SNOSB23G October   2008  – May 2026 LM5575-Q1

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 Shutdown and Standby
      2. 6.3.2 Current Limit
      3. 6.3.3 Soft Start
      4. 6.3.4 Thermal Protection
    4. 6.4 Device Functional Modes
      1. 6.4.1 High-Voltage Start-Up Regulator
      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 BOOST Pin
      6. 6.4.6 Maximum Duty Cycle and Input Dropout Voltage
  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 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 (RT)
        4. 7.2.2.4  L1
        5. 7.2.2.5  C3 (CRAMP)
        6. 7.2.2.6  C9, C10
        7. 7.2.2.7  D1
        8. 7.2.2.8  C1, C2
        9. 7.2.2.9  C8
        10. 7.2.2.10 C7
        11. 7.2.2.11 C4
        12. 7.2.2.12 R5, R6
        13. 7.2.2.13 R1, R2, C12
        14. 7.2.2.14 R7, C11
        15. 7.2.2.15 R4, C5, C6
      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 Examples
      3. 7.4.3 Thermal Considerations
  9. 8 Device and Documentation Support
    1. 8.1 Device Support
      1. 8.1.1 Development Support
        1. 8.1.1.1 Custom Design With WEBENCH® Tools
    2. 8.2 Receiving Notification of Documentation Updates
    3. 8.3 Support Resources
    4. 8.4 Trademarks
    5. 8.5 Electrostatic Discharge Caution
    6. 8.6 Glossary
  10. 9 Revision History
  11. 10Mechanical, Packaging, and Orderable Information

6.4.1 High-Voltage Start-Up Regulator

The LM5575-Q1 contains a dual-mode internal high-voltage start-up regulator that provides the VCC bias supply for the PWM controller and bootstrap MOSFET gate driver. The input pin (VIN) can be connected directly to the input voltage, as high as 75V. For input voltages lower than 9V, a low dropout switch connects VCC directly to VIN. In this supply range, VCC is approximately equal to VIN. For VIN voltage greater than 9V, the low-dropout switch is disabled and the VCC regulator is enabled to maintain VCC at approximately 7V. The wide operating range of 6V to 75V is achieved through the use of this dual-mode regulator.

The output of the VCC regulator is current-limited to 25mA. Upon power up, the regulator sources current into the capacitor connected to the VCC pin. When the voltage at the VCC pin exceeds the VCC UVLO threshold of 5.35V and the SD pin is greater than 1.225V, the output switch is enabled and a soft-start sequence begins. The output switch remains enabled until VCC falls below 5V or the SD pin falls below 1.125V.

An auxiliary supply voltage can be applied to the VCC pin to reduce the IC power dissipation. If the auxiliary voltage is greater than 7.3V, the internal regulator essentially shuts off, reducing the IC power dissipation. The VCC regulator series pass transistor includes a diode between VCC and VIN that must not be forward biased in normal operation. Therefore the auxiliary VCC voltage must never exceed the VIN voltage.

In high-voltage applications, take care to ensure the VIN pin does not exceed the absolute maximum voltage rating of 76V. During line or load transients, voltage ringing on the VIN line that exceeds the absolute maximum ratings can damage the IC. Both careful printed-circuit board layout and the use of quality bypass capacitors located close to the VIN and GND pins are essential.

LM5575-Q1 VIN and VCC SequencingFigure 6-1 VIN and VCC Sequencing