frequency noise attenuation can be achieved by

increasing the size of C2 (see the Functional Diagram).

Extra noise attenuation is not typically required.

Shutdown

The MAX9721’s low-power shutdown mode reduces sup-

ply current to 1µA. Driving SHDN low disables the ampli-

fiers and charge pump. The driver’s output impedance is

typically 50kΩ (MAX9721A), 37.5kΩ (MAX9721B), or

25kΩ (MAX9721C) when in shutdown mode.

Click-and-Pop Suppression

In conventional single-supply audio drivers, the output-

coupling capacitor is a major contributor of audible

clicks and pops. Upon startup, the driver charges the

coupling capacitor to its bias voltage, typically half the

supply. Likewise, on shutdown, the capacitor is dis-

charged to GND. This results in a DC shift across the

capacitor that appears as an audible transient at the

speaker. The MAX9721’s DirectDrive technology elimi-

nates the need for output-coupling capacitors.

The MAX9721 also features extensive click-and-pop

suppression that eliminates any audible transient

sources internal to the device. The Power-Up/Down

Waveform in the Typical Operating Characteristics

shows minimal DC shift and no spurious transients at

the output upon startup or shutdown.

In most applications, the output of the preamplifier dri-

ving the MAX9721 has a DC bias of typically half the

supply. At startup, the input coupling capacitor is

charged to the preamplifier’s DC bias voltage through

the internal input resistor (25kΩ, typ) causing an audi-

ble click/pop. Delaying the rise of SHDN 4 or 5 time

the preamplifier eliminates any click/pop caused by the

input filter (see the Functional Diagram).

Applications Information

Power Dissipation

Linear power amplifiers can dissipate a significant

amount of power under normal operating conditions.

The maximum power dissipation for each package is

given in the Absolute Maximum Ratings section under

Continuous Power Dissipation or can be calculated by

the following equation:

°C/W as specified in the Absolute Maximum Ratings

+59.3°C/W.

The MAX9721 has two power dissipation sources, the

charge pump and the two amplifiers. If the power dissi-

pation exceeds the rated package dissipation, reduce

temperature, or add heatsinking to the device. Large

ing more heat to be transferred from the package to

surrounding air.

P

T- T

DISSPKG(MAX)

J(MAX)

A

JA

=

θ

1V, Fixed-Gain, DirectDrive, Stereo Headphone

Amplifier with Shutdown

8

_______________________________________________________________________________________

Figure 2. Low-Frequency Attenuation for Common DC-Blocking

Capacitor Values

LF ROLLOFF (16Ω LOAD)

FREQUENCY (Hz)

100

-30

-25

-20

-10

-3dB CORNER FOR

100µF IS 100Hz

-15

-5

-3

0

-35

10

1k

33µF

330µF

220µF

100µF

Figure 3. Distortion Contributed By DC-Blocking Capacitors

ADDITIONAL THD+N DUE

TO DC-BLOCKING CAPACITORS

FREQUENCY (Hz)

10k

1k

100

0.001

0.01

0.1

1

10

0.0001

10

100k

TANTALUM

ALUM/ELEC