TPA032D04_16 Datasheet, PDF (13/26 Pages) Texas Instruments – 10-W STEREO CLASS-D AUDIO POWER AMPLIFIER

TPA032D04
10ĆW STEREO CLASSĆD AUDIO POWER AMPLIFIER
SLOS203B − DECEMBER 1999 − REVISED JUNE 2000
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.
In the efficiency equation below, P_L is power across the load and P_SUP is the supply power.
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
both the on-resistance (r_DS(on)) and the switching times of the output transistors are zero.
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 5).
VDD
M1
VA
M2
IL
L
CL
IOUT
RL
C
+
VOUT
−
Figure 5. Half-Bridge Class-D Output Stage
Figures 6 and 7 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
inductor current is flowing through M2 from ground. This means that V_A (the voltage at the drain of M2, as shown
in Figure 5) 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
averages V_A, which makes V_OUT equal to the supply voltage multiplied by the duty cycle.
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