SNVS769J March   2000  – December 2014 LM2940-N , LM2940C

PRODUCTION DATA.  

  1. Features
  2. Applications
  3. Description
  4. Revision History
  5. Pin Configuration and Functions
  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 (5 V and 8 V)
    6. 6.6 Electrical Characteristics (9 V and 10 V)
    7. 6.7 Electrical Characteristics (12 V and 15 V)
    8. 6.8 Typical Characteristics
  7. Detailed Description
    1. 7.1 Overview
    2. 7.2 Functional Block Diagram
    3. 7.3 Feature Description
      1. 7.3.1 Short-Circuit Current Limit
      2. 7.3.2 Overvoltage Shutdown (OVSD)
      3. 7.3.3 Thermal Shutdown (TSD)
    4. 7.4 Device Functional Modes
      1. 7.4.1 Operation with Enable Control
  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 External Capacitors
          1. 8.2.2.1.1 Minimum Capacitance
          2. 8.2.2.1.2 ESR Limits
      3. 8.2.3 Application Curves
  9. Power Supply Recommendations
  10. 10Layout
    1. 10.1 Layout Guidelines
    2. 10.2 Layout Examples
    3. 10.3 Heatsinking
      1. 10.3.1 Heatsinking TO-220 Package Parts
  11. 11Device and Documentation Support
    1. 11.1 Documentation Support
      1. 11.1.1 Related Documentation
    2. 11.2 Related Links
    3. 11.3 Trademarks
    4. 11.4 Electrostatic Discharge Caution
    5. 11.5 Glossary
  12. 12Mechanical, Packaging, and Orderable Information

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10 Layout

10.1 Layout Guidelines

The dynamic performance of the LM2940 is dependent on the layout of the PCB. PCB layout practices that are adequate for typical LDOs may degrade the PSRR, noise, or transient performance of the LM2940. Best performance is achieved by placing CIN and COUT on the same side of the PCB as the LM2940, and as close as is practical to the package. The ground connections for CIN and COUT should be back to the LM2940 ground pin using as wide and short of a copper trace as is practical.

10.2 Layout Examples

layout_WSON-NGN_snvs769.gifFigure 30. LM2940 WSON Layout
layout_SOT223-DCY_snvs769.gifFigure 31. LM2940 SOT-223 Layout
layout_TO263-KTT_snvs769.gifFigure 32. TO-263 Layout

10.3 Heatsinking

A heatsink may be required depending on the maximum power dissipation and maximum ambient temperature of the application. Under all possible operating conditions, the junction temperature must be within the range specified under Absolute Maximum Ratings.

To determine if a heatsink is required, the power dissipated by the regulator, PD, must be calculated.

Figure 33 shows the voltages and currents which are present in the circuit, as well as the formula for calculating the power dissipated in the regulator:

882237.png
IIN = IL + IG
PD = (VIN − VOUT) IL + (VIN) IG
Figure 33. Power Dissipation Diagram

The next parameter which must be calculated is the maximum allowable temperature rise, TR(MAX). This is calculated by using the formula:

Equation 1. TR(MAX) = TJ(MAX) − TA(MAX)

where

  • TJ(MAX) is the maximum allowable junction temperature, which is 125°C for commercial grade parts.
  •    TA(MAX)is the maximum ambient temperature which will be encountered in the application.

Using the calculated values for TR(MAX) and PD, the maximum allowable value for the junction-to-ambient thermal resistance, RθJA, can now be found:

Equation 2. RθJA = TR(MAX) / PD

NOTE

If the maximum allowable value for RθJA is found to be ≥ 23.3°C/W for the TO-220 package (with a heatsink of 21.7°C/W RθSA), ≥ 40.9°C/W for the DDPAK/TO-263 package, or ≥ 59.3°C/W for the SOT-223 package, no heatsink is needed since the package alone will dissipate enough heat to satisfy these requirements.

If the calculated value for RθJA falls below these limits, a heatsink is required.

10.3.1 Heatsinking TO-220 Package Parts

The TO-220 can be attached to a typical heatsink, or secured to a copper plane on a PC board.

If a manufactured heatsink is to be selected, the value of heatsink-to-ambient thermal resistance, RθSA, must first be calculated:

Equation 3. RθSA = RθJA − RθCS − RθJC

where

  • RθJC is defined as the thermal resistance from the junction to the surface of the case. A value of 3°C/W can be assumed for RθJC for this calculation.
  • RθCS is defined as the thermal resistance between the case and the surface of the heatsink. The value of RθCS will vary from about 0.5°C/W to about 2.5°C/W (depending on method of attachment, insulator, etc.). If the exact value is unknown, 2°C/W should be assumed for RθCS.

When a value for RθSA is found using Equation 3, a heatsink must be selected that has a value that is less than or equal to this number.

RθSA is specified numerically by the heatsink manufacturer in the catalog, or shown in a curve that plots temperature rise vs power dissipation for the heatsink.