SLUSEP7 December   2022 LMR51440 , LMR51450

PRODUCTION DATA  

  1. 1 Features
  2. 2 Applications
  3. 3 Description
  4. 4 Revision History
  5. 5 Device Comparison Table
  6. 6 Pin Configuration and Functions
  7. 7 Specifications
    1. 7.1 Absolute Maximum Ratings
    2.     ESD Ratings
    3. 7.2 Recommended Operating Conditions
    4. 7.3 Thermal Information
    5. 7.4 Electrical Characteristics
    6. 7.5 System Characteristics
    7. 7.6 Typical Characteristics
  8. 8 Detailed Description
    1. 8.1 Overview
    2. 8.2 Functional Block Diagram
    3. 8.3 Feature Description
      1. 8.3.1  Fixed Frequency Peak Current Mode Control
      2. 8.3.2  Adjustable Output Voltage
      3. 8.3.3  Enable
      4. 8.3.4  Switching Frequency
      5. 8.3.5  Power-Good Flag Output
      6. 8.3.6  Minimum ON-Time, Minimum OFF-Time, and Frequency Foldback
      7. 8.3.7  Bootstrap Voltage
      8. 8.3.8  Overcurrent and Short-Circuit Protection
      9. 8.3.9  Soft Start
      10. 8.3.10 Thermal Shutdown
    4. 8.4 Device Functional Modes
      1. 8.4.1 Shutdown Mode
      2. 8.4.2 Active Mode
      3. 8.4.3 CCM Mode
      4. 8.4.4 Light-Load Operation (PFM Version)
      5. 8.4.5 Light-Load Operation (FPWM Version)
  9. 9 Application and Implementation
    1. 9.1 Application Information
    2. 9.2 Typical Application
      1. 9.2.1 Design Requirements
      2. 9.2.2 Detailed Design Procedure
        1. 9.2.2.1 Output Voltage Set-Point
        2. 9.2.2.2 Switching Frequency
        3. 9.2.2.3 Inductor Selection
        4. 9.2.2.4 Output Capacitor Selection
        5. 9.2.2.5 Input Capacitor Selection
        6. 9.2.2.6 Bootstrap Capacitor
        7. 9.2.2.7 Undervoltage Lockout Set-Point
      3. 9.2.3 Application Curves
    3. 9.3 Best Design Practices
    4. 9.4 Power Supply Recommendations
    5. 9.5 Layout
      1. 9.5.1 Layout Guidelines
        1. 9.5.1.1 Compact Layout for EMI Reduction
        2. 9.5.1.2 Feedback Resistors
      2. 9.5.2 Layout Example
  10. 10Device and Documentation Support
    1. 10.1 Device Support
      1. 10.1.1 Development Support
        1. 10.1.1.1 Custom Design With WEBENCH® Tools
    2. 10.2 Documentation Support
      1. 10.2.1 Related Documentation
    3. 10.3 Receiving Notification of Documentation Updates
    4. 10.4 Support Resources
    5. 10.5 Trademarks
    6. 10.6 Electrostatic Discharge Caution
    7. 10.7 Glossary
  11. 11Mechanical, Packaging, and Orderable Information

Package Options

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

9.2.2.4 Output Capacitor Selection

The device is designed to be used with a wide variety of LC filters. Minimize the output capacitance to keep cost and size down. The output capacitor or capacitors, COUT, must be chosen with care because it directly affects the steady state output voltage ripple, loop stability, and output voltage overshoot and undershoot during load current transient. The output voltage ripple is essentially composed of two parts. One part is caused by the inductor ripple current flowing through the Equivalent Series Resistance (ESR) of the output capacitors:

Equation 11. ∆VOUT_ESR=∆iL×ESR=KIND×IOUT×ESR

The other part is caused by the inductor current ripple charging and discharging the output capacitors:

Equation 12. ∆VOUT_C=∆iL8×fSW×COUT=KIND×IOUT8×fSW×COUT

The two components of the voltage ripple are not in-phase, therefore, the actual peak-to-peak ripple is less than the sum of the two peaks.

Output capacitance is usually limited by transient performance specifications if the system requires tight voltage regulation with presence of large current steps and fast slew rates. When a large load step occurs, output capacitors provide the required charge before the inductor current can slew to an appropriate level. The control loop of the converter usually requires eight or more clock cycles to regulate the inductor current equal to the new load level during this time. The output capacitance must be large enough to supply the current difference for 6 clock cycles to maintain the output voltage within the specified range. Equation 13 shows the minimum output capacitance needed for a specified VOUT overshoot and undershoot.

Equation 13. COUT>12×6×(IOH-IOL)fSW×∆VOUT_SHOOT

where

  • KIND = Ripple ratio of the inductor current (ΔiL / IOUT)
  • IOL = Low level output current during load transient
  • IOH = High level output current during load transient
  • VOUT_SHOOT = Target output voltage overshoot or undershoot

For this design example, the target output ripple is 25 mV. Assuming ΔVOUT_ESR = ΔVOUT_C = 25mV, choose KIND = 0.4. Equation 11 yields ESR no larger than 12.5 mΩ and Equation 12 yields COUT no smaller than 20 µF. For the target overshoot and undershoot limitation of this design, ΔVOUT_SHOOT = 5% × VOUT = 250 mV. The COUT can be calculated to be no less than 60 µF by Equation 13. In summary, the most stringent criteria for the output capacitor is 60 µF. Considering derating, two 33-µF, 16-V, X7R ceramic capacitor with 5-mΩ ESR is used.