the MAX9724 addresses the problem of the RF suscep-

tibility by rejecting RF noise and preventing it from cou-

pling into the audio band.

The RF susceptibility of an amplifier can be measured

by placing the amplifier in an isolated chamber and sub-

jecting it to an electric field of known strength. If the

electric field is modulated with an audio band signal, a

percentage of the modulated signal will be demodulat-

ed and amplified by the device in the chamber. Figure 2

shows the signal level at the outputs of an unoptimized

amplifier and the MAX9724. The test conditions are

shown in Table 1.

Click-and-Pop Suppression

In conventional single-supply audio amplifiers, the out-

put-coupling capacitor contributes significantly to audi-

ble clicks and pops. Upon startup, the amplifier charges

the coupling capacitor to its bias voltage, typically half

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

charged. This results in a DC shift across the capacitor,

which appears as an audible transient at the speaker.

Since the MAX9724A/ MAX9724B do not require output-

coupling capacitors, this problem does not arise.

Additionally, the MAX9724A/MAX9724B feature exten-

sive click-and-pop suppression that eliminates any audi-

ble transient sources internal to the device.

Typically, the output of the device driving the

MAX9724A/MAX9724B has a DC bias of half the supply

charged to the preamplifier’s DC bias voltage through

series 15kΩ resistor. This DC shift across the capacitor

results in an audible click-and-pop. Delay the rise of

to eliminate clicks-and-pops caused by the input filter.

Shutdown

The MAX9724A/MAX9724B feature a <0.1µA, low-

power shutdown mode that reduces quiescent current

consumption and extends battery life for portable appli-

cations. Drive SHDN low to disable the amplifiers and

the charge pump. In shutdown mode, the amplifier out-

MAX9724B). The amplifiers and charge pump are

enabled once SHDN is driven high.

Applications Information

Power Dissipation

Under normal operating conditions, linear power ampli-

fiers can dissipate a significant amount of power. 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:

P

TT

DISSPKG MAX

J MAX

A

JA

()

()

=

−

θ

60mW, DirectDrive, Stereo Headphone Amplifier

with Low RF Susceptibility and Shutdown

10

______________________________________________________________________________________

TEST PARAMETER

SETTING

RF Field Strength

50V/m

RF Modulation Type

Sine wave

RF Modulation Index

100%

RF Modulation Frequency

1kHz

Table 1. RF Susceptibility Test Conditions

RF CARRIER FREQUENCY (MHz)

1600

2100

1100

600

-80

-60

-40

-20

0

20

40

RF SUSCEPTIBLE

AMPLIFIER

MAX9724

62dB IMPROVEMENT

AT 850MHz

39dB IMPROVEMENT

AT 900MHz

67dB IMPROVEMENT

AT 1800MHz

49dB IMPROVEMENT

AT 1900MHz

-100

100

2600

Figure 2. RF Susceptibility of the MAX9724 and a Typical Headphone Amplifier