TIDUCL3 February   2017

 

  1. 1 Overview
  2. 2 Resources
  3. 3 Features
  4. 4 Applications
  5. 5 Design Images
  6. 6 System Overview
    1. 6.1 System Description
    2. 6.2 Key System Specifications
    3. 6.3 Block Diagram
    4. 6.4 Highlighted Products
      1. 6.4.1 LMT87-Q1
      2. 6.4.2 TLC555-Q1
      3. 6.4.3 OPA2377-Q1
      4. 6.4.4 TL431-Q1
      5. 6.4.5 TPS92691-Q1
  7. 7 System Design Theory
    1. 7.1  PCB and Form Factor
    2. 7.2  Optimizing Board Performance Based on LED String Voltage and Current
    3. 7.3  Switching Frequency
    4. 7.4  Output Overvoltage Protection (OVP)
    5. 7.5  Current Monitoring (IMON)
    6. 7.6  Thermal Foldback
      1. 7.6.1 Changing Thermal Foldback Response
        1. 7.6.1.1 Changing Starting Point for Thermal Foldback
        2. 7.6.1.2 Changing Slope of Thermal Foldback
        3. 7.6.1.3 Constant Current at High Temperatures
      2. 7.6.2 Thermal Foldback Without PWM Dimming
    7. 7.7  Clock Generation (PWM)
    8. 7.8  Onboard Supply and Setting Duty Cycle
    9. 7.9  Buffering, Averaging, and Filtering
    10. 7.10 Boost Converter
  8. 8 Getting Started Hardware
    1. 8.1 Hardware
    2. 8.2 LED Selection
    3. 8.3 J3, LED+, LED– (Boost)
    4. 8.4 J1, POS(+), NEG(–)
    5. 8.5 J4, Temperature Sensor Connection
    6. 8.6 Duty Cycle Adjust
  9. 9 Testing and Results
    1. 9.1 Duty Cycle Accuracy
    2. 9.2 Thermal Foldback Testing
    3. 9.3 EMI Testing
    4. 9.4 Accuracy Calculation
  10. 10Design Files
    1. 10.1 Schematics
    2. 10.2 Bill of Materials
    3. 10.3 PCB Layout Recommendations
      1. 10.3.1 Layout Prints
    4. 10.4 Altium Project
    5. 10.5 Gerber Files
    6. 10.6 Assembly Drawings
  11. 11Related Documentation
    1. 11.1 Trademarks
  12. 12About the Author

8.2 LED Selection

Selecting the DRL LEDs is a key aspect of this TI Design. OSRAM Opto Semiconductor's SYNIOS P2720 LEDs that can come in 300-, 700-, and 1000-mA ranges that allow them to be anywhere from 50 to 315 lumens. These LEDs allow for scalability because they all share the same package, just a different die size. Choosing which of these LEDs depends greatly on the optics that the LED will be placed in; each optic can reduce the light efficiency of the LED, which should be considered in choosing LEDs. Another aspect of LEDs is their thermal roll-off. According to the datasheet of the LEDs used in this TI Design, KW DMLN31.SG[6], the relative luminous flux at high temperatures can decrease to 90% which means that even though they are dissipating more energy at higher temperatures, they are actually dimmer than expected. This must be taken into consideration with the optics so that the LEDs will be able to pass any government standards for automotive lighting such as the SAE J1383. All of these conditions make the equation for luminosity non-linear and heavily dependant on final application factors. Therefore for this TI Design, a first-order estimate of brightness, or the current though the LEDs, is what is used to reflect the brightness of the LEDs. For a final design, the thermal roll-off and optic reflector will need to be considered and a graph of luminosity versus temperature can be performed.