SNVS872D August   2012  – August 2018 LMZ21700

PRODUCTION DATA.  

  1. 1 Features
  2. 2 Applications
  3. 3 Description
    1.     Device Images
      1.      Simplified Schematic
      2.      Efficiency for VIN = 12 V
  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
      1. 7.2.1 Package Construction
    3. 7.3 Feature Description
      1. 7.3.1 Input Undervoltage Lockout
      2. 7.3.2 Enable Input (EN)
      3. 7.3.3 Soft-Start and Tracking Function (SS)
      4. 7.3.4 Power Good Function (PG)
      5. 7.3.5 Output Voltage Setting
      6. 7.3.6 Output Current Limit and Output Short Circuit Protection
      7. 7.3.7 Thermal Protection
    4. 7.4 Device Functional Modes
      1. 7.4.1 PWM Mode Operation
      2. 7.4.2 PSM Operation
  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 Input Capacitor (CIN)
        3. 8.2.2.3 Output Capacitor (COUT)
        4. 8.2.2.4 Softstart Capacitor (CSS)
        5. 8.2.2.5 Power Good Resistor (RPG)
        6. 8.2.2.6 Feedback Resistors (RFBB and RFBT)
      3. 8.2.3 Application Curves
        1. 8.2.3.1 VOUT = 1.2 V
        2. 8.2.3.2 VOUT = 1.8 V
        3. 8.2.3.3 VOUT = 2.5 V
        4. 8.2.3.4 VOUT = 3.3 V
        5. 8.2.3.5 VOUT = 5.0 V
    3. 8.3 Do's and Don'ts
  9. 9 Power Supply Recommendations
    1. 9.1 Voltage Range
    2. 9.2 Current Capability
    3. 9.3 Input Connection
      1. 9.3.1 Voltage Drops
      2. 9.3.2 Stability
  10. 10Layout
    1. 10.1 Layout Guidelines
      1. 10.1.1 Minimize the High di/dt Loop Area
      2. 10.1.2 Protect the Sensitive Nodes in the Circuit
      3. 10.1.3 Provide Thermal Path and Shielding
    2. 10.2 Layout Example
      1. 10.2.1 High Density Layout Example for Space Constrained Applications
        1. 10.2.1.1 35-mm² Solution Size (Single Sided)
  11. 11Device and Documentation Support
    1. 11.1 Device Support
    2. 11.2 Development Support
      1. 11.2.1 Custom Design With WEBENCH® Tools
    3. 11.3 Receiving Notification of Documentation Updates
    4. 11.4 Community Resources
    5. 11.5 Trademarks
    6. 11.6 Electrostatic Discharge Caution
    7. 11.7 Glossary
  12. 12Mechanical, Packaging, and Orderable Information
    1. 12.1 Tape and Reel Information

Package Options

Refer to the PDF data sheet for device specific package drawings

Mechanical Data (Package|Pins)
  • SIL|8
Thermal pad, mechanical data (Package|Pins)
Orderable Information

10.1.3 Provide Thermal Path and Shielding

Using the available layers in the PCB can help provide additional shielding and improved thermal performance. Large unbroken GND copper areas provide good thermal and return current paths. Flood unused PCB area with GND copper. Use thermal vias to connect the GND copper between layers.

The required board area for proper thermal dissipation can be estimated using the power dissipation curves for the desired output voltage and the package thermal resistance vs. board area curve. Refer to the power dissipation graphs in the Typical Characteristics section. Using the power dissipation (PDISS) for the designed input and output voltage and the max operating ambient temperature TA for the application, estimate the required thermal resistance RθJA with the following expression.

Equation 9. RθJA - REQUIRED≤ (125ºC – TA) / PDISS

Then use Figure 80 to estimate the board copper area required to achieve the calculated thermal resistance.

LMZ21700 D012_LMZ21700_PACKAGE_THERMAL_VS_PCB_SNVS872.gifFigure 80. Package Thermal Resistance vs. Board Copper Area, No Air Flow

For example, for a design with 17 V input and 5 V output and 0.65 A load the power dissipation according to Figure 7 is 0.43 W.

For 85°C ambient temperature, the RθJA-REQUIRED is ≤ (125°C - 85°C) / 0.43 W, or ≤ 93°C/W. Looking at Figure 80 the minimum copper area required to achieve this thermal resistance with a 4-layer board and 70 µm (2 oz) copper is approximately 1 cm².