SLAA701B October   2016  – June 2026 TAS5342A , TAS5342LA , TAS5352 , TAS5630B , TPA3220 , TPA3221 , TPA3251 , TPA3255 , TPA3255-Q1

 

  1.   1
  2.   Trademarks
  3.   Abstract
  4. 1LC Filter Design
    1. 1.1 Class-D Output Configurations
      1. 1.1.1 Bridged-Tied Load (BTL)
      2. 1.1.2 Parallel Bridge-Tied Load (PBTL)
      3. 1.1.3 Single-Ended (SE)
    2. 1.2 Class-D Modulation Schemes
      1. 1.2.1 AD (Traditional) Modulation
      2. 1.2.2 BD Modulation
    3. 1.3 Class-D Output LC Filter
      1. 1.3.1 Output LC Filter Frequency Response Properties
      2. 1.3.2 Class-D BTL Output LC Filter Topologies
      3. 1.3.3 Single-Ended Filter Calculations
      4. 1.3.4 Type-1 Filter Analysis
        1. 1.3.4.1 Type-1 Frequency Response Example
      5. 1.3.5 Type-2 Filter Analysis
        1. 1.3.5.1 Type-2 Frequency Response Example
      6. 1.3.6 Hybrid Filter for AD Modulation
        1. 1.3.6.1 Hybrid Filter Frequency Response Example
      7. 1.3.7 AD Modulation With Type-1 or Type-2 Filters
      8. 1.3.8 LC Filter Quick Selection Guide
    4. 1.4 Inductor Selection for High-Performance Class-D Audio
      1. 1.4.1 Inductor Linearity
      2. 1.4.2 Ripple Current
        1. 1.4.2.1 Calculating Ripple Current for a Single-Supply Class-D Amplifier
      3. 1.4.3 Minimum Inductance
      4. 1.4.4 Core Loss
      5. 1.4.5 DC Resistance (DCR)
      6. 1.4.6 Inductor Study With the TPA3251 Device
        1. 1.4.6.1 Results
        2. 1.4.6.2 Conclusion
    5. 1.5 Capacitor Considerations
      1. 1.5.1 Class-D Output Voltage Overview
        1. 1.5.1.1 Ripple Voltage
        2. 1.5.1.2 37
      2. 1.5.2 Capacitor Ratings and Specifications
        1. 1.5.2.1 Maximum Voltage or Rated DC Voltage
        2. 1.5.2.2 ESR and Dissipation Factor
        3. 1.5.2.3 Maximum Temperature Rise (Rated AC Voltage and AC Current)
        4. 1.5.2.4 Pulse Rise Time (dv/dt) or Peak Current (Ipeak)
      3. 1.5.3 Capacitor Types
        1. 1.5.3.1 Selecting a Capacitor Type
        2. 1.5.3.2 Metalized Film Capacitors
          1. 1.5.3.2.1 AC Voltage or Current Rating
          2. 1.5.3.2.2 Temperature Coefficient
        3. 1.5.3.3 Ceramic Capacitors
          1. 1.5.3.3.1 Size
          2. 1.5.3.3.2 DC Bias Voltage
          3. 1.5.3.3.3 Temperature Coefficient
          4. 1.5.3.3.4 Reliability
    6. 1.6 Related Collateral
  5. 2Reference
  6. 3Reference
  7. 4Revision History

1.4.1 Inductor Linearity

The inductance versus current profile for the inductor used in the output LC filter of a class-D amplifier can significantly impact the total harmonic distortion (THD) performance.

An ideal inductor maintains the specified inductance value no matter what current passes through it. However, real-world inductors always have decreasing inductance with increasing current. At some point, the current level saturates the inductor and the inductance falls off severely. This is often specified as Isat. Because inductor linearity is a function of current, inductor distortion is higher with lower-impedance loads.

Typical Inductor Saturation CurveFigure 1-20 Typical Inductor Saturation Curve

Keep in mind that the inductance change at the Isat current rating varies between manufacturers and even inductor types. Some manufacturers specify Isat at a 30% or higher change in inductance. Use of this inductor all the way to the Isat rating for an LC class-D filter results in very poor audio performance.

To illustrate the impact of inductor linearity, different inductors were tested with the ultra-low distortion TPA3251 amplifier. Table 1-6 shows data collected from four different inductors that have good linearity specifications for high-performance class-D audio amplifiers.

The inductance was measured at 1 A of current and again at 20 A of current with a 600-kHz test signal, which is the nominal PWM switching frequency of the TPA3251 amplifier. The average change of inductance was calculated for 10 samples of each inductor.

Table 1-6 Average Change in Inductance for 10 Inductor Samples
ManufacturerPart NumberNominal InductanceAverage Inductance Change (1 A–20 A)
Wurth74436307007 µH0.94%
Wurth7443630100010 µH1.38%
CoilcraftMA5173-AE7 µH1.16%
CoilcraftMA5172-AE10 µH1.55%

From the foregoing data, the 10-µH inductor from Wurth is more linear than the 10-µH inductor from Coilcraft. It is also important to note that the 7-µH and 10-µH inductors from Wurth are wound on the same core. Likewise, the 7-µH and 10-µH from Coilcraft are also wound on the same core. Generally, the higher the inductance (the more turns of wire) for a given core material, size, and geometry, the less linear the inductor.

The inductors tested in Table 1-6 were then populated onto the TPA3251 EVM and tested for total harmonic distortion plus noise (THD+N) performance.

TPA3251EVM THD+N vs Output Power, 4 ΩFigure 1-21 TPA3251EVM THD+N vs Output Power, 4 Ω
TPA3251EVM THD+N vs Signal Frequency, 20 W, 4 ΩFigure 1-22 TPA3251EVM THD+N vs Signal Frequency, 20 W, 4 Ω

In Figure 1-21, the more-linear inductors show improved mid-power THD+N performance. With the Wurth inductors, both the 10-µH and 7-µH offer very high performance and nearly approach the characteristic curve of an ideal amplifier. The distortion performance is noise-limited nearly all the way to clipping as shown by the continuously decreasing slope of the THD+N plot. With a more-linear inductor, we are able to see deeper into the noise floor of the amplifier before the harmonic content of the output signal begins to dominate the noise.

From Figure 1-22, the THD+N versus frequency performance also improves significantly in the 1-kHz to 6-kHz range. This is considered to be the most sensitive frequency region of the human ear since it is right in the middle of our audible bandwidth.

If the LC filter design is based on an amplifier where higher THD performance is acceptable, or if the native THD of the amplifier was higher, the 10-µH from Coilcraft™ may be a suitable candidate. In the end, the designer of the system must make a choice between inductor linearity, cost, and size.