SNVS478G January   2007  – April 2025 LM5574

PRODUCTION DATA  

  1.   1
  2. 1 Features
  3. 2 Applications
  4. 3 Description
  5. 4 Pin Configuration and Functions
  6. 5 Specifications
    1. 5.1 Absolute Maximum Ratings
    2. 5.2 ESD Ratings
    3. 5.3 Recommended Operating Conditions
    4. 5.4 Thermal Information
    5. 5.5 Electrical Characteristics
    6. 5.6 Typical Characteristics
  7. 6 Detailed Description
    1. 6.1 Overview
    2. 6.2 Functional Block Diagram
    3. 6.3 Feature Description
      1. 6.3.1 High Voltage Start-Up Regulator
      2. 6.3.2 Oscillator and Sync Capability
      3. 6.3.3 Error Amplifier and PWM Comparator
      4. 6.3.4 Ramp Generator
      5. 6.3.5 Maximum Duty Cycle, Input Dropout Voltage
      6. 6.3.6 Current Limit
      7. 6.3.7 Soft Start
      8. 6.3.8 Boost Pin
      9. 6.3.9 Thermal Protection
    4. 6.4 Device Functional Modes
  8. 7 Application and Implementation
    1. 7.1 Application Information
    2. 7.2 Typical Application
      1. 7.2.1 Design Requirements
      2. 7.2.2 Detailed Design Procedure
        1. 7.2.2.1  Custom Design With WEBENCH® Tools
        2. 7.2.2.2  External Components
        3. 7.2.2.3  R3 (R)T
        4. 7.2.2.4  L1-Inductor
        5. 7.2.2.5  C3 (C)RAMP
        6. 7.2.2.6  C9-Output Capacitor
        7. 7.2.2.7  D1-Async Diode
        8. 7.2.2.8  C1-Input Capacitor
        9. 7.2.2.9  C8-VCC Capacitor
        10. 7.2.2.10 C7-BST Capacitor
        11. 7.2.2.11 C4- SS Capacitor
        12. 7.2.2.12 R5, R6- Feedback Resistors
        13. 7.2.2.13 R1, R2, C2-SD Pin Components
        14. 7.2.2.14 R4, C5, C6-Compensation Components
        15. 7.2.2.15 Bias Power Dissipation Reduction
      3. 7.2.3 Application Curves
    3. 7.3 Power Supply Recommendations
    4. 7.4 Layout
      1. 7.4.1 Layout Guidelines
      2. 7.4.2 Layout Example
      3. 7.4.3 Power Dissipation
      4. 7.4.4 Thermal Considerations
  9. 8 Device and Documentation Support
    1. 8.1 Device Support
      1. 8.1.1 Third-Party Products Disclaimer
      2. 8.1.2 Development Support
        1. 8.1.2.1 Custom Design With WEBENCH® Tools
    2. 8.2 Documentation Support
      1. 8.2.1 Related Documentation
    3. 8.3 Receiving Notification of Documentation Updates
    4. 8.4 Support Resources
    5. 8.5 Trademarks
    6. 8.6 Electrostatic Discharge Caution
    7. 8.7 Glossary
  10. 9 Revision History
  11. 10Mechanical, Packaging, and Orderable Information

7.2.2.8 C1-Input Capacitor

The regulator supply voltage has a large source impedance at the switching frequency. Good-quality input capacitors are necessary to limit the ripple voltage at the VIN pin while supplying most of the switch current during the on-time. When the buck switch turns on, the current into the VIN pin steps to the lower peak of the inductor current waveform, ramps up to the peak value, then drops to zero at turn-off. The average current into VIN during the on-time is the load current. The input capacitance must be selected for RMS current rating and minimum ripple voltage. A good approximation for the required ripple current rating necessary is IRMS > IOUT / 2.

Quality ceramic capacitors with a low ESR must be selected for the input filter. To allow for capacitor tolerances and voltage effects, one 1.0µF, 100V ceramic capacitor is used. If step input voltage transients are expected near the maximum rating of the LM5574, a careful evaluation of ringing and possible spikes at the device VIN pin must be completed. An additional damping network or input voltage clamp can be required in these cases.