SNVSBK9C November   2019  – September 2020 LM63635-Q1

PRODUCTION DATA  

  1. Features
  2. Applications
  3. Description
  4. Revision History
  5. Device Comparison Table
  6. Pin Configuration and Functions
    1.     Pin Functions
  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 Timing Characteristics
    7. 7.7 Switching Characteristics
    8. 7.8 System Characteristics
    9. 7.9 Typical Characteristics
  8. Detailed Description
    1. 8.1 Overview
    2. 8.2 Functional Block Diagram
    3. 8.3 Feature Description
      1. 8.3.1 Sync/Mode Selection
      2. 8.3.2 Output Voltage Selection
      3. 8.3.3 Switching Frequency Selection
        1. 8.3.3.1 Spread Spectrum Option
      4. 8.3.4 Enable and Start-up
      5. 8.3.5 RESET Flag Output
      6. 8.3.6 Undervoltage Lockout and Thermal Shutdown and Output Discharge
    4. 8.4 Device Functional Modes
      1. 8.4.1 Overview
      2. 8.4.2 Light Load Operation
        1. 8.4.2.1 Sync/FPWM Operation
      3. 8.4.3 Dropout Operation
      4. 8.4.4 Minimum On-time Operation
      5. 8.4.5 Current Limit and Short-Circuit Operation
  9. Application and Implementation
    1. 9.1 Application Information
    2. 9.2 Typical Application
      1. 9.2.1 Design Requirements
      2. 9.2.2 Detailed Design Procedure
        1. 9.2.2.1 Choosing the Switching Frequency
        2. 9.2.2.2 Setting the Output Voltage
          1. 9.2.2.2.1 CFF Selection
        3. 9.2.2.3 Inductor Selection
        4. 9.2.2.4 Output Capacitor Selection
        5. 9.2.2.5 Input Capacitor Selection
        6. 9.2.2.6 CBOOT
        7. 9.2.2.7 VCC
        8. 9.2.2.8 External UVLO
        9. 9.2.2.9 Maximum Ambient Temperature
      3. 9.2.3 Full Feature Design Example
      4. 9.2.4 Application Curves
      5. 9.2.5 EMI Performance Curves
    3. 9.3 What to Do and What Not to Do
  10. 10Power Supply Recommendations
  11. 11Layout
    1. 11.1 Layout Guidelines
      1. 11.1.1 Ground and Thermal Considerations
    2. 11.2 Layout Example
  12. 12Device and Documentation Support
    1. 12.1 Documentation Support
      1. 12.1.1 Related Documentation
    2. 12.2 Receiving Notification of Documentation Updates
    3. 12.3 Support Resources
    4. 12.4 Trademarks
    5. 12.5 Glossary
    6. 12.6 Electrostatic Discharge Caution
  13. 13Mechanical, Packaging, and Orderable Information

Package Options

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

Overview

In typical usage, the device is put in AUTO mode (SYNC/MODE pin = ground). 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 set by the condition of the RT pin (see Section 8.3.3).

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 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 it takes the inductor current 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 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 9.2.4 for output voltage variation with load in PFM mode. Figure 8-6 and Figure 8-7 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 RT pin:

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

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

When the device is placed in forced PWM mode (FPWM), the switching frequency remains constant as programmed by the RT pin for all load conditions. This mode essentially turns off the light-load PFM frequency foldback mode detailed in Section 8.4.2. See Section 8.3.1 and Section 8.4.2.1 for details.

GUID-4E9F30BB-6006-4109-95C2-6B7CE60C6329-low.gifFigure 8-6 Typical PFM Switching Waveforms VIN = 12 V, VOUT = 5 V, IOUT = 10 mA
GUID-8C94D890-0AC6-43C7-8A36-B5A1DD05F1F4-low.gifFigure 8-8 Typical PWM Switching Waveforms With Spread Spectrum VIN = 12 V, VOUT = 5 V, IOUT = 2.5 A, ƒSW = 2100 kHz
GUID-53386A9F-F8AB-4F3C-B07C-8D310115AD28-low.gifFigure 8-7 Typical PWM Switching Waveforms Without Spread Spectrum VIN = 12 V, VOUT = 5 V, IOUT = 3.25 A, ƒSW = 2100 kHz
GUID-730EE54E-2D8C-4A75-A4B2-B7D88B3BE0DA-low.gifFigure 8-9 Typical PWM Switching Waveforms FPWM VIN = 12 V, VOUT = 5 V, IOUT = 0 A, ƒSW = 2100 kHz