SLOA337 October   2024 TAS6584-Q1 , TAS6684-Q1

 

  1.   1
  2.   Abstract
  3. 1Introduction
  4. 2LC Filter Configuration
  5. 3Audio Performance
    1. 3.1 Inductor Performance Guide
    2. 3.2 Capacitor Selection
  6. 4Summary
  7. 5References
  8.   A Gain Compensation Biquads
  9.   Trademarks

2 LC Filter Configuration

The LC filter value is selected for a critically damped, flat pass-band, and phase response. Two considerations when selecting components for LC filter is the cutoff frequency and Q factor or damping ratio. The cutoff frequency of LC filter and inductor value are based on the amplifier switching frequency, the ripple current is reduced such that only a small residual ripple voltage is present after the LC filter. TAS6x84-Q1 supports 384kHz, 480kHz, up to 2MHz high switching frequencies. Find the typical inductor and capacitor values using the calculations in the LC Filter Design application note. The 384kHz or 480kHz switching frequency typically uses a 10μH inductor, while the 2MHz switching frequency design can take advantage of a much smaller and lighter weight inductor in the range of 3.3μH. However, the LC filter configuration is also adjusted according to the power supply voltage, and the end system EMC specifications. Use a fourth-order filter configuration and higher inductance for high-voltage supply applications and special EMC conditions. Table 2-1 provides a quick LC filter selection guide.

Table 2-1 LC Filter Configuration
EMC Condition Switching Frequency LC Filter Configuration Cutoff Frequency, 4Ω Load
L1 C1 L2 C2
Class-H enabled, or ≤ 24V power supply applications 384kHz, 480kHz 10μH 2.2μF none none 41.82kHz
High limitation on fundamental frequency 15μμH 2.2μF none none 29.79kHz
Standard configuration 10μH 1μF 1μH 0.22μF 43.85kHz
High limitation on full band 10μH 1μF 3.3μH 1μF 38.93kHz
Only for ≤ 24V power supply applications 2MHz 5.6μH 1μF 0.68μH 0.22μF 76.34kHz
Only for 14.4V battery power supply applications 3.3μH 1μF 0.68μH 0.22μF 113.19kHz

The frequency response of the LC filter is critical when selecting the component values for the inductor and capacitor. Figure 2-1 is the frequency response of the LC filter configurations with 4Ω load, assuming the inductor is linear and the DC resistance (DCR) is zero. The cutoff frequency of different LC filter is given in Table 2-1. The inductance and capacitor value chosen for 384kHz or 480kHz switching frequency are optimized on 4Ω load, slightly overdamped with 10μH / 1μF + 1μH / 0.22μF LC filter and slightly underdamped with 10μH + 2.2μF LC filter. While the response is a bit high underdamped with 3.3μH / 1μF + 0.68μH / 0.22μF LC filter for 2MHz switching frequency.

Frequency Response of LC Filter - 4Ω Load Figure 2-1 Frequency Response of LC Filter - 4Ω Load

The LC filter response also varies with speaker load impedance. The load impedance determines the damping ratio of the output LC filter and is classified as overdamped, critically damped, or underdamped. The equations for the single-ended LC filter shown in Figure 1-1 follow:

Equation 2. f 0 = ω 0 2 π = 1 2 π L × C   Cutoff   frequency   of   single - ended   LC   filter
Equation 3. ω 0 = 2 π f 0   C o n v e r s i o n   b e t w e e n   r a d i a n s   a n d   f r e q u e n c y   i n   h e r t z
Equation 4. Q = R L C L     Q u a l i t y   F a c t o r   Q
Equation 5. ζ = 1 2 Q = 1 2 × R L C L   D a m p i n g   R a t i o

According to those calculations, load impedance determines the damping ratio of the output LC filter. Figure 2-2 is selected LC filter 10μH / 1μF + 1μH / 0.22μF frequency response with various speaker loads. The frequency response is seriously overdamped with 2Ω load, and seriously underdamped with 8Ω load. At high frequency, the peaks are generally harsh to the human ear and can also trigger the protection circuitry, such as overcurrent, of some amplifiers. However, overdamped filters result in attenuation of high-frequency audio content in the audio band.

LC Filter Response With 10μH / 1μF + 1μH / 0.22μF Figure 2-2 LC Filter Response With 10μH / 1μF + 1μH / 0.22μF

To help compensate for this effect and achieve a flat response, the TAS6x84-Q1 offers integrated and channel-based gain compensation biquads which are configurable by channel and are disabled by default. To enable the desired tuning, the respective coefficients need to be written to the DSP memory. Figure 2-3 and Figure 2-4 show the frequency response differences when enable and without tuning gain compensation biquads with the same load. A flat response is achieved after enabling the integrated compensation with a desirable equalizer setting. The guide of tuning gain compensation biquads is in Appendix A.

LC Filter Response With 10μH / 1μF + 1μH / 0.22μF - 2Ω, 24V PVDD Figure 2-3 LC Filter Response With 10μH / 1μF + 1μH / 0.22μF - 2Ω, 24V PVDD
LC Filter Response With 10μH + 0.22μF - 8Ω, 45V PVDD Figure 2-4 LC Filter Response With 10μH + 0.22μF - 8Ω, 45V PVDD

Figure 2-5 and Figure 2-6 show the TAS6584-Q1 2MHz switching frequency power efficiency at several power supplies. The power efficiency at a high supply voltage results in difficult thermal requirements, so the 2MHz switching frequency is not recommended on high supply voltage applications. At the 2MHz switching frequency, a 3.3μH inductor is only recommended on 14.4V power supply applications. Because of the increase in the power supply, increase the inductance to 5.6μH to reduce ripple current. A fourth-order filter configuration is also recommended at 2MHz switching frequency to meet end-system EMC specifications. While the power supply is higher than 24V, a 384kHz or 480kHz switching frequency is the best design. This document focuses on the LC filter performance on 480kHz switching frequency applications, because ripple current is lower than 384kHz switching frequency with same inductance.

Efficiency vs Output Power - 8Ω, PVDD Figure 2-5 Efficiency vs Output Power - 8Ω, PVDD
Efficiency vs Output Power - 4Ω, PVDD Figure 2-6 Efficiency vs Output Power - 4Ω, PVDD