SNVSAH5A September   2017  – May 2020 LM73605 , LM73606

PRODUCTION DATA.  

  1. 1 Features
  2. 2 Applications
  3. 3 Description
    1.     Device Images
      1.      Simplified Schematic
      2.      Efficiency versus Load Current VOUT = 5 V, fSW = 500 kHz, Auto Mode
  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 Timing Characteristics
    7. 6.7 Switching Characteristics
    8. 6.8 System Characteristics
    9. 6.9 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  Synchronous Step-Down Regulator
      2. 7.3.2  Auto Mode and FPWM Mode
      3. 7.3.3  Fixed-Frequency Peak Current-Mode Control
      4. 7.3.4  Adjustable Output Voltage
      5. 7.3.5  Enable and UVLO
      6. 7.3.6  Internal LDO, VCC_UVLO, and BIAS Input
      7. 7.3.7  Soft Start and Voltage Tracking
      8. 7.3.8  Adjustable Switching Frequency
      9. 7.3.9  Frequency Synchronization and Mode Setting
      10. 7.3.10 Internal Compensation and CFF
      11. 7.3.11 Bootstrap Capacitor and VBOOT-UVLO
      12. 7.3.12 Power-Good and Overvoltage Protection
      13. 7.3.13 Overcurrent and Short-Circuit Protection
      14. 7.3.14 Thermal Shutdown
    4. 7.4 Device Functional Modes
      1. 7.4.1 Shutdown Mode
      2. 7.4.2 Standby Mode
      3. 7.4.3 Active Mode
        1. 7.4.3.1 CCM Mode
        2. 7.4.3.2 DCM Mode
        3. 7.4.3.3 PFM Mode
        4. 7.4.3.4 Fault Protection Mode
  8. 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  Custom Design With WEBENCH® Tools
        2. 8.2.2.2  Output Voltage Setpoint
        3. 8.2.2.3  Switching Frequency
        4. 8.2.2.4  Input Capacitors
        5. 8.2.2.5  Inductor Selection
        6. 8.2.2.6  Output Capacitor Selection
        7. 8.2.2.7  Feedforward Capacitor
        8. 8.2.2.8  Bootstrap Capacitors
        9. 8.2.2.9  VCC Capacitor
        10. 8.2.2.10 BIAS
        11. 8.2.2.11 Soft Start
        12. 8.2.2.12 Undervoltage Lockout Setpoint
        13. 8.2.2.13 PGOOD
      3. 8.2.3 Application Curves
  9. 9 Power Supply Recommendations
  10. 10Layout
    1. 10.1 Layout Guidelines
      1. 10.1.1 Layout For EMI Reduction
      2. 10.1.2 Ground Plane
      3. 10.1.3 Optimize Thermal Performance
    2. 10.2 Layout Example
  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 Documentation Support
      1. 11.2.1 Related Documentation
    3. 11.3 Related Links
    4. 11.4 Receiving Notification of Documentation Updates
    5. 11.5 Support Resources
    6. 11.6 Trademarks
    7. 11.7 Electrostatic Discharge Caution
    8. 11.8 Glossary
  12. 12Mechanical, Packaging, and Orderable Information

Package Options

Mechanical Data (Package|Pins)
Thermal pad, mechanical data (Package|Pins)
Orderable Information

10.1.3 Optimize Thermal Performance

The key to thermal optimization on PCB design is to provide heat transferring paths from the device to the outer large copper area. Use thick copper (2 oz) on high current layer or layers if possible. Use thermal vias under the DAP to transfer heat to other layers. Connect NC pins to the GND net, so that GND copper can run underneath the device to create dog-bone shaped heat sink. Try to leave copper solid on IC side as much as possible above and below the device. Place components and route traces away from major heat transferring paths if possible, to avoid blocking heat dissipation path. Try to leave copper solid, free of components and traces, around the thermal vias on the other side of the board as well. Solid copper behaves as heat sink to spread the heat to a larger area and provide more contact area to the air.

When calculating power dissipation, use the maximum input voltage and the average output current for the application. Many common operating conditions are provided in the Application Curves. Less common applications can be derived through interpolation. In all designs, the junction temperature must be kept below the rated maximum of 125°C.

The thermal characteristics of the LM73605 and LM73606 are specified using the parameter RθJA, which characterize thermal resistance from the junction of the silicon to the ambient in a specific system. Although the value of RθJA 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, you can use Equation 30:

Equation 30. TJ = PIC_LOSS × RθJA + TA

where

  • TJ = Junction temperature in °C
  • PIC_LOSS = VIN × IIN × (1 − efficiency) − 1.1 × IOUT × DCR
  • DCR = Inductor DC parasitic resistance in Ω
  • RθJA = Junction-to-ambient thermal resistance of the device in °C/W
  • TA = Ambient temperature in °C.

The maximum operating junction temperature of the LM73605 and LM73606 is 125°C. RθJA is highly related to PCB size and layout, as well as environmental factors such as heat sinking and air flow. Figure 76 shows measured results of RθJA with different copper area on 2-layer boards and 4-layer boards, with 1-W and 2-W power dissipation on the LM73605 and LM73606.

LM73605 LM73606 Theta-JA-vs-CopperArea.gifFigure 76. Measured RθJA versus PCB Copper Area on 2-Layer Boards and 4-Layer Boards