International Rectifier IRAUDAMP4, Руководство пользователя

Страница: 8 / 44

www.irf.com 7
RD-0617
Performance” section, only limited data for the stand-alone Class D power stage is
given. For example, results for THD+N vs. Output Power are provided, utilizing a range
of different inductors. By changing the inductor and repeating this test, a designer can
quickly evaluate a particular inductor.
Figure 4. Results of THD+N vs. Output Power with Different Output Inductors
Self-Oscillating PWM Modulator
The IRAUDAMP4 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 IRS20955 gate-driver propagation
delay, the IRF6645 switching speed, the time-constant of front-end integrator (e.g. R25 +
R29P, C11 and C13 for CH1) and variations in the supply voltages are critical factors of
the self-oscillating frequency. Under nominal conditions, the switching-frequency is
around 400 kHz with no audio input signal and a +/-35 V supply.
Adjustments of Self-Oscillating Frequency
The PWM switching frequency in this type of self-oscillating switching scheme greatly
impacts the audio performance, both in absolute frequency and frequency relative to the
other channels. In absolute terms, at higher frequencies, distortion due to switching-time
becomes significant, while at lower frequencies, the bandwidth of the amplifier suffers. In
relative terms, interference between channels is most significant if the relative frequency
difference is within the audible range. Normally when adjusting the self-oscillating
frequency of the different channels, it is best to either match the frequencies accurately,
0.0001
100
0.001
0.01
0.1
1
10
%
100m 200m 500m 1 2 5 10 20 50 100 200
W
T T T T T T T T T