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

6.1 System Description

This system has been designed to be a solution to precision pulse width modulation (PWM) dimming daytime running lights and implement thermal foldback without the necessity of using a microcontroller (MCU). The design includes key peripherals like electromagnetic interference (EMI) and electromagnetic compatibility (EMC) filtering voltage conditioning (shunt regulator), thermal foldback, precision clock generation, and LED drive.

The TIDA-01382 has been designed with the following points in consideration:

  • The design must be able to generate a precision PWM signal in the range of 5% to 50% duty cycle.
  • The design must be able to implement thermal foldback.
  • The design must satisfy power requirements for one TPS92691 device driving a string of 1 to 12 LEDs for daytime running lights
  • The design operate over the full range of automotive battery conditions:
    • VIN(min) down to 5 V simulating a cold-cranking condition (ISO 7637-2:2004 pulse 4)
    • VIN(max) up to 18 V simulating the upper range of normal battery operation
  • The design must survive and continue operation through:
    • Load dump (ISO 7637-2:2004 pulses 5a)
    • Double battery condition
  • The output must be protected against short-to-battery and GND voltage.
  • The design must optimize the individual blocks for smallest power dissipation and highest efficiency.
  • The layout of the board must be set up in such a way to minimize the footprint of the solution while maintaining high performance.
  • The design must provide a flexible board interface to either mate to custom board through screw terminals
  • The design provide power for the TLC555-Q1, OPA2377, and LMT87-Q1.