MAX9705AETB/V+T Maxim Integrated, MAX9705AETB/V+T Datasheet - Page 12

• Manufacturer Part Number: MAX9705AETB/V+T
• Description: Audio Amplifiers 2.3W Ultra-Low-EMI Filterless Class D Audio Amplifier
• Manufacturer: Maxim Integrated

Specifications of MAX9705AETB/V+T

• Product: Class-D
• Output Power: 950 mW
• Thd Plus Noise: 0.02 %
• Supply Current: 5.4 mA
• Maximum Power Dissipation: 1951.2 mW
• Audio Load Resistance: 4 Ohms
• Common Mode Rejection Ratio (min): 56 dB

Figure 3. MAX9705 Outputs with No Input Signal
The MAX9705 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 30ms 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 output. The
filters add cost, increase the solution size of the amplifi-
er, and can decrease efficiency and THD+N perfor-
mance. The traditional PWM scheme uses large
differential output swings (2 x V
causes large ripple currents. Any parasitic resistance in
the filter components results in a loss of power, lower-
ing the efficiency.
The MAX9705 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 MAX9705 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
2.3W, Ultra-Low-EMI, Filterless,
Class D Audio Amplifier
12
V OUT+ - V OUT- = 0V
______________________________________________________________________________________
V IN = 0V
OUT+
OUT-
Applications Information
Click-and-Pop Suppression
Filterless Operation
DD
peak-to-peak) and
Figure 4. MAX9705 Efficiency vs. Class AB Efficiency
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 MAX9705 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.
The MAX9705 can be configured as a single-ended
input amplifier by capacitively coupling either input to
GND and driving the other input (Figure 5).
100
90
80
70
60
50
40
30
20
10
0
0
MAX9705
EFFICIENCY vs. OUTPUT POWER
Power-Conversion Efficiency
0.2
OUTPUT POWER (W)
0.4
CLASS AB
DD
/100η), which is on the
0.6
Single-Ended Input
Input Amplifier
V
f
R
Differential Input
IN
DD
L
0.8
= 1kHz
= 8Ω
= 3.3V
1.0