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

7.3.1 Control Circuit Overview

The LM5008A is a buck DC-DC regulator that uses a control scheme in which the on-time varies inversely with line voltage (VIN). Control is based on a comparator and the on-time one-shot, with the output voltage feedback (FB) compared to an internal reference (2.5 V). If the FB level is below the reference the buck switch is turned on for a fixed time determined by the line voltage and a programming resistor (RT). Following the ON period, the switch remains off for at least the minimum off-timer period of 300 ns. If FB is still below the reference at that time, the switch turns on again for another on-time period. This continues until regulation is achieved.

The LM5008A operates in discontinuous conduction mode at light load currents, and continuous conduction mode at heavy load current. In discontinuous conduction mode, current through the output inductor starts at zero and ramps up to a peak during the on-time, then ramps back to zero before the end of the off-time. The next on-time period starts when the voltage at FB falls below the internal reference; until then, the inductor current remains zero. In this mode, the operating frequency is lower than in continuous conduction mode and varies with load current. Therefore, at light loads, the conversion efficiency is maintained because the switching losses reduce with the reduction in load and frequency. The discontinuous operating frequency can be calculated with Equation 1.

Equation 1. LM5008A 30074911.gif

where

  • RL = the load resistance

In continuous conduction mode, current flows continuously through the inductor and never ramps down to zero. In this mode the operating frequency is greater than the discontinuous mode frequency and remains relatively constant with load and line variations. The approximate continuous mode operating frequency can be calculated with Equation 2.

Equation 2. LM5008A 30074912.gif

The output voltage (VOUT) is programmed by two external resistors as shown in the Functional Block Diagram. The regulation point can be calculated with Equation 3.

Equation 3. VOUT = 2.5 × (RFB1 + RFB2) / RFB1

The LM5008A regulates the output voltage based on ripple voltage at the feedback input, requiring a minimum amount of ESR for the output capacitor C2. A minimum of 25 mV to 50 mV of ripple voltage at the feedback pin (FB) is required for the LM5008A. In cases where the capacitor ESR is too small, additional series resistance may be required (R3 in the Functional Block Diagram).

For applications where lower output voltage ripple is required, the output can be taken directly from a low-ESR output capacitor as shown in Figure 7. However, R3 slightly degrades the load regulation.

LM5008A 30074913.gifFigure 7. Low-Ripple Output Configuration