SNVS583H March   2009  – October 2018 LM5008A

PRODUCTION DATA.  

  1. 1 Features
  2. 2 Applications
  3. 3 Description
    1.     Device Images
      1.      Typical Application, Basic Step-Down Regulator
  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 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 Control Circuit Overview
      2. 7.3.2 Start-Up Regulator (VCC)
      3. 7.3.3 Regulation Comparator
      4. 7.3.4 Overvoltage Comparator
      5. 7.3.5 On-Time Generator and Shutdown
      6. 7.3.6 Current Limit
      7. 7.3.7 N-Channel Buck Switch and Driver
      8. 7.3.8 Thermal Protection
    4. 7.4 Device Functional Modes
      1. 7.4.1 Shutdown Mode
      2. 7.4.2 Active Mode
  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 Selection Of External Components
        3. 8.2.2.3 Low-Output Ripple Configurations
      3. 8.2.3 Application Curves
  9. 9 Power Supply Recommendations
  10. 10Layout
    1. 10.1 Layout Guidelines
    2. 10.2 Layout Example
  11. 11Device and Documentation Support
    1. 11.1 Device Support
      1. 11.1.1 Third-Party Products Disclaimer
      2. 11.1.2 Custom Design With WEBENCH® Tools
      3. 11.1.3 Development Support
    2. 11.2 Documentation Support
      1. 11.2.1 Related Documentation
        1. 11.2.1.1 PCB Layout Resources
        2. 11.2.1.2 Thermal Design Resources
    3. 11.3 Receiving Notification of Documentation Updates
    4. 11.4 Community Resources
    5. 11.5 Trademarks
    6. 11.6 Electrostatic Discharge Caution
    7. 11.7 Glossary
  12. 12Mechanical, Packaging, and Orderable Information

Package Options

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

8.2.2.3 Low-Output Ripple Configurations

For applications where low-output ripple is required, the following options can be used to reduce or nearly eliminate the ripple:

  1. Reduced ripple configuration: In Figure 11, Cff is added across RFB2 to AC-couple the ripple at VOUT directly to the FB pin. This allows the ripple at VOUT to be reduced to a minimum of 25 mVp-p by reducing R3, because the ripple at VOUT is not attenuated by the feedback resistors. The minimum value for Cff is determined by Equation 10:
    2. Equation 10. LM5008A 30074920.gif

    where

    • tON(max) is the maximum on-time, which occurs at VIN(min). The next larger standard value capacitor must be used for Cff.
    LM5008A 30074921.gifFigure 11. Reduced Ripple Configuration
  2. Minimum ripple configuration: If the application requires a lower value of ripple (<10 mVp-p), the circuit of Figure 12 can be used. R3 is removed, and the resulting output ripple voltage is determined by the inductor’s ripple current and C2’s characteristics. RA and CA are chosen to generate a sawtooth waveform at their junction, and that voltage is AC-coupled to the FB pin through CB. To determine the values for RA, CA, and CB, use the following procedure in Equation 11:
    3. Equation 11. Calculate VA = VOUT – (VSW × (1 – (VOUT/VIN(min))))

    where

    • VSW is the absolute value of the voltage at the SW pin during the off-time (typically 1 V). VA is the DC voltage at the RA/CA junction, and is used in Equation 12.
    Equation 12. Calculate RA × CA = (VIN(min) – VA) × tON/ΔV

    where

    • tON is the maximum on-time (at minimum input voltage), and ΔV is the desired ripple amplitude at the RA/CA junction (typically 40-50 mV). RA and CA are then chosen from standard value components to satisfy the above product. Typically CA is 1000 pF to 5000 pF, and RA is 10 kΩ to 300 kΩ. CB is then chosen large compared to CA, typically 0.1 µF.
    LM5008A 30074922.gifFigure 12. Minimum Output Ripple Using Ripple Injection
  3. Alternate minimum ripple configuration: The circuit in Figure 13 is the same as that in the Functional Block Diagram, except the output voltage is taken from the junction of R3 and C2. The ripple at VOUT is determined by the inductor’s ripple current and C2’s characteristics. However, R3 slightly degrades the load regulation. This circuit may be suitable if the load current is fairly constant.
    4. LM5008A 30074923.gifFigure 13. Alternate Minimum Output Ripple Configuration