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MAX9724A Datasheet(PDF) 12 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) 12 Page - Maxim Integrated Products

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pump’s load regulation and output resistance. A C1
value that is too small degrades the device’s ability to
provide sufficient current drive, which leads to a loss of
output voltage. Increasing the value of C1 improves load
regulation and reduces the charge-pump output resis-
tance to an extent. See the Output Power vs. Load
Resistance and Charge-Pump Capacitor Size graph in
the
Typical Operating Characteristics. Above 1µF, the
on-resistance of the switches and the ESR of C1 and C2
dominate.
Hold Capacitor (C2)
The hold capacitor value (see the
Functional
Diagram/Typical Operating Circuits) and ESR directly
affect the ripple at PVSS. Increasing the value of C2
reduces output ripple. Likewise, decreasing the ESR of
C2 reduces both ripple and output resistance. Lower
capacitance values can be used in systems with low
maximum output power levels. See the Output Power
vs. Load Resistance and Charge-Pump Capacitor Size
graph in the
Typical Operating Characteristics.
Power-Supply Bypass Capacitor (C3)
The power-supply bypass capacitor (see the
Functional
Diagram/Typical Operating Circuits) lowers the output
impedance of the power supply, and reduces the
impact of the MAX9724A/MAX9724B’s charge-pump
switching transients. Bypass VDD with C3, the same
value as C1, and place it physically close to the VDD
and PGND pins.
Amplifier Gain
The gain of the MAX9724B amplifier is internally set to
-1.5V/V. All gain-setting resistors are integrated into the
device, reducing external component count. The inter-
nally set gain, in combination with DirectDrive, results in
a headphone amplifier that requires only five small
capacitors to complete the amplifier circuit: two for the
charge pump, two for audio input coupling, and one for
power-supply bypassing (see the
Functional
Diagram/Typical Operating Circuits).
The gain of the MAX9724A amplifier is set externally as
shown in Figure 3, the gain is:
AV = -RF/RIN (V/V)
Choose feedback resistor values in the tens of kΩ
range. Lower values may cause excessive power dissi-
pation and require impractically small values of RIN for
large gain settings. The high-impedance state of the
outputs can also be degraded during shutdown mode
if an inadequate feedback resistor is used since the
equivalent output impedance during shutdown is
14kΩ||Rf (RF is equal to 30kΩ for the MAX9724B). The
source resistance of the input device may also need to
be taken into consideration. Since the effective value of
RIN is equal to the sum of the source resistance of the
input device and the value of the input resistor connect-
ed to the inverting terminal of the headphone amplifier
(20kΩ for the MAX9724B), the overall closed-loop gain
of the headphone amplifier can be reduced if the input
resistor is not significantly larger than the source resis-
tance of the input device.
60mW, DirectDrive, Stereo Headphone Amplifier
with Low RF Susceptibility and Shutdown
12
______________________________________________________________________________________
LEFT
AUDIO
INPUT
RIGHT
AUDIO
INPUT
OUTL
INL
INR
OUTR
MAX9724A
RIN
RIN
RF
RF
Figure 3. Gain Setting for the MAX9724A


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