SNAS744B July   2017  – March 2018 LMS3655

PRODUCTION DATA.  

  1. 1 Features
  2. 2 Applications
  3. 3 Description
    1.     Device Images
      1.      Typical Application Circuit
      2.      LMS3655 Efficiency: VOUT = 5 V
  4. 4 Revision History
  5. 5 Device Comparison Table
  6. 6 Pin Configuration and Functions
    1.     Pin Functions
  7. 7 Specifications
    1. 7.1 Absolute Maximum Ratings
    2. 7.2 ESD Ratings
    3. 7.3 Recommended Operating Conditions
    4. 7.4 Thermal Information
    5. 7.5 Thermal Information (for Device Mounted on PCB)
    6. 7.6 Electrical Characteristics
    7. 7.7 System Characteristics
    8. 7.8 Timing Requirements
    9. 7.9 Typical Characteristics
  8. 8 Detailed Description
    1. 8.1 Overview
    2. 8.2 Functional Block Diagram
      1. 8.2.1 Control Scheme
    3. 8.3 Feature Description
      1. 8.3.1 RESET Flag Output
      2. 8.3.2 Enable and Start-Up
      3. 8.3.3 Soft-Start Function
      4. 8.3.4 Current Limit
      5. 8.3.5 Hiccup Mode
      6. 8.3.6 Synchronizing Input
      7. 8.3.7 Undervoltage Lockout (UVLO) and Thermal Shutdown (TSD)
      8. 8.3.8 Input Supply Current
    4. 8.4 Device Functional Modes
      1. 8.4.1 AUTO Mode
      2. 8.4.2 FPWM Mode
      3. 8.4.3 Dropout
      4. 8.4.4 Spread-Spectrum Operation
  9. 9 Application and Implementation
    1. 9.1 Application Information
    2. 9.2 Typical Applications
      1. 9.2.1 General Application
        1. 9.2.1.1 Design Requirements
        2. 9.2.1.2 Detailed Design Procedure
          1. 9.2.1.2.1 External Components Selection
            1. 9.2.1.2.1.1 Input Capacitors
            2. 9.2.1.2.1.2 Output Inductors and Capacitors
              1. 9.2.1.2.1.2.1 Inductor Selection
              2. 9.2.1.2.1.2.2 Output Capacitor Selection
          2. 9.2.1.2.2 FB for Adjustable Output
          3. 9.2.1.2.3 VCC
          4. 9.2.1.2.4 BIAS
          5. 9.2.1.2.5 CBOOT
          6. 9.2.1.2.6 Maximum Ambient Temperature
        3. 9.2.1.3 Application Curves
      2. 9.2.2 Adjustable 5-V Output
        1. 9.2.2.1 Design Requirements
        2. 9.2.2.2 Detailed Design Procedure
        3. 9.2.2.3 Application Curves
      3. 9.2.3 Adjustable 3.3-V Output
        1. 9.2.3.1 Design Requirements
        2. 9.2.3.2 Detailed Design Procedure
        3. 9.2.3.3 Application Curves
      4. 9.2.4 6-V Adjustable Output
        1. 9.2.4.1 Design Requirements
        2. 9.2.4.2 Detailed Design Procedure
        3. 9.2.4.3 Application Curves
    3. 9.3 Do's and Don't's
  10. 10Power Supply Recommendations
  11. 11Layout
    1. 11.1 Layout Guidelines
    2. 11.2 Layout Example
  12. 12Device and Documentation Support
    1. 12.1 Device Support
      1. 12.1.1 Third-Party Products Disclaimer
    2. 12.2 Documentation Support
      1. 12.2.1 Related Documentation
    3. 12.3 Receiving Notification of Documentation Updates
    4. 12.4 Community Resources
    5. 12.5 Trademarks
    6. 12.6 Electrostatic Discharge Caution
    7. 12.7 Glossary
  13. 13Mechanical, Packaging, and Orderable Information

Package Options

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

9.2.3.2 Detailed Design Procedure

  • BIAS is connected to the output. This example assumes that the load is close to the output so no bias resistance is necessary. A 0.1-µF capacitor is still recommended close to the bias pin.
  • FB is connected to the output through a voltage divider in order to create a voltage of 1 V at the FB pin when the output is at 3.3 V. A 22-pF capacitance is added in parallel with the top feedback resistor in order to improve transient behavior. BIAS and FB are connected to the output through separate traces. This is important to reduce noise and achieve good performance. See Layout Guidelines for more details on the proper layout method.
  • SYNC is connected to ground directly as there is no need for this function in this application.
  • EN is connected to VIN so the device operates as soon as the input voltage rises above the VIN-OPERATE threshold.
  • FPWM is connected to GND. This causes the device to operate in AUTO mode. In this mode, the switching frequency is adjusted at light loads to optimize efficiency. As a result the switching frequency changes with the output current until medium load is reached. The part will then switch at the frequency defined by FSW. See Device Functional Modes for more details.
  • A 4.7-µF capacitor is connected between VCC and GND close to the VCC pin. This ensures stable operation of the internal LDO.
  • RESET is biased to an external rail in this example. A pullup resistor is necessary. A 100-kΩ pullup resistor is selected for this application and is generally sufficient. The value can be selected to match the needs of the application but must not lead to excessive current into the RESET pin when RESET is in a low state. Consult Absolute Maximum Ratings for the maximum current allowed. In addition, a low pullup resistor could lead to an incorrect logic level due to the value of RRESET. Consult Electrical Characteristics for details on the RESET pin.
  • It is important to connect small high frequency capacitors CIN_HF1 and CIN_HF2 as close to both inputs PVIN1 and PVIN2 as possible. For the detailed process of choosing input capacitors, refer to Input Capacitors.
  • Output capacitor selection is detailed in Output Capacitor Selection.
  • Inductor selection is detailed in Inductor Selection. In general, a 10-µH inductor is recommended for the nominal adjustable output range of 3.3 V to 5 V. The inductance can vary with the output voltage due to ripple and current limit requirements.