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 MAX9701 still exhibits >80% efficiencies

under the same conditions (Figure 5).

Shutdown

The MAX9701 has a shutdown mode that reduces power

consumption and extends battery life. Driving SHDN low

places the MAX9701 in a low-power (0.1µA) shutdown

Click-and-Pop Suppression

The MAX9701 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 40ms 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 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

Any parasitic resistance in the filter components results

in a loss of power, lowering the efficiency.

The MAX9701 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 MAX9701 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 8

Ω speakers, for portable

audio applications, exhibit series inductances in the

range of 20µH to 100µH.

Output Offset

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 8

Ω load results in 1mA extra

current consumption in a Class AB device. In the Class D

case, an 8mV offset into 8

Ω equates to an additional

power drain of 8µW. Due to the high efficiency of the

Class D amplifier, this represents an additional quiescent

η), which is on the

order of a few µA.

Selectable Gain

The MAX9701 features four selectable gain settings,

minimizing external component count. Gains of 0dB,

3dB, 12dB, and 18dB are set through gain-select

inputs, GAIN1 and GAIN2. GAIN1 and GAIN2 can be

hard-wired or digitally controlled. Table 2 shows the

suggested gain settings to attain a maximum output

power from a given peak input voltage and given load

1.3W, Filterless, Stereo Class D Audio

Power Amplifier

______________________________________________________________________________________

13

0

30

20

10

50

40

90

80

70

60

100

0

0.1

0.2

0.4

0.3

0.5

EFFICIENCY vs. OUTPUT POWER

OUTPUT POWER (W)

MAX9701

CLASS AB

f = 1kHz

Ω

Figure 5. MAX9701 Efficiency vs. Class AB Efficiency

GAIN1

GAIN2

GAIN

(dB)

INPUT

(

Ω)

(mW)

0

0

+18

0.305

4

1100

1

0

+12

0.615

4

1100

01

+6

1.213

4

1100

11

0

2.105

4

1100

0

0

+18

0.345

8

725

1

0

+12

0.686

8

725

01

+6

1.360

8

725

11

0

2.705

8

725

Table 2. Gain Settings