MAX9720 查看數據表（PDF） - Maxim Integrated

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MAX9720

50mW, DirectDrive, Stereo Headphone Amplifier with SmartSense and Shutdown

Maxim Integrated 

50mW, DirectDrive, Stereo Headphone

Amplifier with SmartSense and Shutdown

• The voltage coefficient of the capacitor, the change

in capacitance due to a change in the voltage

across the capacitor, distorts the audio signal. At

frequencies around the -3dB point, the reactance of

the capacitor dominates, and the voltage coefficient

appears as frequency-dependent distortion. Figure

4 shows the THD+N introduced by two different

capacitor dielectrics. Note that around the -3dB

point, THD+N increases dramatically.

The combination of low-frequency attenuation and fre-

quency-dependent distortion compromises audio

reproduction. DirectDrive improves low-frequency

reproduction in portable audio equipment that empha-

sizes low-frequency effects such as multimedia laptops

and MP3, CD, and DVD players.

Charge Pump

The MAX9720 features a low-noise charge pump. The

320kHz switching frequency is well beyond the audio

range, and does not interfere with the audio signals.

The switch drivers feature a controlled switching speed

that minimizes noise generated by turn-on and turn-off

transients. Limiting the switching speed of the charge

pump minimizes the di/dt noise caused by the parasitic

bond wire and trace inductance. Although not typically

required, additional high-frequency ripple attenuation

can be achieved by increasing the size of C2 (see

Typical Application Circuit).

MICROPHONE

AMPLIFIER

OUTPUT

AUDIO

INPUT

MICROPHONE

AMPLIFIER

MICROPHONE

BIAS

SmartSense

The SmartSense feature detects a short on either out-

put and automatically reconfigures the MAX9720 for

optimum power savings. If an output short circuit is

detected during the SmartSense routine, the shorted

channel is disabled, ALERT is asserted, and the device

is set to mono mode (assuming the other channel is not

shorted). SmartSense works by applying an inaudible

3µs test voltage pulse to the load. The resulting current

from the test pulse and load is sensed. If the load

impedance is less than 4Ω, the output is determined to

be a short.

LOW-FREQUENCY ROLLOFF

(RL = 16Ω)

0

-3

-6

DirectDrive

-9

330µF

-12

220µF

-15

100µF

-18

-21

33µF

-24

-27

-30

0.01

0.1

1

10

100

FREQUENCY (Hz)

Figure 3. Low-Frequency Attenuation of Common DC-Blocking

Capacitor Values

ADDITIONAL THD+N DUE

TO DC-BLOCKING CAPACITORS

10

1

AUDIO

INPUT

MAX9720

HEADPHONE DRIVER

Figure 2. Earbud Speaker/Microphone Combination Headset

Configuration

0.1

TANTALUM

0.01

0.001

ALUM/ELEC

0.0001

10

100

1k

10k

100k

FREQUENCY (Hz)

Figure 4. Distortion Contributed by DC-Blocking Capacitors

12 ______________________________________________________________________________________

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