
MAX4409
80mW, DirectDrive, Stereo Headphone
Amplifier with Common-Mode Sense
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Choose RIN according to the Gain-Setting Resistors sec-
tion. Choose the CIN such that f-3dB is well below the
lowest frequency of interest. Setting f-3dB too high
affects the low-frequency response of the amplifier. Use
capacitors whose dielectrics have low-voltage coeffi-
cients, such as tantalum or aluminum electrolytic.
Capacitors with high-voltage coefficients, such as
ceramics, may result in increased distortion at low fre-
quencies.
Charge-Pump Capacitor Selection
Use capacitors with an ESR less than 100m
Ω for opti-
mum performance. Low-ESR ceramic capacitors mini-
mize the output resistance of the charge pump. For
best performance over the extended temperature
range, select capacitors with an X7R dielectric. Table 1
lists suggested manufacturers.
Flying Capacitor (C1)
The value of the flying capacitor (C1) affects the load
regulation and output resistance of the charge pump. 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
resistance to an extent. See the Output Power vs.
Charge-Pump Capacitance and Load Resistance
graph in the Typical Operating Characteristics. Above
2.2F, the on-resistance of the switches and the ESR of
C1 and C2 dominate.
Output Capacitor (C2)
The output capacitor value and ESR directly affect the
ripple at PVSS. Increasing the value of C2 reduces out-
put ripple. Likewise, decreasing the ESR of C2 reduces
both ripple and output resistance. Lower capacitance
values can be used in systems with low maximum out-
put power levels. See the Output Power vs. Charge-
Pump Capacitance and Load Resistance graph in the
Typical Operating Characteristics.
Power-Supply Bypass Capacitor
The power-supply bypass capacitor (C3) lowers the out-
put impedance of the power supply, and reduces the
impact of the MAX4409’s charge-pump switching tran-
sients. Bypass PVDD with C3, the same value as C1, and
place it physically close to the PVDD and PGND pins.
Common-Mode Noise Rejection
Figure 6 shows a theoretical connection between two
devices, for example, a notebook computer (transmit-
ter, on the left) and an amplifier (receiver, on the right).
The application includes the headphone socket used
as a line output to a home hi-fi system, for example. In
the upper diagram, any difference between the two
GND references (represented by VNOISE) causes cur-
rent to flow through the screen of cable between the
two devices. This can cause noise pickup at the receiv-
er due to the potential divider action of the audio
screen cable impedance and the GND wiring of the
amplifier.
Introducing impedance between the jack socket and
GND of the notebook helps (as shown in the lower dia-
gram). This has the following effect:
Current flow (from GND potential differences) in the
cable screen is reduced, which is a safety issue.
It allows the MAX4409 differential sensing to reduce
the GND noise seen by the receiver (amplifier).
The other side effect is the differential HP jack sensing
corrects the headphone crosstalk (from introducing the
resistance on the jack GND return). Only one channel
is depicted in Figure 6.
Figure 6 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.
Layout and Grounding
Proper layout and grounding are essential for optimum
performance. Connect PGND and SGND together at a
single point on the PC board. Connect all components
associated with the charge pump (C2 and C3) to the
PGND plane. Connect PVDD and SVDD together at the
device. Connect PVSS and SVSS together at the
device. Bypassing of both supplies is accomplished by
charge-pump capacitors C2 and C3 (see Typical
Table 1. Suggested Capacitor Manufacturers
SUPPLIER
PHONE
FAX
WEBSITE
Taiyo Yuden
800-348-2496
847-925-0899
www.t-yuden.com
TDK
847-803-6100
847-390-4405
www.component.tdk.com
Note: Please indicate you are using the MAX4409 when contacting these component suppliers.