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

7.3.4 Low Dropout Operation and Bootstrap Voltage (BOOT)

The TPS54540B provides an integrated bootstrap voltage regulator. A small capacitor between the BOOT and SW pins provides the gate-drive voltage for the high-side MOSFET. The BOOT capacitor is refreshed when the high-side MOSFET is off and the external low-side diode conducts. The recommended value of the BOOT capacitor is 0.1 μF. TI recommends a ceramic capacitor with an X7R or X5R grade dielectric with a voltage rating of 10 V or higher for stable performance over temperature and voltage.

When operating with a low voltage difference from input to output, the high-side MOSFET of the TPS54540B operates at 100% duty cycle as long as the BOOT to SW pin voltage is greater than 2.1 V. When the voltage from BOOT to SW drops below 2.1 V, the high-side MOSFET is turned off, and an integrated low side MOSFET pulls SW low to recharge the BOOT capacitor. To reduce the losses of the small low-side MOSFET at high output voltages, it is disabled at 24-V output and re-enabled when the output reaches 21.5 V.

Because the gate drive current sourced from the BOOT capacitor is small, the high-side MOSFET can remain on for many switching cycles before the MOSFET is turned off to refresh the capacitor. Thus, the effective duty cycle of the switching regulator can be high, approaching 100%. The effective duty cycle of the converter during dropout is mainly influenced by the voltage drops across the power MOSFET, the inductor resistance, the low-side diode voltage, and the printed-circuit-board resistance.

Equation 1 calculates the minimum input voltage required to regulate the output voltage and ensure normal operation of the device. This calculation must include tolerance of the component specifications and the variation of these specifications at their maximum operating temperature in the application.

Equation 1. TPS54540B q_Vin_min_slvsbb4.gif

where

  • VF = Schottky diode forward voltage
  • Rdc = DC resistance of inductor and PCB
  • RDS(on) = High-side MOSFET RDS(on)

At heavy loads, the minimum input voltage must be increased to ensure a monotonic start-up. Equation 2 can be used to calculate the minimum input voltage for this condition.

Equation 2. TPS54540B q_vout(max)_slvsdp8.gif

where

  • D(max) ≥ 0.9
  • IB2SW = 100 µA
  • tSW = 1 / fSW(MHz)
  • VB2SW = VBOOT + VF
  • VBOOT = (1.41 × VIN – 0.554 – VF / tSW – 1.847 × 103 × IB2SW) / (1.41 + 1 / tSW)*
  • RDS(on) = 1 / (–0.3 × VB2SW2 + 3.577 × VB2SW – 4.246)