SNVSC42A September   2023  – July 2024 LMQ64480-Q1 , LMQ644A0-Q1 , LMQ644A2-Q1

PRODUCTION DATA  

  1.   1
  2. 1 Features
  3. 2 Applications
  4. 3 Description
  5. 4 Device Comparison Table
  6. 5 Pin Configuration and Functions
    1. 5.1 Wettable Flanks
  7. 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
  8. 7 Detailed Description
    1. 7.1 Overview
    2. 7.2 Functional Block Diagram
    3. 7.3 Feature Description
      1. 7.3.1  Input Voltage Range (VIN)
      2. 7.3.2  Enable EN Pin and Use as VIN UVLO
      3. 7.3.3  Output Voltage Selection and Soft Start
      4. 7.3.4  SYNC Allows Clock Synchronization and Mode Selection
      5. 7.3.5  Clock Locking
      6. 7.3.6  Adjustable Switching Frequency
      7. 7.3.7  Power-Good Output Voltage Monitoring
      8. 7.3.8  Internal LDO, VCC UVLO, and BIAS Input
      9. 7.3.9  Bootstrap Voltage and VCBOOT-UVLO (CB1 and CB2 Pin)
      10. 7.3.10 CONFIG Device Configuration Pin
      11. 7.3.11 Spread Spectrum
      12. 7.3.12 Soft Start and Recovery From Dropout
      13. 7.3.13 Overcurrent and Short-Circuit Protection
      14. 7.3.14 Hiccup
      15. 7.3.15 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 Peak Current Mode Operation
        2. 7.4.3.2 Auto Mode Operation
          1. 7.4.3.2.1 Diode Emulation
        3. 7.4.3.3 FPWM Mode Operation
        4. 7.4.3.4 Minimum On-time (High Input Voltage) Operation
        5. 7.4.3.5 Dropout
        6. 7.4.3.6 Recovery from Dropout
        7. 7.4.3.7 Other Fault Modes
  9. 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  Choosing the Switching Frequency
        2. 8.2.2.2  Setting the Output Voltage
        3. 8.2.2.3  Inductor Selection
        4. 8.2.2.4  Output Capacitor Selection
        5. 8.2.2.5  Input Capacitor Selection
        6. 8.2.2.6  BOOT Capacitor
        7. 8.2.2.7  VCC
        8. 8.2.2.8  CFF and RFF Selection
        9. 8.2.2.9  SYNCHRONIZATION AND MODE
        10. 8.2.2.10 External UVLO
        11. 8.2.2.11 Typical Thermal Performance
      3. 8.2.3 Application Curves
    3. 8.3 Power Supply Recommendations
    4. 8.4 Layout
      1. 8.4.1 Layout Guidelines
        1. 8.4.1.1 Ground and Thermal Considerations
      2. 8.4.2 Layout Example
  10. 9 Device and Documentation Support
    1. 9.1 Device Support
      1. 9.1.1 Third-Party Products Disclaimer
    2. 9.2 Receiving Notification of Documentation Updates
    3. 9.3 Support Resources
    4. 9.4 Trademarks
    5. 9.5 Electrostatic Discharge Caution
    6. 9.6 Glossary
  11. 10Revision History
  12. 11Mechanical, Packaging, and Orderable Information

8.2.2.3 Inductor Selection

The main parameters for selecting the inductor are the inductance and saturation current. The inductance is based on the desired peak-to-peak ripple current. The inductance is normally chosen to be in the range of 20% to 50% of the maximum output current. Experience shows that a good value for inductor ripple current is 30% of the maximum load current for systems with a fixed input voltage. This example uses VIN = 13.5 V, which is closer to the nominal voltage of a 12-V car battery. When selecting the ripple current for applications with much smaller maximum load than the maximum available from the device, the maximum device current must be used for this calculation. Equation 5 can be used to determine the value of the inductance. The constant K is the percentage of peak-to-peak inductor current ripple to rated output current. For this 6-A, 2100-kHz, 3.3-V example, K = 0.25 is chosen and the closest standard value of 1 μH was selected.

Equation 5. L=VOUTVIN×VIN - VOUT fSW×K ×IOUT_MAX

Ideally, the saturation current rating of the inductor must be at least as large as the high-side switch current limit, IHS. This rating makes sure that the inductor does not saturate, even during a soft-short condition on the output. A hard short causes the LMQ644xx to enter hiccup mode (see Section 7.3.14). A soft short can hold the output current near the current limit without triggering hiccup. When the inductor core material saturates, the inductance can fall to a very low value, causing the inductor current to rise very rapidly. Although the valley current limit, ILS, is designed to reduce the risk of current runaway, a saturated inductor can cause the current to rise to high values very rapidly. This event can lead to component damage, so the inductor not saturating is crucial. Inductors with a ferrite core material have very hard saturation characteristics, but usually have lower core losses than powdered iron cores. Powered iron cores exhibit a soft saturation, allowing some relaxation in the saturation current rating of the inductor. However, they have more core losses at frequencies typically above 1 MHz. To avoid subharmonic oscillation, the inductance value must not be less than that given in Equation 6. The maximum inductance is limited by the minimum current ripple required for the current mode control to perform correctly. As a rule-of-thumb, the minimum inductor ripple current must be no less than about 10% of the device maximum rated current under nominal conditions.

Equation 6. L>VOUT fSW×IRATED