Rainbow Electronics MAX9704 - Manual (Page 9)

Shutdown
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.
Click-and-Pop Suppression
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
SS should
be at least 180nF with a voltage rating of at least 7V.
Applications Information
Filterless Operation
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
swings (2 ✕ V DD peak-to-peak) and causes large ripple
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.
Gain Selection
Table 2 shows the suggested gain settings to attain a
maximum output power from a given peak input voltage
and given load.
Output Offset
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
DD /100 ✕ η),
which is in the order of a few microamps.
MAX9703/MAX9704
15W, Filterless, Spread-Spectrum
Mono/Stereo Class D Amplifiers
_______________________________________________________________________________________ 9
Figure 2. MAX9704 Efficiency vs. Class AB Efficiency
0
30
20
10
40
50
60
70
80
90
100
0 6 8 10 12 14 16 18 2 4 20
EFFICIENCY vs. OUTPUT POWER
OUTPUT POWER (W)
EFFICIENCY (%)
V DD = 15V
f = 1kHz
R
L = 8Ω
MAX9704
CLASS AB
Table 2. Gain Settings
GAIN (dB)
INPUT DIFF
(V RMS )
R L (
Ω
)
P OUT AT 10%
THD+N (W)
13.0 0.67 4 9
16.0 0.48 4 9
19.1 0.33 4 9
29.6 0.10 4 9
13.0 1.23 8 15
16.0 0.86 8 15
19.1 0.61 8 15
29.6 0.19 8 15