SNVS715B March   2011  – June 2019 LM21212-2

PRODUCTION DATA.  

  1. 1 Features
  2. 2 Applications
  3. 3 Description
    1.     Device Images
      1.      Simplified Application Circuit
  4. 4 Revision History
  5. 5 Description, continued
  6. 6 Pin Configuration and Functions
    1.     Pin Descriptions
  7. 7 Specifications
    1. 7.1 Absolute Maximum Ratings
    2. 7.2 ESD Ratings
    3. 7.3 Recommended Operating Ratings
    4. 7.4 Electrical Characteristics
    5. 7.5 Typical Characteristics
  8. 8 Detailed Description
    1. 8.1 Overview
    2. 8.2 Functional Block Diagram
    3. 8.3 Feature Description
      1. 8.3.1 Precision Enable
      2. 8.3.2 UVLO
      3. 8.3.3 Current Limit
      4. 8.3.4 Short-Circuit Protection
      5. 8.3.5 Thermal Protection
      6. 8.3.6 Power-Good Flag
      7. 8.3.7 Light Load Operation
    4. 8.4 Device Functional Modes
  9. 9 Application and Implementation
    1. 9.1 Application Information
    2. 9.2 Typical Application
      1. 9.2.1 Typical Application 1
        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 Output Voltage
          3. 9.2.1.2.3 Precision Enable
          4. 9.2.1.2.4 Soft Start
          5. 9.2.1.2.5 Resistor-Adjustable Frequency
          6. 9.2.1.2.6 Inductor Selection
          7. 9.2.1.2.7 Output Capacitor Selection
          8. 9.2.1.2.8 Input Capacitor Selection
          9. 9.2.1.2.9 Control Loop Compensation
        3. 9.2.1.3 Application Curves
      2. 9.2.2 Typical Application Schematic 2
        1. 9.2.2.1 Design Requirements
        2. 9.2.2.2 Detailed Design Procedure
  10. 10Layout
    1. 10.1 Layout Considerations
    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
        1. 11.1.2.1 Custom Design With WEBENCH® Tools
    2. 11.2 Receiving Notification of Documentation Updates
    3. 11.3 Community Resources
    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

10.3 Thermal Considerations

The thermal characteristics of the LM21212-2 are specified using the parameter θJA, which relates the junction temperature to the ambient temperature. Although the value of θJA is dependant on many variables, it still can be used to approximate the operating junction temperature of the device.

To obtain an estimate of the device junction temperature, one may use the following relationship:

Equation 20. LM21212-2 30155145.gif

where

  • TJ is the junction temperature in °C
  • θJA is the junction to ambient thermal resistance for the LM21212-2
  • TA is the ambient temperature in °C

and

Equation 21. LM21212-2 30155146.gif

where

  • PIN is the input power in Watts (PIN = VIN x IIN)
  • IOUT is the output load current in A

It is important to always keep the operating junction temperature (TJ) below 125°C for reliable operation. If the junction temperature exceeds 165°C the device will cycle in and out of thermal shutdown. If thermal shutdown occurs it is a sign of inadequate heatsinking or excessive power dissipation in the device.

Figure 36, shown below, provides a better approximation of the θJA for a given PCB copper area. The PCB used in this test consisted of 4 layers: 1 oz. copper was used for the internal layers while the external layers were plated to 2 oz. copper weight. To provide an optimal thermal connection, a 3 × 4 array of 8 mil. vias under the thermal pad were used, and an additional sixteen 8 mil. vias under the rest of the device were used to connect the 4 layers.

LM21212-2 30155142.gifFigure 36. Thermal Resistance vs PCB Area (4-Layer Board)

Figure 37 shows a plot of the maximum ambient temperature vs output current for the typical application circuit shown in , assuming a θJA value of 24°C/W.

LM21212-2 30155144.gifFigure 37. Maximum Ambient Temperature vs Output Current (0 LFM)