SLVSD01B September   2015  – May 2019 TPS57140-EP

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 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  Bootstrap Voltage (Boot)
      4. 7.3.4  Low-Dropout Operation
      5. 7.3.5  Error Amplifier
      6. 7.3.6  Voltage Reference
      7. 7.3.7  Adjusting the Output Voltage
      8. 7.3.8  Enable and Adjusting UVLO
      9. 7.3.9  Slow-Start or Tracking Pin (SS/TR)
      10. 7.3.10 Overload Recovery Circuit
      11. 7.3.11 Constant Switching Frequency and Timing Resistor (RT/CLK Pin)
      12. 7.3.12 Overcurrent Protection and Frequency Shift
      13. 7.3.13 Selecting the Switching Frequency
      14. 7.3.14 How to Interface to RT/CLK Pin
      15. 7.3.15 Power Good (PWRGD Pin)
      16. 7.3.16 Overvoltage Transient Protection (OVTP)
      17. 7.3.17 Thermal Shutdown
      18. 7.3.18 Small-Signal Model for Loop Response
      19. 7.3.19 Simple Small-Signal Model for Peak-Current-Mode Control
      20. 7.3.20 Small-Signal Model for Frequency Compensation
    4. 7.4 Device Functional Modes
      1. 7.4.1 Sequencing
      2. 7.4.2 Pulse-Skip Eco-mode Control Scheme
  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  Selecting the Switching Frequency
        2. 8.2.2.2  Output Inductor Selection (LO)
        3. 8.2.2.3  Output Capacitor
        4. 8.2.2.4  Catch Diode
        5. 8.2.2.5  Input Capacitor
        6. 8.2.2.6  Slow-Start Capacitor
        7. 8.2.2.7  Bootstrap Capacitor Selection
        8. 8.2.2.8  UVLO Set Point
        9. 8.2.2.9  Output Voltage and Feedback Resistors Selection
        10. 8.2.2.10 Compensation
      3. 8.2.3 Application Curves
  9. 9 Power Supply Recommendations
  10. 10Layout
    1. 10.1 Layout Guidelines
    2. 10.2 Layout Example
    3. 10.3 Power-Dissipation Estimate
  11. 11Device and Documentation Support
    1. 11.1 Receiving Notification of Documentation Updates
    2. 11.2 Community Resources
    3. 11.3 Trademarks
    4. 11.4 Electrostatic Discharge Caution
    5. 11.5 Glossary
  12. 12Mechanical, Packaging, and Orderable Information

Package Options

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

8.2.2.4 Catch Diode

The TPS57140-EP requires an external catch diode between the PH pin and GND. The selected diode must have a reverse voltage rating equal to or greater than Vinmax. The peak current rating of the diode must be greater than the maximum inductor current. The diode should also have a low forward voltage. Schottky diodes are typically a good choice for the catch diode due to their low forward voltage. The lower the forward voltage of the diode, the higher the efficiency of the regulator is.

Typically, the higher the voltage and current ratings the diode has, the higher the forward voltage is. Because the design example has an input voltage up to 18 V, select a diode with a minimum of 20-V reverse voltage.

For the example design, Schottky diode selection is the B220A for its lower forward voltage, and it comes in a larger package size, which has good thermal characteristics over small devices. The typical forward voltage of the B220A is 0.5 V.

The diode selection must also have an appropriate power rating. The diode conducts the output current during the off-time of the internal power switch. The off-time of the internal switch is a function of the maximum input voltage, the output voltage, and the switching frequency. The output current during the off-time multiplied by the forward voltage of the diode equals the conduction losses of the diode. At higher switch frequencies, take the ac losses of the diode into account. The ac losses of the diode are due to the charging and discharging of the junction capacitance and reverse recovery. Use Equation 37 to calculate the total power dissipation, conduction losses plus ac losses, of the diode.

The B220A has a junction capacitance of 120 pF. Using Equation 37, the selected diode dissipates 0.632 W. This power dissipation, depending on mounting techniques, should produce a 16°C temperature rise in the diode when the input voltage is 18 V and the load current is 1.5 A.

If the power supply spends a significant amount of time at light load currents or in sleep mode, consider using a diode which has a low leakage current and slightly higher forward voltage drop.

Equation 37. TPS57140-EP q_pd_lvs795.gif