SNVS528H October   2007  – January 2016 LM20143 , LM20143-Q1

PRODUCTION DATA.  

  1. 1 Features
  2. 2 Applications
  3. 3 Description
  4. 4 Revision History
  5. 5 Pin Configuration and 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 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 Peak Current Mode Control
      2. 7.3.2 Precision Enable
      3. 7.3.3 Current Limit
      4. 7.3.4 Pre-Bias Start Up Capability
      5. 7.3.5 Soft-Start and Voltage Tracking
      6. 7.3.6 Power Good and Overvoltage Fault Handling
      7. 7.3.7 UVLO
      8. 7.3.8 Thermal Protection
    4. 7.4 Device Functional Modes
      1. 7.4.1 Light Load Operation
      2. 7.4.2 Tracking an External Supply
      3. 7.4.3 Using Precision Enable and Power Good
  8. 8 Application and Implementation
    1. 8.1 Application Information
    2. 8.2 Typical Applications
      1. 8.2.1 3.3-V or 5-V Supply Rail Design
        1. 8.2.1.1 Design Requirements
        2. 8.2.1.2 Detailed Design Procedure
          1. 8.2.1.2.1  Duty Cycle Calculation
          2. 8.2.1.2.2  Inductor Selection (L)
          3. 8.2.1.2.3  Output Capacitor Selection (COUT)
          4. 8.2.1.2.4  Input Capacitor Selection (CIN)
          5. 8.2.1.2.5  Setting the Output Voltage (RFB1, RFB2)
          6. 8.2.1.2.6  Adjusting the Operating Frequency (RT)
          7. 8.2.1.2.7  AVIN Filtering Components (CF and RF)
          8. 8.2.1.2.8  Sub-Regulator Bypass Capacitor (CVCC)
          9. 8.2.1.2.9  Setting the Start Up Time (CSS)
          10. 8.2.1.2.10 Loop Compensation (RC1, CC1)
        3. 8.2.1.3 Application Curves
      2. 8.2.2 5-V Supply Rail Design
        1. 8.2.2.1 Design Requirements
        2. 8.2.2.2 Detailed Design Procedure
      3. 8.2.3 3-V Supply Rail Design
        1. 8.2.3.1 Design Requirements
        2. 8.2.3.2 Detailed Design Procedure
  9. 9 Power Supply Recommendations
  10. 10Layout
    1. 10.1 Layout Guidelines
    2. 10.2 Layout Example
    3. 10.3 Thermal Considerations
  11. 11Device and Documentation Support
    1. 11.1 Device Support
      1. 11.1.1 Third-Party Products Disclaimer
      2. 11.1.2 Development Support
    2. 11.2 Documentation Support
      1. 11.2.1 Related Documentation
    3. 11.3 Related Links
    4. 11.4 Trademarks
    5. 11.5 Electrostatic Discharge Caution
    6. 11.6 Glossary
  12. 12Mechanical, Packaging, and Orderable Information

Package Options

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

9 Power Supply Recommendations

The LM20143 converter is designed to operate from either a 3.3-V or 5-V rail. The input supply must be compatible with the Absolute Maximum Ratings and Recommended Operating Conditions. Further more the input supply must be able to supply the required average input current to the regulated load. The average input current can be estimated with Equation 15.

Equation 15. LM20143 LM20143-Q1 30030518.gif

where

  • η is efficiency

If the regulator is connected to the input supply through long wires or PCB traces with large impedance, special care is required to achieve good performance. The parasitic inductance and resistance of the input cables may have an adverse effect on the operation of the regulator. The parasitic inductance, in combination with the low ESR ceramic input capacitors, can form an under-damped resonant circuit. This circuit may cause overvoltage transients at the PVIN pin each time the input supply is cycled on and off. The parasitic resistance causes the PVIN voltage to dip when the load on the regulator is switched on or exhibits a transient. If the regulator is operating close to the minimum input voltage, this dip may cause false UVLO fault triggering and system reset. The best way to solve these types of issues is to reduce the distance from the input supply to the regulator and/or use an aluminum or tantalum input capacitor in parallel with the ceramics. The moderate ESR of the electrolytic capacitors helps to damp the input resonant circuit and reduce any voltage overshoots. A value in the range of 20 μF to 100 μF is usually sufficient to provide input damping and help to hold the input voltage steady during large load transients.

Sometimes, an EMI input filter is used in front of the regulator. This can lead to instability, as well as some of the effects mentioned above, unless it is carefully designed. The user guide Simple Success with Conducted EMI for DC-DC Converters, SNVA489, provides helpful suggestions when designing an input filter for any switching regulator.