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The TPA005D12 Class-D Audio Power Amplifier Evaluation Module
3-6
Details
3.2.2 Overview of Class-D Audio Amplifiers
Class-D audio amplifiers are very similar in operation to switch-mode power
supplies in that both compare an input signal with a reference to create an error
voltage that controls a pulse-width modulator (PWM) circuit. The PWM circuit
then produces an output signal at constant frequency and with a duty cycle that
varies according to the input signal. A block diagram of the major components
that make up the amplifier is shown in Figure 3–5.
Figure 3–5. Class-D Functional Diagram
Audio
Analog
Source
Comparator
V
ERROR
H-Bridge
LPF
V
CONTROL
Load
Ramp
Generator
PWM
Control
V
OUT
V
IN
V
RAMP
The audio input signal (V
IN
) is applied to a very fast comparator along with a
ramp signal (V
RAMP
) created by the ramp generator. Each time the triangle
wave from the ramp generator crosses the audio input signal level, the
comparator sends an error signal (V
ERROR
) to the PWM control circuit. The
PWM control signal (V
CONTROL
) then regulates the duty cycle of the
high-current DMOS power transistors of the H-bridge, providing the output
signal (V
OUT
).
These transistors operate in either the cutoff or saturated regions, rather than
the linear region, which is where class AB amplifiers operate. This reduces
switching and conduction losses, reducing the power dissipated by the power
transistors and allowing more power to be delivered to the load. An
inductor-capacitor (LC) low-pass filter (LPF) then removes the high frequency
switching component from V
OUT
, leaving an amplified version of the original
input signal. Examples of these waveforms are shown in Figure 3–6.