80mW, DirectDrive, Stereo Headphone

Amplifier with Common-Mode Sense

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13

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 100mΩ 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

2.2µF, 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

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-

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

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

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.