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

7.6.1.1 Changing Starting Point for Thermal Foldback

The TPS92691-Q1 has an internal clamp on the "IADJ" pin at 2.55 V and an absolute maximum input voltage at the pin of 8.8 V. Because there is room there to scale up the input into the "IADJ" pin, the simplest way to change the starting point for thermal foldback is introducing a non-inverting amplifier, as shown in Figure 11, with a gain found using Equation 9.

TIDA-01382 non-inverting_amplifier.gifFigure 11. Non-Inverting Amplifier
Equation 9. TIDA-01382 tida-01382-equation-07.gif

With an output voltage from the LMT87 of 3.16 V at –40°C, the maximum scaling factor possible is 2.5 in order to not violate the maximum voltage at the "IADJ" pin. Therefore, if a scaling factor of 2.5 was used, the temperature at which the current would start to derate would be 117ºC. Also if no scaling is used, the temperature at which the current starts to derate is 7ºC. Equation 10 and Equation 11 show how to choose at what temperature the derating should start.

NOTE

This is a first-order linear approximation. If being exact is required, consult the LMT87-Q1 datasheet.

Temperature range from –40°C to T:

Equation 10. TIDA-01382 tida-01382-equation-08.gif

Temperature range from T to 125°C:

Equation 11. TIDA-01382 tida-01382-equation-09.gif

Where "T" is the temperature when the current will start derating and K is the scaling factor introduced by the non-inverting amplifier. The scaling factor needed is determined by Equation 12:

Equation 12. TIDA-01382 tida-01382-equation-10.gif

Using these equations, the derating is set so that it occurs at whatever temperature chosen ±3°C. In order to build this application, choose an op amp specifically for this non-inverting amplifier. There are several factors that will effect which op amp would be a good candidate. The input offset specification is the key factor on how accurately the current would reflect the temperature. This would mean an op amp like the OP07 or LT1013. Another aspect would be a low common-mode input voltage with rail-to-rail output so that the VTEMP could go down to GND and the output of the op amp would accurately reflect that. Op amps such as the TLV341 or the OP314 would work in this case.