TPA032D02

10-W STEREO CLASS-D AUDIO POWER AMPLIFIER

SLOS243A – DECEMBER 1999 – REVISED MARCH 2000

13

POST OFFICE BOX 655303

APPLICATION INFORMATION

efficiency of class-D vs linear operation

Amplifier efficiency is defined as the ratio of output power delivered to the load to power drawn from the supply.

Efficiency

+ h +

P

L

P

SUP

A high-efficiency amplifier has a number of advantages over one with lower efficiency. One of these advantages

is a lower power requirement for a given output, which translates into less waste heat that must be removed

from the device, smaller power supply required, and increased battery life.

Audio power amplifier systems have traditionally used linear amplifiers, which are well known for being

inefficient. Class-D amplifiers were developed as a means to increase the efficiency of audio power amplifier

systems.

A linear amplifier is designed to act as a variable resistor network between the power supply and the load. The

transistors operate in their linear region and voltage that is dropped across the transistors (in their role as

variable resistors) is lost as heat, particularly in the output transistors.

The output transistors of a class-D amplifier switch from full OFF to full ON (saturated) and then back again,

spending very little time in the linear region in between. As a result, very little power is lost to heat because the

transistors are not operated in their linear region. If the transistors have a low on-resistance, little voltage is

dropped across them, further reducing losses. The ideal class-D amplifier is 100% efficient, which assumes that

the ideal class-D amplifier

To illustrate how the output transistors of a class-D amplifier operate, a half-bridge application is examined first

(see Figure 4).

VDD

VOUT

L

CL

RL

IL

IOUT

+

–

VA

M2

M1

C

Figure 4. Half-Bridge Class-D Output Stage

Figures 5 and 6 show the currents and voltages of the half-bridge circuit. When transistor M1 is on and M2 is

off, the inductor current is approximately equal to the supply current. When M2 switches on and M1 switches

off, the supply current drops to zero, but the inductor keeps the inductor current from dropping. The additional

in Figure 4) transitions between the supply voltage and slightly below ground. The inductor and capacitor form

a low-pass filter, which makes the output current equal to the average of the inductor current. The low pass filter