5V, Differential Input, DirectDrive, 130mW
Stereo Headphone Amplifiers with Shutdown
such as the headphone socket used as a line output to
10kΩ
a home hi-fi system. In the upper diagram, any differ-
ence between the two GND references (represented by
V
) causes current to flow through the screen of
NOISE
1μF
10kΩ
cable between the two devices. This can cause noise
pickup at the receiver due to the potential divider
action of the audio screen cable impedance and the
GND wiring of the amplifier.
INR
AUDIO
INPUT
OUTR
10kΩ
10kΩ
Introducing impedance between the jack socket and
GND of the notebook helps (as shown in the lower dia-
gram). This has the following effect:
INL
Current flow (from GND potential differences) in the
cable screen is reduced, which is a safety issue.
OUTL
It allows the MAX9722A/MAX9722B differential sensing
to reduce the GND noise seen by the receiver (amplifier).
The other side effect is that the differential headphone
jack sensing corrects the headphone crosstalk (from
introducing the resistance on the jack GND return).
Only one channel is depicted in Figure 9.
MAX9722A
Figure 10. MAX9722 BTL Configuration
Figure 9 has some example numbers for resistance,
but the audio designer has control over only one series
resistance applied to the headphone jack return. Note
that this resistance can be bypassed for ESD purposes
at frequencies much higher than audio if required. The
upper limit for this added resistance is the amount of
output swing the headphone amplifier tolerates when
driving low-impedance loads. Any headphone return
current appears as a voltage across this resistor.
TOTAL HARMONIC DISTORTION PLUS
NOISE vs. OUTPUT VOLTAGE
100
10
V
= 5V
DD
A = -1V/V
V
OUTPUTS DRIVING
PIEZOELECTRIC SPEAKER
1
Piezoelectric Speaker Amplifier
Low-profile piezoelectric speakers can provide quality
sound for portable electronics. However, piezoelectric
speakers typically require large voltage swings
f = 1kHz
0.1000
0.0100
0.0010
0.0001
(>8V ) across the speaker element to produce usable
P-P
f = 20Hz
sound pressure levels. Power sources in portable
devices are usually low voltage in nature. Operating
from batteries, conventional amplifiers cannot provide
sufficient voltage swing to drive a piezoelectric speak-
er. However, the MAX9722’s DirectDrive architecture
can be configured to drive a piezoelectric speaker with
f = 100Hz
0
2
4
6
8
10
12
14
OUTPUT POWER (mW)
up to 12V
while operating from a single 5V supply.
P-P
Figure 11. MAX9722 THD+N vs. Output Voltage
The stereo MAX9722 features an inverting charge
pump that takes the positive +5V supply and creates a
negative -5V supply. Each output of the MAX9722 can
channel. The left channel is configured as a unity-gain
follower, inverting the output of the right channel and
driving the other leg of the speaker. Use precision
resistors to set the gain of the left channel to ensure low
distortion and good matching.
swing 6V . This may be sufficient to drive a piezo-
P-P
electric speaker. If a higher output voltage is desired,
configuring the MAX9722A as a bridge-tied load (BTL)
amplifier (Figure 10) doubles the maximum output
The MAX9722 was tested with a Panasonic WM-R57A
piezoelectric speaker, and the resulting THD+N curves
are shown (Figures 11 and 12). Note in both graphs, as
frequency increases, the THD+N increases. This is due
swing as seen by the load to 12V . In a BTL configu-
P-P
ration, the right channel of the MAX9722 serves as the
master amplifier, setting the gain of the device, driving
one side of the speaker, and providing signal to the left
14 ______________________________________________________________________________________
MAXIM [ MAXIM INTEGRATED PRODUCTS ]
