[Design View / Design Solution]
Improve EMC In Class D Amplifier Applications
Besides reducing EMI, new modulation techniques and filter architectures provide the cost benefits and audio performance of class AB designs.
Some products with low switching frequencies (200 to 300 kHz) won't work well with this approach. For those products, then, you may have to adopt other methods and topologies. The MAX9704 supplies the best results when set for fixed-frequency mode (FFM) operation at 940 kHz (FS1 = low, FS2 = high) (Fig. 5). For this particular part, FFM sets the switching period at a constant value—pin-selectable between three values—to suit a given application.
Figures 6and 7 show time-domain performance for the Figure 4 filter when driven by the MAX9704 class D amplifier. A resistive 8-Ω load was used in both cases. Figure 6 shows the FILT1 and FILT2 nodes overlaid (top traces), and the resulting 1-kHz differential output waveform (lower trace). Noise on the upper traces is the residual of the output switching after filtering (supply voltage is 15 V).
Figure 7 shows a detail on the trace of Figure 6. Note that the ripple, mostly from the 940-kHz switching frequency, appears as common-mode on both channels. Note also the absence of higher harmonics, which shows the effective suppression of EMI frequencies. (Radiated-EMI measurements usually start above 30 MHz.)
The filter designs in this article all assume a resistive load of 8 Ω. Voice-coil inductance causes the impedance of most wide-range moving-coil loudspeakers to rise above 20 kHz or so. That property makes efficient " filterless" operation possible, but you must account for the rising impedance when optimizing the component values for any additional EMI output filtering.
For audio designers, the need for filtering also arises in the lab when attempting to evaluate and characterize class D amplifier performance. Even if the end product can pass EMC tests without filters, the amplifier selection and evaluation pose problems. Many audio analyzers intended for measuring total harmonic distortion plus noise (THD+N) or amplitude response from conventional audio amplifiers can provide false results when driven by a "filterless" class D amplifier.
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