SNAS714C November   2016  – August 2021 LMS3635-Q1 , LMS3655-Q1


  1. Features
  2. Applications
  3. Description
  4. Revision History
  5. Device Comparison Tables
  6. Pin Configuration and Functions
  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. 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. Application and Implementation
    1. 9.1 Application Information
    2. 9.2 Typical Applications
      1. 9.2.1 General Application
        1. Design Requirements
        2. Detailed Design Procedure
          1. Custom Design With WEBENCH® Tools
          2. External Components Selection
            1. Input Capacitors
            2. Output Inductors and Capacitors
              1. Inductor Selection
              2. Output Capacitor Selection
          3. Setting the Output Voltage
          4. FB for Adjustable Output
          5. VCC
          6. BIAS
          7. CBOOT
          8. Maximum Ambient Temperature
        3. Application Curves
      2. 9.2.2 Fixed 5-V Output for USB-Type Applications
        1. Design Requirements
        2. Detailed Design Procedure
        3. Application Curves
      3. 9.2.3 Fixed 3.3-V Output
        1. Design Requirements
        2. Detailed Design Procedure
        3. Application Curves
      4. 9.2.4 6-V Adjustable Output
        1. Design Requirements
        2. Detailed Design Procedure
        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. 12.1.2 Development Support
        1. Custom Design With WEBENCH® Tools
    2. 12.2 Documentation Support
      1. 12.2.1 Related Documentation
    3. 12.3 Receiving Notification of Documentation Updates
    4. 12.4 Support 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

Detailed Design Procedure

  • BIAS is connected to the output. This example assumes that the load is connected to the output through long wires so a 3-Ω resistor is inserted to minimize risks of damage to the part during load shorts. In addition 0.1-µF capacitor is required close to the bias pin.
  • FB is connected directly to the output. BIAS and FB are connected to the output through separate traces. This is important to reduce noise and achieve good performances. See Section 11.1 for more details on the proper layout method.
  • SYNC is connected to ground through a pulldown resistor, and an external synchronization signal can be applied. The pulldown resistor ensures that the pin is not floating when the SYNC pin is not driven by any source.
  • 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 VIN. This causes the device to operate in FPWM mode. In this mode, the device remains in CCM operation regardless of the output current and is ensured to be within the boundaries set by FSW. To prevent frequency foldback behavior at low duty cycles, provide a 200-mA load. The drawback is that the efficiency is not optimized for light loads. See Section 8.4 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 the output in this example. A pullup resistor is necessary. A 100-kΩ 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 Section 7.1 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 Section 7.6 for details on the RESET pin.
  • Input capacitor selection is detailed in Section It is important to connect small high-frequency capacitors CIN_HF1 and CIN_HF2 as close to both inputs PVIN1 and PVIN2 as possible.
  • Output capacitor selection is detailed in Section
  • Inductor selection is detailed in Section In general, a 10-µH inductor is recommended for the fixed output options. For the adjustable output configurations, the inductance can vary with the output voltage due to ripple and current limit requirements.