Texas Instruments TPA005D12 Overview of Class-D Audio Amplifiers, ±5. Class-D Functional Diagram

The TPA005D12 Class-D Audio Power Amplifier Evaluation Module

3.2.2Overview 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

Ramp

Generator

VIN

VRAMP

VERROR	PWM	VCONTROL
Comparator	Control	H-Bridge
		VOUT
	LPF	Load

The audio input signal (VIN) is applied to a very fast comparator along with a

ramp signal (VRAMP) 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 (VERROR) to the PWM control circuit. The PWM control signal (VCONTROL) then regulates the duty cycle of the high-current DMOS power transistors of the H-bridge, providing the output

signal (VOUT).

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 VOUT, leaving an amplified version of the original input signal. Examples of these waveforms are shown in Figure 3±6.

3-6 Details