TIDUFC9 May   2025

 

  1.   1
  2.   Description
  3.   Resources
  4.   Features
  5.   Applications
  6.   6
  7. 1System Description
    1. 1.1 Key System Specifications
  8. 2System Overview
    1. 2.1 Block Diagram
    2. 2.2 Design Considerations
      1. 2.2.1 Codec Design
      2. 2.2.2 Class-D Amplifier
        1. 2.2.2.1 Audio Filter Design
      3. 2.2.3 Power Design
      4. 2.2.4 EMC, EMI Design Considerations
    3. 2.3 Highlighted Products
      1. 2.3.1 TAC5312-Q1
      2. 2.3.2 TAS5441-Q1
      3. 2.3.3 LMR43620-Q1
      4. 2.3.4 TPS7A52-Q1
      5. 2.3.5 TPD2E007
  9. 3Hardware, Testing Requirements, and Test Results
    1. 3.1 Hardware Requirements
      1. 3.1.1 Board Connection
      2. 3.1.2 Configuring the Board
    2. 3.2 Software Requirements
      1. 3.2.1 Firmware for Bench Tests
    3. 3.3 Test Setup
    4. 3.4 Test Results
      1. 3.4.1 Audio Performance
      2. 3.4.2 Power Tests
      3. 3.4.3 EMI, EMC Test Results
  10. 4Design and Documentation Support
    1. 4.1 Design Files
      1. 4.1.1 Schematics
      2. 4.1.2 Bill of Materials
      3. 4.1.3 PCB Layout Recommendations
        1. 4.1.3.1 Layout Prints
      4. 4.1.4 Altium Project
      5. 4.1.5 Gerber Files
      6. 4.1.6 Assembly Drawings
    2. 4.2 Documentation Support
    3. 4.3 Support Resources
    4. 4.4 Trademarks
  11. 5About the Author

2.2.2.1 Audio Filter Design

To improve the audio quality and to reduce electromagnetic emissions, an output filter is needed for the TAS5441-Q1. These output filters can use many components, for instance, Figure 2-3 shows an LC reconstruction filter with common-mode and differential capacitors and RC snubbers.

TIDA-060048 Class-D Amplifier LC Filter With RC Snubber Figure 2-3 Class-D Amplifier LC Filter With RC Snubber

The RC snubbers help reduce electromagnetic emissions, however, snubbers are not needed for many applications. The radiated emissions testing done on this design did not use the RC snubbers. Figure 2-4 shows the output filter without the RC snubber network.

TIDA-060048 Class-D Amplifier LC Filter Figure 2-4 Class-D Amplifier LC Filter

When designing the output filter, consider the filter as two single-ended outputs. For this reason, RL is considered to be 2Ω instead of 4Ω in the following design equations. A second-order Butterworth filter provides a flat pass-band response and a reasonable sharp roll off. Critically damp the filter, which occurs when:

Equation 1. Q = 1 2

The equations for C and L can be derived from the following two equations.

Equation 2. Q = R L × C L
Equation 3. ω c 2 = 1 C × L

Both the inductor and capacitor are chosen by choosing the cutoff frequency of the filter. For audio voice applications, 30kHz or above is a reasonable number. The equation to choose the inductance is:

Equation 4. L = R L × 2 ω c
Equation 5. L = 2 Ω × 2 2 × π × 34 k H z = 10 μ H

Further considerations for the inductor include shielding to improve EMC performance, saturation current, and low core losses. The TAS5441-Q1 overcurrent shutdown protection is 2.4A to 3.5A. Reaching the saturation current of an inductor lowers the inductance, so choosing the saturation level to be around or above the shutdown current is a safe choice. The inductor chosen for this design is the VLS6045EX-100M-H which is magnetically shielded, has low DC resistance, and has a typical saturation current of 3.9A.

Choose the capacitors using the following equation:

Equation 6. C = 1 ω c × R L × 2
Equation 7. C = 1 2 × π × 34 k H z × 4 × 2 = 1 . 66 μ F

To improve EMC performance, the cutoff frequency can be lowered slightly more by raising the capacitor slightly. For this design, a capacitor of 2.2µF is used, which results in the frequency response shown in Figure 2-5.


TIDA-060048 Frequency Response of LC Filter, L = 10µH, C = 2.2µF, R = 4Ω, 8Ω BTL

Figure 2-5 Frequency Response of LC Filter, L = 10µH, C = 2.2µF, R = 4Ω, 8Ω BTL

While this change assists with EMC performance, the change does drop the gain at 20kHz to 2dB instead of 1dB. Tradeoffs must be made between audio quality and EMC testing.

For improved EMC performance, the RC snubbers can be added back in, but this design is tested without them.

For the full derivation of the design equations see the LC Filter Design application report.