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IRAUDAMP6 REV 1
.0
Output Filter Design and Preamplifier
The audio performance of the IRAUDAMP6 depends on a number of different factors. The section
entitled, “Typical Performance” presents performance measurements based on the overall system,
including the preamp and output filter. While the preamp and output filter are not part of the Class
D power stage, they have a significant effect on the overall performance.
Output filter
The amplified PWM output is reconstructed back to an analog signal by the output LC LPF.
Demodulation LC low-pass filter (LPF) formed by L4 and C34, filters out the Class D switching
carrier signal leaving the audio output at the speaker load. A single stage output filter can be used
with switching frequencies of 400 kHz and greater; a design with a lower switching frequency may
require an additional stage of LPF.
Since the output filter is not included in the control loop of the IRAUDAMP6, the reference design
cannot compensate for performance deterioration due to the output filter. Therefore, it is important
to understand what characteristics are preferable when designing the output filter:
1) The DC resistance of the inductor should be minimal and be within 20 m_Ohm or less.
2) The linearity of the output inductor and capacitor should be high with respect to load
current and voltage.
Preamplifier
The preamp allows partial gain of the input signal, and in the IRAUDAMP6, controls the volume.
The preamp itself will add distortion and noise to the input signal, resulting in a gain through the
Class D output stage and appearing at the output. Even a few micro-volts of noise can add
significantly to the output noise of the overall amplifier. In fact, the output noise from the preamp
contributes more than half of the overall noise to the system.
It is possible to evaluate the performance without the preamp and volume control, by moving
resistors R154and R155 to R157 and R156, respectively. This effectively bypasses the preamp
and connects the RCA inputs directly to the Class D power stage input. Improving the selection of
preamp and/or output filter, will improve the overall system performance to approach that of the
stand-alone
Class D power stage.
Self-Oscillating PWM Modulator
The IRAUDAMP6 Class D audio power amplifier features a self-oscillating type PWM modulator
for the lowest component count and robust design. This topology represents an analog version of
a second-order sigma-delta modulation having a Class D switching stage inside the loop. The
benefit of the sigma-delta modulation, in comparison to the carrier-signal based modulation, is that
all the error in the audible frequency range is shifted to the inaudible upper-frequency range by
nature of its operation. Also, sigma-delta modulation allows a designer to apply a sufficient
amount of correction.
The self-oscillating frequency is determined by the total delay time inside the control loop of the
system. The delay of the logic circuits, the IRS20957 gate-driver propagation delay, the IRF6785
switching speed, the time-constant of front-end integrator (e.g. R50 + R49, C38 and C42 for CH1)