max9773etgt Maxim Integrated Products, Inc., max9773etgt Datasheet - Page 13

• Manufacturer Part Number: max9773etgt
• Description: Max9773 1.8w, Filterless, Ultra-low Emi, Stereo Class D Audio Power Amplifier
• Manufacturer: Maxim Integrated Products, Inc.

Efficiency of a Class D amplifier is due to the switching
operation of the output stage transistors. In a Class D
amplifier, the output transistors act as current-steering
switches and consume negligible additional power.
Any power loss associated with the Class D output
stage is mostly due to the I*R loss of the MOSFET on-
resistance, and quiescent-current overhead.
The theoretical best efficiency of a linear amplifier is
78%; however, that efficiency is only exhibited at peak
output powers. Under normal operating levels (typical
music reproduction levels), efficiency falls below 30%,
whereas the MAX9773 still exhibits >80% efficiencies
under the same conditions (Figure 5).
The MAX9773 has a shutdown mode that reduces power
consumption and extends battery life. Driving SHDN low
places the MAX9773 in a low-power (0.1µA) shutdown
mode. Connect SHDN to V
The MAX9773 features comprehensive click-and-pop
suppression that eliminates audible transients on startup
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, preventing
clicks and pops when the H-bridge is subsequently
enabled. For 80ms following startup, a soft-start function
gradually unmutes the input amplifiers.
Traditional Class D amplifiers require an output filter to
recover the audio signal from the amplifier’s PWM output.
Figure 4. MAX9773 Outputs with No Input Signal
V OUT_+ - V OUT_- = 0V
V IN_ = 0V
OUT_+
OUT_-
Applications Information
______________________________________________________________________________________
Click-and-Pop Suppression
Stereo Class D Audio Power Amplifier
DD
for normal operation.
Filterless Operation
1.8W, Filterless, Ultra-Low EMI,
Efficiency
Shutdown
The filters add cost, increase the solution size of the
amplifier, and can decrease efficiency. The traditional
PWM scheme uses large differential output swings (2 x
V
resistance in the filter components results in a loss of
power, lowering the efficiency.
The MAX9773 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 MAX9773 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 can be dam-
aged. For optimum results, use a speaker with a series
inductance >10µH. Typical 8Ω speakers, for portable
audio applications, exhibit series inductances in the
range of 20µH to 100µH.
Unlike a Class AB amplifier, the output offset voltage of a
Class D amplifier does not noticeably increase quiescent
current draw when a load is applied. This is due to the
power conversion of the Class D amplifier. For example,
an 8mV DC 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
order of a few µA.
Figure 5. MAX9773 Efficiency vs. Class AB Efficiency
DD(P-P)
) and causes large ripple currents. Any parasitic
100
90
80
70
60
50
40
30
20
10
0
0
EFFICIENCY vs. OUTPUT POWER
0.1
0.2
OUTPUT POWER (W)
DD
MAX9773
CLASS AB
0.3
/ 100 x η), which is on the
0.4
0.5
V
f = 1kHz
R
DD
L
Output Offset
- 8Ω
= 3.3V
0.6
0.7
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