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

8.3.8 Input Supply Current

The LMS3655 is designed to have very low input supply current when regulating light loads. This is achieved by powering much of the internal circuitry from the output. The BIAS pin is the input to the LDO that powers the majority of the control circuits. By connecting the BIAS input to the output of the regulator, this current acts as a small load on the output. This current is reduced by the ratio of VOUT / VIN, just like any other load.

IQ_VIN is defined as the current consumed by a converter using a LMS3655 device while regulating without a load. To calculate the theoretical total quiescent current, the below equation can be used with parameters from the Electrical Characteristics and System Characteristics tables. While operating without a load, the LMS3655 only powers itself. The device draws power from three sources: the VIN pin (IQ), the EN pin (IEN), and the BIAS pin (IB). Because the BIAS input is connected to the output of the circuit, the power consumed is converted from input power with an effective efficiency, ηeff. Here, effective efficiency is the added input power needed when lightly loading the converter of the LMS3655 device and is divided by the corresponding additional load. This allows unloaded current to be calculated in Equation 2:

Equation 2. LMS3655 eq01_snas660.gif

where

  • IQ_VIN is the current consumed by the operating (switching) buck converter while unloaded.
  • IQ is the current drawn by the LMS3655 from its VIN terminal. See IQ in Electrical Characteristics.
  • IEN is current drawn by the LMS3655 from its EN terminal. Include this current if EN is connected to VIN. See IEN in Electrical Characteristics. Note that this current drops to a very low value if connected to a voltage less than 5 V.
  • IB is bias current drawn by the unloaded LMS3655. See IB in System Characteristics.
  • Idiv is the current drawn by the feedback voltage divider used to set output voltage.
  • ηeff is the light load efficiency of the Buck converter with IQ_VIN removed from the input current of the buck converter.

NOTE

The EN pin consumes a few micro-amperes when tied to high; see IEN. Add IEN to IQ as shown in Equation 2 if EN is tied to VIN. If EN is tied to a voltage less than 5 V, virtually no current is consumed allowing EN to be used as an UVLO pin once a voltage divider is added.