SBVS066S December   2005  – November 2024 TPS74401

PRODUCTION DATA  

  1.   1
  2. 1 Features
  3. 2 Applications
  4. 3 Description
  5. 4 Pin Configuration and Functions
  6. 5 Specifications
    1. 5.1 Absolute Maximum Ratings
    2. 5.2 ESD Ratings
    3. 5.3 Recommended Operating Conditions
    4. 5.4 Thermal Information
    5. 5.5 Electrical Characteristics
    6. 5.6 Typical Characteristics
  7. 6 Detailed Description
    1. 6.1 Overview
    2. 6.2 Functional Block Diagram
    3. 6.3 Feature Description
      1. 6.3.1 Enable, Shutdown
      2. 6.3.2 Power-Good (VQFN Package Only)
      3. 6.3.3 Internal Current Limit
      4. 6.3.4 Thermal Protection
    4. 6.4 Device Functional Modes
      1. 6.4.1 Normal Operation
      2. 6.4.2 Dropout Operation
      3. 6.4.3 Disabled
    5. 6.5 Programming
      1. 6.5.1 Programmable Soft-Start
      2. 6.5.2 Sequencing Requirements
  8. 7 Application and Implementation
    1. 7.1 Application Information
      1. 7.1.1 Input, Output, and Bias Capacitor Requirements
      2. 7.1.2 Transient Response
      3. 7.1.3 Dropout Voltage
      4. 7.1.4 Output Noise
    2. 7.2 Typical Applications
      1. 7.2.1 Setting the TPS74401
        1. 7.2.1.1 Design Requirements
        2. 7.2.1.2 Detailed Design Procedure
        3. 7.2.1.3 Application Curves
      2. 7.2.2 Using an Auxiliary Bias Rail
      3. 7.2.3 Without an Auxiliary Bias
    3. 7.3 Power Supply Recommendations
    4. 7.4 Layout
      1. 7.4.1 Layout Guidelines
        1. 7.4.1.1 Power Dissipation
        2. 7.4.1.2 Thermal Considerations
      2. 7.4.2 Layout Example
  9. 8 Device and Documentation Support
    1. 8.1 Documentation Support
      1. 8.1.1 Related Documentation
      2. 8.1.2 Device Nomenclature
    2. 8.2 Device Support
      1. 8.2.1 Development Support
        1. 8.2.1.1 Evaluation Modules
        2. 8.2.1.2 Spice Models
    3. 8.3 Receiving Notification of Documentation Updates
    4. 8.4 Support Resources
    5. 8.5 Trademarks
    6. 8.6 Electrostatic Discharge Caution
    7. 8.7 Glossary
  10. 9 Revision History
  11. 10Mechanical, Packaging, and Orderable Information

7.2.1.2 Detailed Design Procedure

The first step for this design is to examine the maximum load current along with the input and output voltage requirements, to determine if the device thermal and dropout voltage requirements can be met. At 3A, the input dropout voltage of the TPS74401 family is a maximum of 240mV over temperature. As a result, the dropout headroom is sufficient for operation over both input and output voltage accuracy.

The maximum power dissipated in the linear regulator is the maximum voltage dropped across the pass element from the input to the output multiplied by the maximum load current. In this example, the maximum voltage drop across in the pass element is (1.8V – 1.5V), giving a VDROP = 300mV. The power dissipated can than be estimated by the equation PDISS = IL(max) × VDROP = approximately 600mW. This calculation gives an efficiency of nearly 83.3% by using Equation 3.

When the power dissipated in the linear regulator is known, the corresponding junction temperature increase can be calculated. To estimate the junction temperature increase above ambient, the power dissipated must be multiplied by the junction-to-ambient thermal resistance. For thermal resistance information, see the Thermal Information table. For this example, using the KTW package, the junction temperature rise is calculated to be 21.2°C. The maximum junction temperature increase is calculated by adding the junction temperature rise to the maximum ambient temperature. In this example, the maximum junction temperature is 46.2°C. Keep in mind that the junction temperature must be less than 125°C for reliable operation. Additional ground planes, added thermal vias, and air flow all help to improve the thermal transfer characteristics of the system.

The next step is to determine the bias voltage or if a separate source is needed for the bias voltage. Because VIN is less than VOUT plus the VBIAS dropout, VBIAS must be an independent supply. VBIAS = VOUT + 1.62V = 3.12V; the system has a 3.3V rail to use for this supply and also to provide some limited headroom for VBIAS. The 5V rail is a better choice to improve the performance of the LDO, so the 5V rail is used.