Datasheet

MAX9742
Single-/Dual-Supply, Stereo 16W,
Class D Amplifier with Differential Inputs
14 ______________________________________________________________________________________
efficiency of a linear amplifier is 78%; however, that effi-
ciency is only exhibited at peak output powers. Under
normal operating levels (typical music reproduction lev-
els), efficiency falls below 30%, whereas the MAX9742
still exhibits 80% efficiency under the same conditions.
Since the output transistors switch the output to either
V
DD
or V
SS
(ground for single-supply operation), the
resulting output of a Class D amplifier is a high-fre-
quency square wave. This square wave is pulse-width-
modulated by the audio input signal. In the MAX9742,
the pulse-width modulation (PWM) is accomplished by
comparing the input audio signal to an internally gener-
ated triangle wave oscillator. The resulting duty cycle of
the square wave is proportional to the level of the input
signal. When the input signal is at 0V, the duty cycle of
the MAX9742 output is equal to 50%. To extract the
amplified audio signal from this PWM waveform, the
output of the MAX9742 is fed to an external LC lowpass
filter (see the Single-Ended LC Output Filter Design (L
F
and C
F
) section). The LC filter works as an averaging
circuit for the PWM output voltage waveform. The
resulting averaged output voltage is equal to the ampli-
fied audio signal. Figure 3a illustrates the resulting
PWM output waveform due to the varying input signal
level, and Figure 3b shows the recovered amplified
input signal after filtering.
L
F
C
F
OUT_
V
DD
V
SS
V
SS
DUAL-SUPPLY CONFIGURATION SHOWN
7V REGULATOR
(WITH RESPECT TO V
SS
)
GATE DRIVE LOGIC
C
BOOT
0.1µF
D
BOOT
1N4148
REGLS
C
REGLS
1µF
V
REGLS
NSENSE
(INTERNALLY
CONNECTED TO V
SS
)
MAX9742
Figure 2. Simplified Block Diagram of the MAX9742 Output Stage