max9722beue Maxim Integrated Products, Inc., max9722beue Datasheet - Page 9

• Manufacturer Part Number: max9722beue
• Description: 5v, Differential Input, Directdrive, 130mw Stereo Headphone Amplifiers With Shutdown Products
• Manufacturer: Maxim Integrated Products, Inc.

Conventional single-supply headphone amplifiers have
their outputs biased about a nominal DC voltage (typi-
cally half the supply) for maximum dynamic range.
Large coupling capacitors are needed to block this DC
bias from the headphone. Without these capacitors, a
significant amount of DC current flows to the head-
phone, resulting in unnecessary power dissipation and
possible damage to both the headphone and the head-
phone amplifier.
Maxim’s patented DirectDrive architecture uses a
charge pump to create an internal negative supply volt-
age, allowing the MAX9722A/MAX9722B outputs to be
biased about GND. With no DC component, there is no
need for the large DC-blocking capacitors. Instead of
two large (220µF, typ) tantalum capacitors, the
MAX9722A/MAX9722B charge pump requires two
small ceramic capacitors, conserving board space,
reducing cost, and improving the frequency response
of the headphone amplifier. See the Output Power vs.
Charge-Pump Capacitance and Load Resistance
graph in the Typical Operating Characteristics for
details of the possible capacitor sizes. There is a low
DC voltage on the amplifier outputs due to amplifier off-
set. However, the offset of the MAX9722A is typically
0.5mV, which, when combined with a 32Ω load, results
in less than 15.6µA of DC current flow to the head-
phones. Previous attempts to eliminate the output-cou-
pling capacitors involved biasing the headphone return
(sleeve) to the DC-bias voltage of the headphone
amplifiers. This method raises some issues:
• The sleeve is typically grounded to the chassis.
• During an ESD strike, the amplifier’s ESD structures
• When using the headphone jack as a line out to other
• When using a combination microphone and speaker
In addition to the cost and size disadvantages of the DC-
blocking capacitors required by conventional head-
Using this biasing approach, the sleeve must be iso-
lated from system ground, complicating product
design.
are the only path to system ground. Thus, the amplifi-
er must be able to withstand the full ESD strike.
equipment, the bias voltage on the sleeve may con-
flict with the ground potential from other equipment,
resulting in possible damage to the amplifiers.
headset, the microphone typically requires a GND
reference. The amplifier DC bias on the sleeve con-
flicts with the microphone requirements (Figure 3).
Stereo Headphone Amplifiers with Shutdown
5V, Differential Input, DirectDrive, 130mW
_______________________________________________________________________________________
Low-Frequency Response
DirectDrive
phone amplifiers, these capacitors limit the amplifier’s
low-frequency response and can distort the audio signal:
1) The impedance of the headphone load and the DC-
where R
C
The highpass filter is required by conventional single-
ended, single power-supply headphone amplifiers to
block the midrail DC-bias component of the audio sig-
nal from the headphones. The drawback to the filter is
that it can attenuate low-frequency signals. Larger val-
ues of C
larger, more expensive capacitors. Figure 4 shows the
relationship between the size of C
low-frequency attenuation. Note that the -3dB point for
a 16Ω headphone with a 100µF blocking capacitor is
100Hz, well within the normal audio band, resulting in
low-frequency attenuation of the reproduced signal.
Figure 3. Earbud Speaker/Microphone Combination Headset
Configuration
OUT
MICROPHONE
blocking capacitor form a highpass filter with the
-3dB point set by:
AMPLIFIER
is the value of the DC-blocking capacitor.
OUTPUT
AUDIO
AUDIO
INPUT
INPUT
OUT
L
is the impedance of the headphone and
reduce this effect but result in physically
f
-3
dB
HEADPHONE DRIVER
MICROPHONE
AMPLIFIER
=
MAX9722
2
π
R C
L OUT
1
MICROPHONE
OUT
BIAS
and the resulting
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