SNVS470I January   2007  – November 2025 LM25576

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
    4. 6.4 Device Functional Modes
      1. 6.4.1 Shutdown and Stand-by Mode
      2. 6.4.2 Oscillator and Sync Capability
      3. 6.4.3 Error Amplifier and PWM Comparator
      4. 6.4.4 RAMP Generator
      5. 6.4.5 Maximum Duty Cycle and Input Drop-Out Voltage
      6. 6.4.6 Current Limit
      7. 6.4.7 Soft Start
      8. 6.4.8 Boost Pin
      9. 6.4.9 Thermal Protection
  8. 7 Application and Implementation
    1. 7.1 Application Information
      1. 7.1.1 Bias Power Dissipation Reduction
    2. 7.2 Typical Application
      1. 7.2.1 Typical Schematic for High Frequency (1 MHz) Application
      2. 7.2.2 Typical Schematic for Buck and Boost (Inverting) Application
      3. 7.2.3 Design Requirements
        1. 7.2.3.1  R3 (RT)
        2. 7.2.3.2  L1
        3. 7.2.3.3  C3 (CRAMP)
        4. 7.2.3.4  C9, C10
        5. 7.2.3.5  D1
        6. 7.2.3.6  C1, C2
        7. 7.2.3.7  C8
        8. 7.2.3.8  C7
        9. 7.2.3.9  C4
        10. 7.2.3.10 R5, R6
        11. 7.2.3.11 R1, R2, C12
        12. 7.2.3.12 R7, C11
        13. 7.2.3.13 R4, C5, C6
      4. 7.2.4 Detailed Design Procedure
        1. 7.2.4.1 Custom Design With WEBENCH® Tools
      5. 7.2.5 Application Curves
    3. 7.3 Power Supply Recommendations
    4. 7.4 Layout
      1. 7.4.1 Layout Guidelines
        1. 7.4.1.1 PCB Layout
      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 Developmental Support
        1. 8.1.1.1 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

6.4.8 Boost Pin

The LM25576 integrates an N-Channel buck switch and associated floating high voltage level shift / gate driver. This gate driver circuit works in conjunction with an internal diode and an external bootstrap capacitor. A 0.022 µF ceramic capacitor, connected with short traces between the BST pin and SW pin, is recommended. During the off-time of the buck switch, the SW pin voltage is approximately –0.5 V and the bootstrap capacitor is charged from Vcc through the internal bootstrap diode. When operating with a high PWM duty cycle, the buck switch is forced off each cycle for 500 ns to make sure that the bootstrap capacitor is recharged.

Under very light load conditions or when the output voltage is precharged, the SW voltage does not remain low during the off-time of the buck switch. If the inductor current falls to zero and the SW pin rises, the bootstrap capacitor does not receive sufficient voltage to operate the buck switch gate driver. For these applications, the PRE pin can be connected to the SW pin to pre-charge the bootstrap capacitor. The internal precharge MOSFET and diode connected between the PRE pin and PGND turns on each cycle for 265 ns just prior to the onset of a new switching cycle. If the SW pin is at a normal negative voltage level (continuous conduction mode), then no current flows through the precharge MOSFET/diode.