MAX9704ETJ Maxim, MAX9704ETJ Datasheet - Page 9

• Manufacturer Part Number: MAX9704ETJ
• Description: 15W / Filterless / Spread-Spectrum Mono/Stereo Class D Amplifiers
• Manufacturer: Maxim

The MAX9703/MAX9704 have a shutdown mode that
reduces power consumption and extends battery life.
Driving SHDN low places the device in low-power
(0.2µA) shutdown mode. Connect SHDN to a logic high
for normal operation.
The MAX9703/MAX9704 feature comprehensive click-
and-pop suppression that eliminates audible transients
on startup and shutdown. While in shutdown, the H-
bridge is pulled to GND through 330kΩ. During startup,
or power-up, the input amplifiers are muted and an inter-
nal loop sets the modulator bias voltages to the correct
levels, preventing clicks and pops when the H-bridge is
subsequently enabled. Following startup, a soft-start
function gradually unmutes the input amplifiers. The
value of the soft-start capacitor has an impact on the
click/pop levels. For optimum performance, C
be at least 180nF with a voltage rating of at least 7V.
Traditional class D amplifiers require an output filter to
recover the audio signal from the amplifier’s PWM out-
put. The filters add cost, increase the solution size of
the amplifier, and can decrease efficiency. The tradi-
tional PWM scheme uses large differential output
Figure 2. MAX9704 Efficiency vs. Class AB Efficiency
100
90
80
70
60
50
40
30
20
10
0
Applications Information
0
_______________________________________________________________________________________
EFFICIENCY vs. OUTPUT POWER
MAX9704
2
Click-and-Pop Suppression
4
6
OUTPUT POWER (W)
8
Filterless Operation
10 12 14 16 18
CLASS AB
15W, Filterless, Spread-Spectrum
V
f = 1kHz
R
DD
L
Mono/Stereo Class D Amplifiers
= 8Ω
= 15V
Shutdown
20
SS
should
swings (2
currents. Any parasitic resistance in the filter compo-
nents results in a loss of power, lowering the efficiency.
The MAX9703/MAX9704 do not require an output filter.
The devices rely on the inherent inductance of the
speaker coil and the natural filtering of both the speak-
er and the human ear to recover the audio component
of the square-wave output. Eliminating the output filter
results in a smaller, less-costly, more-efficient solution.
Because the frequency of the MAX9703/MAX9704 out-
put is well beyond the bandwidth of most speakers,
voice coil movement due to the square-wave frequency
is very small. Although this movement is small, a speak-
er not designed to handle the additional power can be
damaged. For optimum results, use a speaker with a
series inductance > 30µH. Typical 8Ω speakers exhibit
series inductances in the range of 30µH to 100µH.
Optimum efficiency is achieved with speaker induc-
tances > 60µH.
Table 2 shows the suggested gain settings to attain a
maximum output power from a given peak input voltage
and given load.
Unlike a class AB amplifier, the output offset voltage of
class D amplifiers does not noticeably increase quies-
cent current draw when a load is applied. This is due to
the power conversion of the class D amplifier. For
example, an 8mVDC offset across an 8Ω load results in
1mA extra current consumption in a class AB device. In
the class D case, an 8mV offset into 8Ω equates
to an additional power drain of 8µW. Due to the high
efficiency of the class D amplifier, this represents an
additional quiescent current draw of: 8µW/(V
which is in the order of a few microamps.
Table 2. Gain Settings
GAIN (dB)
13.0
16.0
19.1
29.6
13.0
16.0
19.1
29.6
V
DD
INPUT DIFF
peak-to-peak) and causes large ripple
(V
0.67
0.48
0.33
0.10
1.23
0.86
0.61
0.19
RMS
)
R
L
4
4
4
4
8
8
8
8
( )
Gain Selection
Output Offset
P
DD
THD+N (W)
OUT
/100
15
15
15
15
AT 10%
9
9
9
9
η),
9