SNVSAA7B December   2015  – July 2021 LM53625-Q1 , LM53635-Q1

PRODUCTION DATA  

  1. 1 Features
  2. 2 Applications
  3. 3 Description
  4. 4 Revision History
  5. 5 Device Comparison
  6. 6 Pin Configuration and 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 Electrical Characteristics
    6. 7.6 System Characteristics
    7. 7.7 Timing Characteristics
    8. 7.8 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 Input Voltage Frequency Foldback
    5. 8.5 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
              1. 9.2.1.2.1.1.1 Input Capacitor Selection
            2. 9.2.1.2.1.2 Output Inductors and Capacitors Selection
              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 Setting the Output Voltage
            1. 9.2.1.2.2.1 FB for Adjustable Versions
          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 Fixed 5-V Output for USB-Type Applications
        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 Fixed 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 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 What to Do and What Not to Do
  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 Support Resources
    5. 12.5 Trademarks
    6. 12.6 Electrostatic Discharge Caution
    7. 12.7 Glossary
  13. 13Mechanical, Packaging, and Orderable Information
9.2.1.2.1.2.1 Inductor Selection

The LM53625/35-Q1 is optimized for a nominal inductance of 2.2 μH for the 5-V and 3.3-V versions. This gives a ripple current that is approximately 20% to 30% of the full load current of 3.5 A. For output voltages greater than 5 V, a proportionally larger inductor can be used, thus keeping the ratio of inductor current slope to internal compensating slope constant.

The most important inductor parameters are saturation current and parasitic resistance. Inductors with a saturation current of between 7 A and 8 A are appropriate for most applications when using the LM53625/35-Q1. Of course, the inductor parasitic resistance must be as low as possible to reduce losses at heavy loads. Table 9-2 gives a list of several possible inductors that can be used with the LM53625/35-Q1.

The LM53625 and LM53635 devices run in current mode and with internal compensation. This compensation is stable with inductance between 1.5 µH and 10 µH. For most applications, use 2.2 µH with the fixed 5-V and 3.3-V versions of the LM53625 and LM53635 devices. Adjustable devices operate at the same frequency under high input-voltage conditions as devices set to deliver 3.3 V (see Figure 9-28). Inductor current ripple at high input voltages can become excessive when using a 2.2-µH inductor with an adjustable device that is delivering output voltage above 6 V. A 4.7-µH inductor might be necessary. Inductance that is too high is not recommended as it can result in poor load transient behavior and instability for extreme inductance choice. See Table 9-2 for typical recommended values.

The inductor must be rated to handle the peak load current plus the ripple current — take care when reviewing the different saturation current ratings specified by different manufacturers. Saturation current ratings are typically specified at 25°C, so ratings at maximum ambient temperature of the application should be requested from the manufacturer. For the LM53635, TI recommends a saturation current of 7.5 A or higher, and for the LM53625, a saturation current of 6.5 A or higher is recommended

Table 9-2 Recommended Inductors
MANUFACTURERPART NUMBERSATURATION CURRENTDC RESISTANCE
Würth74406500226 A15 mΩ
CoilcraftDO3316T-222MLB7.8 A11 mΩ
CoiltronicsMPI4040R3-2R2-R7.9 A48 mΩ
VishayIHLP2525CZER2R2M018 A18 mΩ
VishayIHLP2525BDER2R2M016.5 A28 mΩ

The designer should choose the inductors that best match the system requirements. A very wide range of inductors are available as regarding physical size, height, maximum current (thermally limited, and inductance loss limited), series resistance, maximum operating frequency, losses, and so forth. In general, inductors of smaller physical size have higher series resistance (DCR) and implicitly lower overall efficiency is achieved. Very low-profile inductors may have even higher series resistance. TI recommends finding the best compromise between system performance and cost.