier
      6. 7.3.6  Adjusting the Output Voltage
      7. 7.3.7  Enable and Adjusting Undervoltage Lockout
      8. 7.3.8  Internal Soft Start
      9. 7.3.9  Constant Switching Frequency and Timing Resistor (RT/CLK) pin)
      10. 7.3.10 Accurate Current-Limit Operation and Maximum Switching Frequency
      11. 7.3.11 Synchronization to RT/CLK pin
      12. 7.3.12 Overvoltage Protection
      13. 7.3.13 Thermal Shutdown
      14. 7.3.14 Small Signal Model for Loop Response
      15. 7.3.15 Simple Small Signal Model for Peak-Current-Mode Control
      16. 7.3.16 Small Signal Model for Frequency Compensation
    4. 7.4 Device Functional Modes
      1. 7.4.1 Operation with VIN < 4.5 V (Minimum VIN)
      2. 7.4.2 Operation with EN Control
  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  Custom Design with WEBENCH® Tools
        2. 8.2.2.2  Selecting the Switching Frequency
        3. 8.2.2.3  Output Inductor Selection (LO)
        4. 8.2.2.4  Output Capacitor
        5. 8.2.2.5  Catch Diode
        6. 8.2.2.6  Input Capacitor
        7. 8.2.2.7  Bootstrap Capacitor Selection
        8. 8.2.2.8  Undervoltage Lockout Set Point
        9. 8.2.2.9  Output Voltage and Feedback Resistors Selection
        10. 8.2.2.10 Minimum VIN
        11. 8.2.2.11 Compensation
        12. 8.2.2.12 Discontinuous Conduction Mode and Eco-mode Boundary
        13. 8.2.2.13 Power Dissipation Estimate
        14. 8.2.2.14 Safe Operating Area
      3. 8.2.3 Application Curves
    3. 8.3 Other System Examples
      1. 8.3.1 Inverting Power
      2. 8.3.2 Split-Rail Power Supply
  9. 9 Power Supply Recommendations
  10. 10Layout
    1. 10.1 Layout Guidelines
    2. 10.2 Layout Examples
    3. 10.3 Estimated Circuit Area
  11. 11Device and Documentation Support
    1. 11.1 Device Support
      1. 11.1.1 Third-Party Products Disclaimer
      2. 11.1.2 Custom Design with WEBENCH® Tools
    2. 11.2 Receiving Notification of Documentation Updates
    3. 11.3 Community Resources
    4. 11.4 Trademarks
    5. 11.5 Electrostatic Discharge Caution
  12. 12Mechanical, Packaging, and Orderable Information

Package Options

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

8.2.2.11 Compensation

There are several methods to design compensation for dc/dc regulators. The method presented here is easy to calculate and ignores the effects of the slope compensation that is internal to the device. Becausae the slope compensation is ignored, the actual crossover frequency is lower than the crossover frequency used in the calculations. This method assumes the crossover frequency is between the modulator pole and the ESR zero and the ESR zero is at least 10 times greater the modulator pole.

To get started, the modulator pole, ƒp(mod), and the ESR zero, ƒz1 must be calculated using Equation 44 and Equation 45. For COUT, use a derated value of 130 μF. Use equations Equation 46 and Equation 47 to estimate a starting point for the crossover frequency, ƒco. For the example design, ƒp(mod) is 1850 Hz and ƒz(mod) is 610 kHz. Equation 45 is the geometric mean of the modulator pole and the ESR zero, and Equation 47 is the mean of modulator pole and half of the switching frequency. Equation 46 yields 34 kHz and Equation 47 gives 19 kHz. Use the geometric mean value of Equation 46 and Equation 47 for an initial crossover frequency. For this example, after lab measurement, the crossover frequency target was increased to 30 kHz for an improved transient response.

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.

Equation 44. TPS54540B q_41_lvsBx7.gif
Equation 45. TPS54540B q_42_lvsBx7.gif
Equation 46. TPS54540B q_43_lvsBx7.gif
Equation 47. TPS54540B q_44_lvsBx7.gif

To determine the compensation resistor, R4, use Equation 48. The typical power stage transconductance, gmps, is 17 A/V. The output voltage, VO, reference voltage, VREF, and amplifier transconductance, gmea, are 3.3 V, 0.8 V and 350 μA/V, respectively. R4 is calculated to be 17 kΩ and a standard value of 16.9 kΩ is selected. Use Equation 49 to set the compensation zero to the modulator pole frequency. Equation 49 yields 5100 pF for compensating capacitor C5. 4700 pF is used for this design.

Equation 48. TPS54540B q_46_lvsbx7.gif
Equation 49. TPS54540B q_C5_lvsBx7.gif

A compensation pole can be implemented if desired by adding capacitor C8 in parallel with the series combination of R4 and C5. Use the larger value calculated from Equation 50 and Equation 51 for C8 to set the compensation pole. The selected value of C8 is 47 pF for this design example.

Equation 50. TPS54540B q_48_lvsBx7.gif
Equation 51. TPS54540B q_49_lvsBx7.gif