SNVSCO0 January   2024 LM63635C-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
  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 Timing Characteristics
    7. 6.7 Switching Characteristics
    8. 6.8 System Characteristics
    9. 6.9 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 Output Voltage Selection
      2. 7.3.2 Switching Frequency Selection
        1. 7.3.2.1 Spread Spectrum Option
      3. 7.3.3 Enable and Start-Up
      4. 7.3.4 RESET Flag Output
      5. 7.3.5 Undervoltage Lockout, Thermal Shutdown, and Output Discharge
    4. 7.4 Device Functional Modes
      1. 7.4.1 Overview
      2. 7.4.2 Light Load Operation
      3. 7.4.3 Dropout Operation
      4. 7.4.4 Minimum On-Time Operation
      5. 7.4.5 Current Limit and Short-Circuit 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 Choosing the Switching Frequency
        2. 8.2.2.2 Inductor Selection
        3. 8.2.2.3 Output Capacitor Selection
        4. 8.2.2.4 Input Capacitor Selection
        5. 8.2.2.5 CBOOT
        6. 8.2.2.6 VCC
        7. 8.2.2.7 External UVLO
        8. 8.2.2.8 Maximum Ambient Temperature
      3. 8.2.3 Application Curves
      4. 8.2.4 EMI Performance Curves
    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 Device Nomenclature
    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 Glossary
    7. 9.7 Electrostatic Discharge Caution
  11. 10Revision History
  12. 11Mechanical, Packaging, and Orderable Information

Package Options

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

7.4.1 Overview

In auto mode, the device moves between PWM and PFM as the load changes. At light loads, the regulator operates in PFM where the switching frequency is varied to regulate the output voltage. At higher loads, the mode changes to PWM with the switching frequency of the selected device.

In PWM mode, the regulator operates as a current mode, constant frequency converter using PWM to regulate the output voltage. While operating in this mode, the output voltage is regulated by switching at a constant frequency and modulating the duty cycle to control the power to the load. This action provides excellent line and load regulation and low output voltage ripple.

In PFM mode, the high-side MOSFET is turned on in a burst of one or more pulses to provide energy to the load. The duration of the burst depends on how long the inductor current takes to reach IPEAK-MIN. The periodicity of these bursts is adjusted to regulate the output, while diode emulation (DEM) is used to maximize efficiency (see the Glossary). This mode provides high light-load efficiency by reducing the amount of input supply current required to regulate the output voltage at small loads. This action trades off very good light-load efficiency for larger output voltage ripple and variable switching frequency. Also, a small increase in output voltage occurs at light loads. See Section 8.2.3 for output voltage variation with load in PFM mode. Figure 7-5 and Figure 7-6 show the typical switching waveforms in PFM and PWM.

There are four cases where the switching frequency does not conform to the condition set by the switching frequency:

  • Light load operation (auto mode)
  • Dropout
  • Minimum on-time operation
  • Current limit

Under all of these cases, the switching frequency folds back, meaning less than the programmed frequency. During these conditions, by definition, the output voltage remains in regulation, except for current limit operation.

GUID-81AF8A4F-6682-400D-9595-3976226C0932-low.gifFigure 7-5 Typical PFM Switching Waveforms VIN = 12V, VOUT = 5V, IOUT = 10mA
GUID-A4E7882F-1044-472E-BC9E-99C39F765B98-low.gifFigure 7-7 Typical PWM Switching Waveforms With Spread Spectrum VIN = 12V, VOUT = 5V, IOUT = 2.5A, ƒSW = 2100kHz
GUID-BFADF15A-6148-4157-B8D5-10F38E1A89AF-low.gifFigure 7-6 Typical PWM Switching Waveforms Without Spread Spectrum VIN = 12V, VOUT = 5V, IOUT = 3.25A, ƒSW = 2100kHz