SNVS118F december   1999  – may 2023 LM2594 , LM2594HV

PRODUCTION DATA  

  1.   1
  2. 1 Features
  3. 2 Applications
  4. 3 Description
  5. 4 Revision History
  6. 5 Description (continued)
  7. 6 Pin Configuration and Functions
  8. 7 Specifications
    1. 7.1  Absolute Maximum Ratings
    2. 7.2  ESD Ratings
    3. 7.3  Recommended Operating Conditions
    4. 7.4  Thermal Information
    5. 7.5  Electrical Characteristics – 3.3 V
    6. 7.6  Electrical Characteristics – 5 V
    7. 7.7  Electrical Characteristics – 12 V
    8. 7.8  Electrical Characteristics – Adjustable
    9. 7.9  Electrical Characteristics – All Output Voltage Versions
    10. 7.10 Typical Characteristics
  9. 8 Detailed Description
    1. 8.1 Overview
    2. 8.2 Functional Block Diagram
    3. 8.3 Feature Description
      1. 8.3.1 Delayed Start-Up
      2. 8.3.2 Undervoltage Lockout
      3. 8.3.3 Inverting Regulator
      4. 8.3.4 Inverting Regulator Shutdown Methods
    4. 8.4 Device Functional Modes
      1. 8.4.1 Discontinuous Mode Operation
  10. 9 Application and Implementation
    1. 9.1 Application Information
      1. 9.1.1 Input Capacitor (CIN)
      2. 9.1.2 Output Capacitor (COUT)
      3. 9.1.3 Catch Diode
      4. 9.1.4 Inductor Selection
      5. 9.1.5 Output Voltage Ripple and Transients
      6. 9.1.6 Open Core Inductors
    2. 9.2 Typical Applications
      1. 9.2.1 Series Buck Regulator (Fixed Output)
        1. 9.2.1.1 Design Requirements
        2. 9.2.1.2 Detailed Design Procedure
          1. 9.2.1.2.1 Custom Design with WEBENCH® Tools
          2. 9.2.1.2.2 Inductor Selection (L1)
          3. 9.2.1.2.3 Output Capacitor Selection (COUT)
          4. 9.2.1.2.4 Catch Diode Selection (D1)
          5. 9.2.1.2.5 Input Capacitor (CIN)
        3. 9.2.1.3 Application Curves
      2. 9.2.2 Series Buck Regulator (Adjustable Output)
        1. 9.2.2.1 Design Requirements
        2. 9.2.2.2 Detailed Design Procedure
          1. 9.2.2.2.1 Programming Output Voltage
          2. 9.2.2.2.2 Inductor Selection (L1)
          3. 9.2.2.2.3 Output Capacitor Selection (COUT)
          4. 9.2.2.2.4 Feedforward Capacitor (CFF)
          5. 9.2.2.2.5 Catch Diode Selection (D1)
          6. 9.2.2.2.6 Input Capacitor (CIN)
        3. 9.2.2.3 Application Curves
    3. 9.3 Power Supply Recommendations
    4. 9.4 Layout
      1. 9.4.1 Layout Guidelines
      2. 9.4.2 Layout Example
      3. 9.4.3 Thermal Considerations
  11. 10Device and Documentation Support
    1. 10.1 Device Support
      1. 10.1.1 Development Support
        1. 10.1.1.1 Custom Design with WEBENCH® Tools
    2. 10.2 Receiving Notification of Documentation Updates
    3. 10.3 Support Resources
    4. 10.4 Trademarks
    5. 10.5 Electrostatic Discharge Caution
    6. 10.6 Glossary
  12. 11Mechanical, Packaging, and Orderable Information

Package Options

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

9.1.6 Open Core Inductors

Another possible source of increased output ripple voltage or unstable operation is from an open core inductor. Ferrite bobbin or stick inductors have magnetic lines of flux flowing through the air from one end of the bobbin to the other end. These magnetic lines of flux induces a voltage into any wire or PCB copper trace that comes within the inductor's magnetic field. The strength of the magnetic field, the orientation and location of the PC copper trace to the magnetic field, and the distance between the copper trace and the inductor, determine the amount of voltage generated in the copper trace. Another way of looking at this inductive coupling is to consider the PCB copper trace as one turn of a transformer (secondary) with the inductor winding as the primary. Many millivolts can be generated in a copper trace located near an open core inductor which can cause stability problems or high output ripple voltage problems.

If unstable operation is seen, and an open core inductor is used, it is possible that the location of the inductor with respect to other PC traces can be the problem. To determine if this is the problem, temporarily raise the inductor away from the board by several inches and then check circuit operation. If the circuit now operates correctly, then the magnetic flux from the open core inductor is causing the problem. Substituting a closed core inductor such as a torroid or E-core corrects the problem, or re-arranging the PC layout can be necessary. Magnetic flux cutting the IC device ground trace, feedback trace, or the positive or negative traces of the output capacitor must be minimized.

Sometimes, locating a trace directly beneath a bobbin inductor provides good results, provided it is exactly in the center of the inductor (because the induced voltages cancel themselves out), but if it is off center one direction or the other, then problems can arise. If flux problems are present, even the direction of the inductor winding can make a difference in some circuits.

This discussion on open core inductors is not to frighten the user, but to alert the user on what kind of problems to watch out for when using them. Open core bobbin or stick inductors are an inexpensive, simple way of making a compact efficient inductor, and they are used by the millions in many different applications.