Rainbow Electronics MAX9712 - Manual (Page 11)

MAX9712
500mW, Low EMI, Filterless,
Class D Audio Amplifier
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Shutdown
The MAX9712 has a shutdown mode that reduces power
consumption and extends battery life. Driving SHDN low
places the MAX9712 in a low-power (0.1µA) shutdown
mode. Connect SHDN to V
DD
for normal operation.
Click-and-Pop Suppression
The MAX9712 features comprehensive click-and-pop
suppression that eliminates audible transients on start-
up and shutdown. While in shutdown, the H-bridge is in
a high-impedance state. During startup, or power-up,
the input amplifiers are muted and an internal loop sets
the modulator bias voltages to the correct levels, pre-
venting clicks and pops when the H-bridge is subse-
quently enabled. For 35ms following startup, a soft-start
function gradually unmutes the input amplifiers.
Applications Information
Filterless Operation
Traditional class D amplifiers require an output filter to
recover the audio signal from the amplifier’s output. The
filters add cost, increase the solution size of the amplifi-
er, and can decrease efficiency. The traditional PWM
scheme uses large differential output swings (2 x V
DD
peak-to-peak) and causes large ripple currents. Any
parasitic resistance in the filter components results in a
loss of power, lowering the efficiency.
The MAX9712 does not require an output filter. The
device relies on the inherent inductance of the speaker
coil and the natural filtering of both the speaker 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 MAX9712 output 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 speaker not
designed to handle the additional power may be dam-
aged. For optimum results, use a speaker with a series
inductance >10µH. Typical 8
Ω speakers exhibit series
inductances in the range of 20µH to 100µH.
Power Conversion Efficiency
Unlike a class AB amplifier, the output offset voltage of
a class D amplifier does not noticeably increase quies-
cent current draw when a load is applied. This is due to
Figure 3. MAX9712 with 76mm of Speaker Cable
30.0 60.0 80.0 100.0 120.0 140.0 160.0 180.0 280.0 300.0 220.0 200.0 240.0 260.0
10.0
15.0
20.0
25.0
30.0
35.0
40.0
45.0
50.0
AMPLITUDE (dB
µ V/m)
FREQUENCY (MHz)
Figure 4. MAX9712 Outputs with No Input Signal
VIN = 0V
OUT-
OUT+
VOUT+ - VOUT- = 0V
Figure 5. MAX9712 Efficiency vs. Class AB Efficiency
0
30
20
10
50
40
90
80
70
60
100
0 0.1 0.2 0.3 0.4 0.5
EFFICIENCY vs. OUTPUT POWER
OUTPUT POWER (W)
EFFICIENCY (%)
MAX9712
CLASS AB
V DD = 3.3V
f = 1kHz
R
L - 8 Ω