SNVSA85C October   2015  – January 2017 LM27761

PRODUCTION DATA.  

  1. 1 Features
  2. 2 Applications
  3. 3 Description
    1.     Device Images
      1.      Typical Application
  4. 4 Revision History
  5. 5 Pin Configuration and Functions
    1.     Pin 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 Diagram
    3. 7.3 Feature Description
      1. 7.3.1 Undervoltage Lockout
      2. 7.3.2 Input Current Limit
      3. 7.3.3 PFM Operation
      4. 7.3.4 Output Discharge
      5. 7.3.5 Thermal Shutdown
    4. 7.4 Device Functional Modes
      1. 7.4.1 Shutdown Mode
      2. 7.4.2 Enable Mode
  8. 8 Application and Implementation
    1. 8.1 Application Information
    2. 8.2 Typical Application - Regulated Voltage Inverter
      1. 8.2.1 Design Requirements
      2. 8.2.2 Detailed Design Procedure
        1. 8.2.2.1 Custom Design With WEBENCH® Tools
        2. 8.2.2.2 Charge-Pump Voltage Inverter
        3. 8.2.2.3 Negative Low-Dropout Linear Regulator
        4. 8.2.2.4 Power Dissipation
        5. 8.2.2.5 Output Voltage Setting
        6. 8.2.2.6 External Capacitor Selection
          1. 8.2.2.6.1 Charge-Pump Output Capacitor
          2. 8.2.2.6.2 Input Capacitor
          3. 8.2.2.6.3 Flying Capacitor
          4. 8.2.2.6.4 LDO Output Capacitor
      3. 8.2.3 Application Curves
  9. 9 Power Supply Recommendations
  10. 10Layout
    1. 10.1 Layout Guidelines
    2. 10.2 Layout Example
  11. 11Device and Documentation Support
    1. 11.1 Device Support
      1. 11.1.1 Development Support
        1. 11.1.1.1 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

8.2.2.4 Power Dissipation

The allowed power dissipation for any package is a measure of the ability of the device to pass heat from the junctions of the device to the heatsink and the ambient environment. Thus, the power dissipation is dependent on the ambient temperature and the thermal resistance across the various interfaces between the die junction and ambient air.

The maximum allowable power dissipation can be calculated by Equation 2:

Equation 2. PD-MAX = (TJ-MAX – TA) / RθJA

The actual power being dissipated in the device can be represented by Equation 3:

Equation 3. PD = PIN – POUT = [VIN × (–IOUT + IQ) – (VOUT × IOUT)]

Equation 2 and Equation 3 establish the relationship between the maximum power dissipation allowed due to thermal consideration, the voltage drop across the device, and the continuous current capability of the device. These equations must be used to determine the optimum operating conditions for the device in a given application.

In lower power dissipation applications the maximum ambient temperature (TA-MAX) may be increased. In higher power dissipation applications the maximum ambient temperature(TA-MAX) may have to be derated. TA-MAX can be calculated using Equation 4:

Equation 4. TA-MAX = TJ-MAX-OP – (RθJA × PD-MAX)

where

  • TJ-MAX-OP = maximum operating junction temperature (125°C)
  • PD-MAX = the maximum allowable power dissipation
  • RθJA = junction-to-ambient thermal resistance of the package

Alternately, if TA-MAX cannot be derated, the power dissipation value must be reduced. This can be accomplished by reducing the input voltage as long as the minimum VIN is not violated, or by reducing the output current, or some combination of the two.