max9746 Maxim Integrated Products, Inc., max9746 Datasheet - Page 11

• Manufacturer Part Number: max9746
• Description: Max9746 1.2w, Low-emi, Filterless, Class D Audio Amplifier
• Manufacturer: Maxim Integrated Products, Inc.

Efficiency of a Class D amplifier is attributed to the
region of 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 out-
put stage is mostly due to the I
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 MAX9746 still exhibits near 90% efficien-
cies under the same conditions (Figure 4).
The MAX9746 has a shutdown mode that reduces power
consumption and extends battery life. Driving SHDN low
places the MAX9746 in a low-power (0.1µA) shutdown
mode. Connect SHDN to V
The MAX9746 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.
Figure 4. MAX9746 Efficiency vs. Class AB Efficiency
100
90
80
70
60
50
40
30
20
10
0
0
EFFICIENCY vs. OUTPUT POWER
______________________________________________________________________________________
Click-and-Pop Suppression
OUTPUT POWER (W)
0.2
DD
MAX9746
for normal operation.
CLASS AB
✕
R loss of the MOSFET
0.4
V
f = 1kHz
R
DD
L
= 8Ω
= 3.3V
Efficiency
Shutdown
0.6
1.2W, Low-EMI, Filterless,
Class D Audio Amplifier
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
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 MAX9746 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 MAX9746 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 exhibit series
inductances in the 20µH to 100µH range.
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
the power conversion of the Class D amplifier. For exam-
ple, 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 addi-
tional power drain of 8µW. Due to the high efficiency of
the Class D amplifier, this represents an additional quies-
cent-current draw of 8µW/(V
order of a few microamps.
The MAX9746 features a differential input structure,
making it compatible with many CODECs, and offering
improved noise immunity over a single-ended input
amplifier. In devices such as cellular phones, high-fre-
quency signals from the RF transmitter can be picked
up by the amplifier’s input traces. The signals appear at
the amplifier’s inputs as common-mode noise. A differ-
ential input amplifier amplifies the difference of the two
inputs; any signal common to both inputs is canceled.
Applications Information
Power-Conversion Efficiency
DD
Filterless Operation
/100η), which is in the
Input Amplifier
Differential Input
DD
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