MAX9724A(2007) 查看數據表（PDF） - Maxim Integrated

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MAX9724A
(Rev.:2007)

60mW, DirectDrive, Stereo Headphone Amplifier with Low RF Susceptibility and Shutdown

Maxim Integrated 

60mW, DirectDrive, Stereo Headphone Amplifier

with Low RF Susceptibility and Shutdown

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.

Table 1. RF Susceptibility Test Conditions

TEST PARAMETER

RF Field Strength

RF Modulation Type

RF Modulation Index

RF Modulation Frequency

SETTING

50V/m

Sine wave

100%

1kHz

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

voltage. At startup, the input-coupling capacitor is

charged to the preamplifier’s DC-bias voltage through

the input and feedback resistors of the MAX9724A/

MAX9724B, resulting in a DC shift across the capacitor

and an audible click/pop. Delay the rise of SHDN 4 to 5

time constants based on RIN and CIN, relative to the

startup of the preamplifier, 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-

put impedance is set to 14kΩ||RF (RF is 30kΩ for the

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:

PDISSPKG(MAX)

=

TJ(MAX) −

θJA

TA

40

62dB IMPROVEMENT 39dB IMPROVEMENT

AT 850MHz

AT 900MHz

20 RF SUSCEPTIBLE

67dB IMPROVEMENT

AMPLIFIER

AT 1800MHz

0

49dB IMPROVEMENT

AT 1900MHz

-20

-40

-60

MAX9724

-80

-100

100

600

1100

1600

2100

2600

RF CARRIER FREQUENCY (MHz)

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

8 _______________________________________________________________________________________

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