Output Power

The MAX9721’s output power increases when the left

and right audio signals differ in magnitude and/or

phase. Figure 4 shows the two extreme cases for in-

and out-of-phase input signals. The output power of a

typical stereo application lies between the two extremes

shown in Figure 4. The MAX9721 is specified to output

20mW per channel when both inputs are in-phase.

Powering Other Circuits from

the Negative Supply

The MAX9721 internally generates a negative supply

vided the current drawn from the charge pump does

not exceed 1mA. Headphone driver output power and

THD+N will be adversely affected if more than 1mA is

cal application for the negative supply.

a 1µF capacitor from C1P to C1N for best charge-pump

operation.

Component Selection

Input Filtering

setting resistor form a highpass filter that removes any

DC bias from an input signal (see the Functional

an optimum DC level. The -3dB point of the highpass

filter, assuming zero-source impedance, is given by:

frequency response. Use capacitors with low-voltage

coefficient dielectrics. Film or C0G dielectric capacitors

are good choices for AC-coupling capacitors. Capacitors

with high-voltage coefficients, such as ceramics, can

result in increased distortion at low frequencies.

Charge-Pump Capacitor Selection

Use capacitors with less than 100mΩ of ESR. Low-ESR

ceramic capacitors minimize the output impedance of the

charge pump. Capacitors with an X7R dielectric provide

the best performance over the extended temperature

range. Table 1 lists suggested capacitor manufacturers.

Flying Capacitor (C1)

The value of C1 affects the charge pump’s load regula-

tion and output impedance. Choosing C1 too small

degrades the MAX9721’s ability to provide sufficient

current drive and leads to a loss of output voltage.

Increasing the value of C1 improves load regulation

and reduces the charge-pump output impedance. See

the Output Power vs. Charge-Pump Capacitance and

Load Impedance graph in the Typical Operating

Characteristics.

Hold Capacitor (C2)

The hold capacitor’s value and ESR directly affect the

ple. Choosing a capacitor with lower ESR reduces rip-

ple and output impedance. Lower capacitance values

can be used in systems with low maximum output

power levels. See the Output Power vs. Charge-Pump

Capacitance and Load Impedance graph in the Typical

Operating Characteristics.

f

2

25k

C

-3dB

IN

=

××

1

πΩ

1V, Fixed-Gain, DirectDrive, Stereo Headphone

Amplifier with Shutdown

_______________________________________________________________________________________

9

Figure 4. Output Power vs. Supply Voltage with Inputs In-/Out-

of-Phase

0

15

10

5

20

25

30

35

40

45

50

0.9

1.1

1.3

1.5

OUTPUT POWER vs. SUPPLY VOLTAGE

WITH INPUTS IN- AND OUT-OF-PHASE

SUPPLY VOLTAGE (V)

THD+N = 1%

INPUTS 180° OUT-OF-PHASE

INPUTS IN-PHASE