SLUSE50 November   2023 TPS92642-Q1

PRODUCTION DATA  

  1.   1
  2. 1 Features
  3. 2 Applications
  4. 3 Description
  5. 4 Pin Configuration and Functions
  6. 5 Specifications
    1. 5.1 Absolute Maximum Ratings
    2. 5.2 ESD Ratings
    3. 5.3 Recommended Operating Conditions
    4. 5.4 Thermal Information
    5. 5.5 Electrical Characteristics
    6. 5.6 Typical Characteristics
  7. 6 Detailed Description
    1. 6.1 Overview
    2. 6.2 Functional Block Diagram
    3. 6.3 Feature Description
      1. 6.3.1  Internal Regulator
      2. 6.3.2  Buck Converter Switching Operation
      3. 6.3.3  Bootstrap Supply
      4. 6.3.4  Switching Frequency and Adaptive On-Time Control
      5. 6.3.5  Minimum On-Time, Off-Time, and Inductor Ripple
      6. 6.3.6  LED Current Regulation and Error Amplifier
      7. 6.3.7  Start-Up Sequence
      8. 6.3.8  Analog Dimming and Forced Continuous Conduction Mode
      9. 6.3.9  External PWM Dimming and Input Undervoltage Lockout (UVLO)
      10. 6.3.10 Pulse Duty Cycle Limit Circuit
      11. 6.3.11 Output Short and Open-Circuit Faults
      12. 6.3.12 Overcurrent Protection
      13. 6.3.13 Thermal Shutdown
      14. 6.3.14 Fault Indicator and Diagnostics Summary
    4. 6.4 Device Functional Modes
  8. 7 Application and Implementation
    1. 7.1 Application Information
      1. 7.1.1  Duty Cycle Considerations
      2. 7.1.2  Switching Frequency Selection
      3. 7.1.3  LED Current Programming
      4. 7.1.4  Inductor Selection
      5. 7.1.5  Output Capacitor Selection
      6. 7.1.6  Input Capacitor Selection
      7. 7.1.7  Bootstrap Capacitor Selection
      8. 7.1.8  Compensation Capacitor Selection
      9. 7.1.9  Input Dropout and Undervoltage Protection
      10. 7.1.10 Pulse Duty Cycle Limit Circuit
      11. 7.1.11 Protection Diodes
    2. 7.2 Typical Application
      1. 7.2.1 Design Requirements
      2. 7.2.2 Detailed Design Procedure
        1. 7.2.2.1 Calculating Duty Cycle
        2. 7.2.2.2 Calculating Minimum On-Time and Off-Time
        3. 7.2.2.3 Minimum Switching Frequency
        4. 7.2.2.4 LED Current Set Point
        5. 7.2.2.5 Inductor Selection
        6. 7.2.2.6 Output Capacitor Selection
        7. 7.2.2.7 Bootstrap Capacitor Selection
        8. 7.2.2.8 Compensation Capacitor Selection
        9. 7.2.2.9 VIN Dropout Protection and PWM Dimming
      3. 7.2.3 Application Curves
    3. 7.3 Power Supply Recommendations
    4. 7.4 Layout
      1. 7.4.1 Layout Guidelines
        1. 7.4.1.1 Compact Layout for EMI Reduction
          1. 7.4.1.1.1 Ground Plane
      2. 7.4.2 Layout Example
  9. 8 Device and Documentation Support
    1. 8.1 Receiving Notification of Documentation Updates
    2. 8.2 Support Resources
    3. 8.3 Trademarks
    4. 8.4 Electrostatic Discharge Caution
    5. 8.5 Glossary
  10. 9 Revision History
  11. 10Mechanical, Packaging, and Orderable Information

Package Options

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

6.3.5 Minimum On-Time, Off-Time, and Inductor Ripple

Buck converter operation is impacted by minimum on-time, minimum off-time, and minimum peak-to-peak inductor ripple limitations. The converter reaches the minimum on-time of 96 ns (typical) when operating with high input voltage and low-output voltage. In this control scheme, the off-time continues to increase and the switching frequency reduces to regulate the inductor current and LED current to the desired value.

Equation 5. fSWMIN=VOUTMINTONMIN×VINMAX;tON=tONMIN

The converter reaches the minimum off-time of 91 ns (typical) when operating in dropout (low input voltage and high output voltage). As the on-time and off-time are fixed, the duty cycle is constant and the buck converter operates in open-loop mode. The inductor current and LED current are not in regulation.

The behavior and response of valley comparator is dependent on sensed peak-to-peak voltage ripple, ΔV(CSP-CSN), and is a function of current sense resistor, RCS, and peak-to-peak inductor current ripple, ΔiL(PK-PK). To ensure periodic switching, the sensed peak-to-peak ripple must exceed the minimum value. At high (near 100%) or low (near 0%) duty cycles, the inductor current ripple may not be sufficient to ensure periodic switching. Under such operating conditions, the converter transitions from periodic switching to a burst sequence, forcing multiple on-time and off-time cycles at a rate higher than the programmed frequency. Although the converter may not operate in a periodic manner, the closed-loop control continues regulating the average LED current with a larger ripple value corresponding to higher peak-to-peak inductor ripple. TI recommends choosing an inductor, output capacitor, and switching frequency to ensure minimum sensed peak-to-peak ripple voltage under nominal operating condition is greater than 8 mV. The Application and Implementation section summarizes the detailed design procedure.