JAJSL23B January   2021  – January 2022 TPS785-Q1

PRODUCTION DATA  

  1. 1 特長
  2. 2 アプリケーション
  3. 3 概要
  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 Diagrams
    3. 7.3 Feature Description
      1. 7.3.1 Foldback Current Limit
      2. 7.3.2 Output Enable
      3. 7.3.3 Active Discharge
      4. 7.3.4 Undervoltage Lockout (UVLO) Operation
      5. 7.3.5 Dropout Voltage
      6. 7.3.6 Thermal Shutdown
    4. 7.4 Device Functional Modes
      1. 7.4.1 Device Functional Mode Comparison
      2. 7.4.2 Normal Operation
      3. 7.4.3 Dropout Operation
      4. 7.4.4 Disabled
  8. 8 Application and Implementation
    1. 8.1 Application Information
      1. 8.1.1 Recommended Capacitor Types
      2. 8.1.2 Input and Output Capacitor Requirements
      3. 8.1.3 Adjustable Device Feedback Resistors
      4. 8.1.4 Load Transient Response
      5. 8.1.5 Exiting Dropout
      6. 8.1.6 Dropout Voltage
      7. 8.1.7 Reverse Current
      8. 8.1.8 Feed-Forward Capacitor (CFF)
      9. 8.1.9 Power Dissipation (PD)
        1. 8.1.9.1 Estimating Junction Temperature
        2. 8.1.9.2 Recommended Area for Continuous Operation
        3. 8.1.9.3 Power Dissipation versus Ambient Temperature
    2. 8.2 Typical Application
      1. 8.2.1 Design Requirements
      2. 8.2.2 Detailed Design Procedure
      3. 8.2.3 Application Curve
  9. 9 Power Supply Recommendations
  10. 10Layout
    1. 10.1 Layout Guidelines
      1. 10.1.1 Additional Layout Considerations
    2. 10.2 Layout Examples
  11. 11Device and Documentation Support
    1. 11.1 Device Support
      1. 11.1.1 Device Nomenclature
    2. 11.2 Documentation Support
      1. 11.2.1 Related Documentation
    3. 11.3 Receiving Notification of Documentation Updates
    4. 11.4 サポート・リソース
    5. 11.5 Trademarks
    6. 11.6 Electrostatic Discharge Caution
    7. 11.7 Glossary
  12. 12Mechanical, Packaging, and Orderable Information

8.1.9 Power Dissipation (PD)

Circuit reliability demands that proper consideration be given to device power dissipation, location of the circuit on the printed circuit board (PCB), and correct sizing of the thermal plane. The PCB area around the regulator must be as free as possible of other heat-generating devices that cause added thermal stresses.

As a first-order approximation, power dissipation in the regulator depends on the input-to-output voltage difference and load conditions. Use Equation 4 to approximate PD:

Equation 4. PD = (VIN – VOUT) × IOUT

Power dissipation can be minimized, and thus greater efficiency achieved, by proper selection of the system voltage rails. Proper selection allows the minimum input-to-output voltage differential to be obtained. The low dropout of the TPS785-Q1 allows for maximum efficiency across a wide range of output voltages.

The main heat conduction path for the device is through the thermal pad on the package. As such, the thermal pad must be soldered to a copper pad area under the device. This pad area contains an array of plated vias that conduct heat to any inner plane areas or to a bottom-side copper plane.

The maximum power dissipation determines the maximum allowable junction temperature (TJ) for the device. According to Equation 5, power dissipation and junction temperature are most often related by the junction-to-ambient thermal resistance (RθJA) of the combined PCB and device package and the temperature of the ambient air (TA). Equation 6 rearranges Equation 5 for output current.

Equation 5. TJ = TA + (RθJA × PD)
Equation 6. IOUT = (TJ – TA) / [RθJA × (VIN – VOUT)]

Unfortunately, this thermal resistance (RθJA) is highly dependent on the heat-spreading capability built into the particular PCB design, and therefore varies according to the total copper area, copper weight, and location of the planes. The RθJA recorded in the Recommended Operating Conditions table is determined by the JEDEC standard, PCB, and copper-spreading area, and is only used as a relative measure of package thermal performance. For a well-designed thermal layout, RθJA is actually the sum of the VSON package junction-to-case (bottom) thermal resistance (RθJC(bot)) plus the thermal resistance contribution by the PCB copper.