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

7.9 Buffering, Averaging, and Filtering

The OPA2377-Q1 dual op amp, low-noise, rail-to-rail input and output, and low offset, makes this device ideal for these type of applications. The output stage of the TLE555-Q1 as Section 6.4.2 describes is hooked up through a 1-K resistor to the PWM input of the TPS92691 device, which dims the LEDs and also connects to a buffer stage and a second-order filter formed by the dual OPA2377 op amp (see Figure 20). The buffer stage is placed to avoid changing impedance from input to output so as not to interfere with the shape or timing of the square wave generated by the TLC555. The waveform after the buffer stage is then averaged by the next op amp stage, which forms a second-order filter that is compared against a reverence voltage set by the resistor divider from the precision reference. The output of the second-order filter is hooked up to the CONT pin of the TLC555-Q1 closing the loop. The CONT input of the TLC555 allows the upper and lower trigger threshold of the timing duty cycle to be changed by varying the voltage level at this pin, which is performed by the output of the second-order filter.

TIDA-01382 tida-01382-buffer.gifFigure 20. Buffer

Use the following parameters for this TI Design example:

  • Gain = 19.2 V/V (inverting gain)
    • Low-pass cutoff frequency = 31.67 Hz
  • Second-order Chebyshev filter response with a 3-dB gain peaking in the passband

Figure 21 shows the infinite-gain multiple-feedback circuit for a low-pass network function.

TIDA-01382 tida-01382-second-order-filter-averaging.gifFigure 21. Second-Order Filter and Averaging

Use Equation 17 to calculate the voltage transfer function:

Equation 17. TIDA-01382 EQ_02_TIDUC97.gif

This circuit produces a signal inversion. For this circuit, the gain at DC and the low-pass cutoff frequency are calculated by Equation 18:

Equation 18. TIDA-01382 EQ_03_TIDUC97.gif

Where:

  • Gain = 19.2
  • fc = 31.67 Hz

Software tools are readily available to simplify filter design. WEBENCH® Filter Designer is a simple, powerful, and easy-to-use active filter design program. This program enables designers to create optimized filter designs using a selection of TI op amps and passive components from TI's vendor partners.