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MAX9724A Datasheet(PDF) 10 Page - Maxim Integrated Products

Part No. MAX9724A
Description  60mW, DirectDrive, Stereo Headphone Amplifier with Low RF Susceptibility and Shutdown
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Maker  MAXIM [Maxim Integrated Products]
Homepage  http://www.maxim-ic.com
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MAX9724A Datasheet(HTML) 10 Page - Maxim Integrated Products

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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
voltage. At startup, the input-coupling capacitor, CIN, is
charged to the preamplifier’s DC bias voltage through
the MAX9724A/MAX9724B input resistor, RIN, and a
series 15kΩ resistor. This DC shift across the capacitor
results in an audible click-and-pop. Delay the rise of
SHDN 4 to 5 time constants based on RIN x 15kΩ x CIN
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:
where TJ(MAX) is +150°C, TA is the ambient tempera-
ture, and
θJA is the reciprocal of the derating factor in
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


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