SLVSDV8 July   2017 TPS54424

PRODUCTION DATA.  

  1. 1 Features
  2. 2 Applications
    1.     Simplified Schematic
  3. 3 Description
    1.     Efficiency
  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 Switching Characteristics
    7. 6.7 Timing Requirements
    8. 6.8 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  Continuous Conduction Mode Operation (CCM)
      3. 7.3.3  VIN Pins and VIN UVLO
      4. 7.3.4  Voltage Reference and Adjusting the Output Voltage
      5. 7.3.5  Error Amplifier
      6. 7.3.6  Enable and Adjustable UVLO
      7. 7.3.7  Soft Start and Tracking
      8. 7.3.8  Safe Start-up into Pre-Biased Outputs
      9. 7.3.9  Power Good
      10. 7.3.10 Sequencing (SS/TRK)
      11. 7.3.11 Adjustable Switching Frequency (RT Mode)
      12. 7.3.12 Synchronization (CLK Mode)
      13. 7.3.13 Bootstrap Voltage and 100% Duty Cycle Operation (BOOT)
      14. 7.3.14 Output Overvoltage Protection (OVP)
      15. 7.3.15 Overcurrent Protection
        1. 7.3.15.1 High-side MOSFET Overcurrent Protection
        2. 7.3.15.2 Low-side MOSFET Overcurrent Protection
    4. 7.4 Device Functional Modes
  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  Switching Frequency
        3. 8.2.2.3  Output Inductor Selection
        4. 8.2.2.4  Output Capacitor
        5. 8.2.2.5  Input Capacitor
        6. 8.2.2.6  Output Voltage Resistors Selection
        7. 8.2.2.7  Soft-start Capacitor Selection
        8. 8.2.2.8  Undervoltage Lockout Set Point
        9. 8.2.2.9  Bootstrap Capacitor Selection
        10. 8.2.2.10 PGOOD Pull-up Resistor
        11. 8.2.2.11 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 Alternate Layout Example
  11. 11Device and Documentation Support
    1. 11.1 Document Support
      1. 11.1.1 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
    6. 11.6 Glossary
  12. 12Mechanical, Packaging, and Orderable Information

Package Options

Refer to the PDF data sheet for device specific package drawings

Mechanical Data (Package|Pins)
  • RNV|18
Thermal pad, mechanical data (Package|Pins)
Orderable Information

8.2.2.3 Output Inductor Selection

To calculate the value of the output inductor, use Equation 14. KIND is a ratio that represents the amount of inductor ripple current relative to the maximum output current. The inductor ripple current is filtered by the output capacitor. Therefore, choosing high inductor ripple currents impacts the selection of the output capacitor since the output capacitor must have a ripple current rating equal to or greater than the inductor ripple current. Additionally with current mode control the sensed inductor current ripple is used in the PWM modulator. Choosing small inductor ripple currents can degrade the transient response performance or introduce jitter in the high-side MOSFET on-time. The inductor ripple, KIND, is normally from 0.2 to 0.4 for the majority of applications giving a peak to peak ripple current range of 0.8 A to 1.6 A. For applications requiring operation near the minimum on-time, with on-times less than 200 ns, the target Iripple must be 1.2 A or larger for best performance. For other applications the target Iripple should be 0.8 A or larger.

For this design example, KIND = 0.3 is used and the inductor value is calculated to be 1.92 μH. The nearest standard value 1.8 µH is selected. It is important that the RMS current and saturation current ratings of the inductor not be exceeded. The RMS and peak inductor current can be found from Equation 16 and Equation 17. For this design, the RMS inductor current is 4.0 A and the peak inductor current is 4.6 A. The chosen inductor is a Würth Elektronik 74438357018. It has a saturation current rating of 8.0 A (20% inductance loss) and a RMS current rating of 5.8 A (40 °C temperature rise). The DC series resistance is 18 mΩ typical.

The current flowing through the inductor is the inductor ripple current plus the output current. During power up, faults or transient load conditions, the inductor current can increase above the calculated peak inductor current level calculated in Equation 17. In transient conditions, the inductor current can increase up to the switch current limit of the device. For this reason, the most conservative approach is to specify the ratings of the inductor based on the switch current limit rather than the steady-state peak inductor current.

Equation 14. TPS54424 eq12_lo_lvs946.gif

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Equation 15. TPS54424 eq13_iripp_lvs946.gif

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Equation 16. TPS54424 eq14_ilrms_lvs946.gif

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Equation 17. TPS54424 eq15_ilpeak_lvs946.gif