SNVSCH2 September   2024 LM65645-Q1

ADVANCE INFORMATION  

  1.   1
  2. 1 Features
  3. 2 Applications
  4. 3 Description
  5. 4 Device Comparison Table
  6. 5 Pin Configuration and Functions
  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 System Characteristics
  8. 7 Detailed Description
    1. 7.1 Overview
    2. 7.2 Functional Block Diagram
    3. 7.3 Feature Descriptions
      1. 7.3.1 Output Voltage Selection
      2. 7.3.2 EN Pin and Use as VIN UVLO
      3. 7.3.3 Mode Selection
        1. 7.3.3.1 MODE/SYNC Pin Uses for Synchronization
        2. 7.3.3.2 Clock Locking
      4. 7.3.4 Adjustable Switching Frequency
      5. 7.3.5 Dual Random Spread Spectrum (DRSS)
      6. 7.3.6 Internal LDO, VCC UVLO, and BIAS Input
      7. 7.3.7 Bootstrap Voltage (BST Pin)
      8. 7.3.8 Soft Start and Recovery From Dropout
      9. 7.3.9 Safety Features
        1. 7.3.9.1 Power-Good Monitor
        2. 7.3.9.2 Overcurrent and Short-Circuit Protection
        3. 7.3.9.3 Hiccup
        4. 7.3.9.4 Thermal Shutdown
    4. 7.4 Device Functional Modes
      1. 7.4.1 Shutdown Mode
      2. 7.4.2 Active Mode
        1. 7.4.2.1 Peak Current Mode Operation
        2. 7.4.2.2 Auto Mode Operation
          1. 7.4.2.2.1 Diode Emulation
        3. 7.4.2.3 FPWM Mode Operation
  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 Custom Design With WEBENCH® Tools
        2. 8.2.2.2 Choosing the Switching Frequency
        3. 8.2.2.3 FB for Adjustable or Fixed Output Voltage Mode
        4. 8.2.2.4 Inductor Selection
        5. 8.2.2.5 Output Capacitor Selection
        6. 8.2.2.6 Input Capacitor Selection
        7. 8.2.2.7 CBOOT
        8. 8.2.2.8 External UVLO
        9. 8.2.2.9 Maximum Ambient Temperature
    3. 8.3 Best Design Practices
    4. 8.4 Power Supply Recommendations
    5. 8.5 Layout
      1. 8.5.1 Layout Guidelines
        1. 8.5.1.1 Ground and Thermal Considerations
      2. 8.5.2 Layout Example
  10. 9 Device and Documentation Support
    1. 9.1 Device Support
      1. 9.1.1 Third-Party Products Disclaimer
      2. 9.1.2 Development Support
        1. 9.1.2.1 Custom Design With WEBENCH® Tools
    2. 9.2 Documentation Support
      1. 9.2.1 Related Documentation
    3. 9.3 Receiving Notification of Documentation Updates
    4. 9.4 Support Resources
    5. 9.5 Trademarks
    6. 9.6 Electrostatic Discharge Caution
    7. 9.7 Glossary
  11. 10Revision History
  12. 11Mechanical, Packaging, and Orderable Information
    1. 11.1 Tape and Reel Information

Package Options

Mechanical Data (Package|Pins)
  • RZT|20
Thermal pad, mechanical data (Package|Pins)
Orderable Information

8.2.2.4 Inductor Selection

The parameters for selecting the inductor are the inductance and saturation current. The inductance is based on the desired peak-to-peak ripple current and is normally chosen to be in the range of 20% to 40% of the maximum output current rating of the device. Experience shows that the best value for inductor ripple current is 30% of the maximum output current rating. Larger values of ripple current can restrict the maximum output current, before current limit is reached. Smaller values of ripple current reduce the SNR of the current mode controller and can lead to increased jitter in the duty cycle. Both the inductor and switching frequency tolerance have an impact on the selection of ripple current, and, therefore, inductor value. Use the maximum device current rating when calculating the ripple current for applications with much smaller maximum load than the maximum available from the device. The ratio of inductor ripple current over maximum output current is designated as K. Equation 7 is used to determine the value of inductance

Equation 7. L= VIN-VOUT×VOUTVIN×K×IOUT-rated×FSW

The typical input voltage for the application is usually used in Equation 7. However, if the application requires and very wide range of input voltages, then some voltage near the upper end of the range can be used. In any case, after the inductor has been selected, the ripple current must be checked at the maximum input voltage. Too large a ripple current can limit the maximum output current, as mentioned above. Use Equation 8 to check for these concerns.

Equation 8. IOUT-max≅IHS-LIM-12×VIN-VOUT×VOUTVIN×L×FSW

Ideally, the saturation current rating of the inductor is at least as large as the high-side switch current limit, IHS-LIM. This size makes sure that the inductor does not saturate even during a short circuit on the output. When the inductor core material saturates, the inductance falls to a very low value, causing the inductor current to rise very rapidly. Although the valley current limit is designed to reduce the risk of current run-away, a saturated inductor can cause the current to rise to high values very rapidly. This rise can lead to component damage. 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 current rating of the inductor. However, powered iron cores have more core losses at frequencies above about 1MHz In any case, the inductor saturation current must not be less than the maximum peak inductor current at full load.

To avoid subharmonic oscillation, the inductance value must not be less than that given in Equation 9. This limit applies to applications where the switch duty cycle becomes greater than or equal to 50%, under any operating condition.

Equation 9. Lmin≥M×VOUTFSW

where

  • M = 0.47 for the 2.5A device
  • M = 0.35 for the 3.5A device
  • M = 0.29 for the 4.5A device

The maximum inductance is limited by the minimum current ripple required for the current mode control to perform correctly. As a rule, the minimum inductor ripple current must be no less than about 10% of the device maximum rated current under nominal conditions.

For this example, assuming a 24V input, Equation 7 gives a value of 1.7μH. Use the next standard value of 2.2uH. Alternatively,Table 8-1 can be used to select the inductor value for a typical input voltage of 12V.