MAX9721 查看數據表（PDF） - Maxim Integrated

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MAX9721

1V, Fixed-Gain, DirectDrive, Stereo Headphone Amplifier with Shutdown

Maxim Integrated 

1V, Fixed-Gain, DirectDrive, Stereo Headphone

Amplifier with Shutdown

OUTPUT POWER vs. SUPPLY VOLTAGE

WITH INPUTS IN- AND OUT-OF-PHASE

50

45

fIN = 1kHz

RL = 16Ω

40 THD+N = 1%

35

INPUTS 180° OUT-OF-PHASE

30

25

20

15

10

INPUTS IN-PHASE

5

0

0.9

1.1

1.3

1.5

SUPPLY VOLTAGE (V)

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

of-Phase

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

voltage (PVSS) to provide the ground-referenced output

signal. Other devices can be powered from PVSS pro-

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

drawn from PVSS. Using PVSS as an LCD bias is a typi-

cal application for the negative supply.

PVSS is unregulated and proportional to VDD. Connect

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

operation.

Component Selection

Input Filtering

The AC-coupling capacitor (CIN) and an internal gain-

setting resistor form a highpass filter that removes any

DC bias from an input signal (see the Functional

Diagram). CIN allows the MAX9721 to bias the signal to

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

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

f-3dB =

2π

×

1

25kΩ

×

CIN

Choose CIN so f-3dB is well below the lowest frequency of

interest. Setting f-3dB too high affects the amplifier’s low-

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

ripple at PVSS. Increasing the value of C2 reduces rip-

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.

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