SNVSBK9C November   2019  – September 2020 LM63635-Q1


  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. 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. 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. Choosing the Switching Frequency
        2. Setting the Output Voltage
          1. CFF Selection
        3. Inductor Selection
        4. Output Capacitor Selection
        5. Input Capacitor Selection
        6. CBOOT
        7. VCC
        8. External UVLO
        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

Current Limit and Short-Circuit Operation

The LM63635-Q1 incorporates both peak and valley inductor current limits to provide protection to the device from overloads and short circuits and limit the maximum output current. Valley current limit prevents inductor current run-away during short circuits on the output, while both peak and valley limits work together to limit the maximum output current of the converter. A "hiccup" type mode is also incorporated for sustained short circuits. Finally, a zero current detector is used on the low-side power MOSFET to implement DEM at light loads (see the Glossary). The nominal value of this limit is about 0 A.

As the device is overloaded, a point is reached where the valley of the inductor current cannot reach below ILS-LIMIT before the next clock cycle. When this occurs, the valley current limit control skips that cycle, causing the switching frequency to drop. Further overload causes the switching frequency to continue to drop, but the output voltage remains in regulation. As the overload is increased, both the inductor current ripple and peak current increase until the high-side current limit, ISC, is reached. When this limit is activated, the switch duty cycle is reduced and the output voltage falls out of regulation. This represents the maximum output current from the converter and is given approximately by Equation 3. The output voltage and switching frequency continue to drop as the device moves deeper into overload while the output current remains at approximately IOMAX. If the inductor ripple current is large, the high-side current limit can be tripped before the low-side limit is reached. In this case, Equation 4 gives the approximate maximum output current.

Equation 3. GUID-090141BD-B1E6-4061-9650-CE45EFFBFB8C-low.gif
Equation 4. GUID-692A14DB-784E-4993-955C-FA72F35721F4-low.gif

If a severe overload or short circuit causes the FB voltage to fall below VHICCUP, the convert enters "hiccup" mode. VHICCUP represents about 40% of the nominal programmed output voltage. In this mode, the device stops switching for tOC, or about 100 ms, and then goes through a normal restart with soft start. If the short-circuit condition remains, the device runs in current limit for a little longer than tOC_active, or about 23 ms, and then shuts down again. This cycle repeats (as shown in Figure 8-13) as long as the short-circuit condition persists. This mode of operation reduces the temperature rise of the device during a sustained short on the output. The output current in this mode is approximately 20% of IOMAX. Once the output short is removed and the tOC delay is passed, the output voltage recovers normally as shown in Figure 8-14.

See Figure 8-15 for the overall output voltage versus output current characteristic.

GUID-57DF2C61-3E7E-4744-BCC8-D399AF242E54-low.gifFigure 8-13 Inductor Current Burst in Short-Circuit Mode
GUID-3C8F68F1-353B-4BD2-9DD3-A73F6F14AEDB-low.gifFigure 8-14 Short-Circuit Transient and Recovery
GUID-B2BCB077-B3DB-4C5D-961A-67891D550AD6-low.gifFigure 8-15 Output Voltage versus Output Current in Current Limit