1.2W, Low-EMI, Filterless,

Class D Audio Amplifier

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11

Shutdown

The MAX9700 has a shutdown mode that reduces power

consumption and extends battery life. Driving

SHDN low

places the MAX9700 in a low-power (0.1µA) shutdown

mode. Connect

Click-and-Pop Suppression

The MAX9700 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

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 MAX9700 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 MAX9700 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.

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. MAX9700 EMI Spectrum

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

FREQUENCY (MHz)

Figure 4. MAX9700 Outputs with No Input Signal

VIN = 0V

OUT-

OUT+

VOUT+ - VOUT- = 0V

Figure 5. MAX9700 Efficiency vs. Class AB Efficiency

0

30

20

10

50

40

90

80

70

60

100

0

0.1

0.2

0.4

0.6

0.3

0.5

0.7

EFFICIENCY vs. OUTPUT POWER

OUTPUT POWER (W)

MAX9700

CLASS AB

f = 1kHz

Ω