max9724c Maxim Integrated Products, Inc., max9724c Datasheet - Page 10

• Manufacturer Part Number: max9724c
• Description: Low Rf Susceptibility Directdrive Stereo Head- Phone Amplifier With 1.8v Compatible Shutdown
• Manufacturer: Maxim Integrated Products, Inc.

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 is demodulated and
amplified by the device in the chamber. Figure 2 shows
the signal level at the outputs of an unoptimized amplifi-
er and the MAX9724. The test conditions are shown in
Table 1.
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 MAX9724C/MAX9724D do not require output-
coupling capacitors, this problem does not arise.
Additionally, the MAX9724C/MAX9724D feature exten-
sive click-and-pop suppression that eliminates any audi-
ble transient sources internal to the device.
Low RF Susceptibility DirectDrive Stereo Head-
phone Amplifier with 1.8V Compatible Shutdown
Table 1. RF Susceptibility Test Conditions
Figure 2. RF Susceptibility of the MAX9724 and a Typical Headphone Amplifier
10
RF Field Strength
RF Modulation Type
RF Modulation Index
RF Modulation Frequency
______________________________________________________________________________________
TEST PARAMETER
Click-and-Pop Suppression
-100
-20
-40
-60
-80
40
20
0
100
MAX9724
62dB IMPROVEMENT
AT 850MHz
RF SUSCEPTIBLE
AMPLIFIER
Sine wave
SETTING
600
50V/m
100%
1kHz
RF CARRIER FREQUENCY (MHz)
39dB IMPROVEMENT
AT 900MHz
1100
67dB IMPROVEMENT
AT 1800MHz
1600
Typically, the output of the device driving the
MAX9724C/MAX9724D has a DC bias of half the supply
voltage. At startup, the input-coupling capacitor, C
charged to the preamplifier’s DC bias voltage through
the MAX9724C/MAX9724D input resistor, R
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 R
to eliminate clicks-and-pops caused by the input filter.
The MAX9724C/MAX9724D 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||R
MAX9724D). The amplifiers and charge pump are
enabled once SHDN is driven high.
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 T
ture, and θ
49dB IMPROVEMENT
AT 1900MHz
J(MAX)
2100
JA
P
DISSPKG MAX
is the reciprocal of the derating factor in
is +150°C, T
Applications Information
2600
(
)
=
A
T
J MAX
is the ambient tempera-
(
Power Dissipation
F
θ
JA
(R
)
−
F
T
IN
is 30kΩ for the
A
x 15kΩ x C
Shutdown
IN
, and a
IN
, is
IN