MAX9722A(2003) データシートの表示（PDF） - Maxim Integrated

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MAX9722A
(Rev.:2003)

5V, Differential Input, DirectDrive, 130mW Stereo Headphone Amplifiers with Shutdown

Maxim Integrated 

5V, Differential Input, DirectDrive, 130mW

Stereo Headphone Amplifiers with Shutdown

Shutdown

The MAX9722A/MAX9722B feature shutdown control

allowing audio signals to be shut down or muted.

Driving SHDN low disables the amplifiers and the

charge pump, sets the amplifier output impedance to

10kΩ, and reduces the supply current. In shutdown

mode, the supply current is reduced to 0.1µA. The

charge pump is enabled once SHDN is driven high.

Applications Information

Power Dissipation

Under normal operating conditions, linear power ampli-

fiers can dissipate a significant amount of power. The

maximum power dissipation for each package is given

in the Absolute Maximum Ratings section under

Continuous Power Dissipation or can be calculated by

the following equation:

PDISSPKG(MAX)

=

TJ(MAX) - TA

θJA

where TJ(MAX) is +145°C, TA is the ambient tempera-

ture, and θJA is the reciprocal of the derating factor in

°C/W as specified in the Absolute Maximum Ratings

section. For example, θJA of the thin QFN package is

+63.8°C/W, and 99.3°C/W for the TSSOP package.

The MAX9722A/MAX9722B have two power dissipation

sources: the charge pump and two amplifiers. If power

dissipation for a given application exceeds the maxi-

mum allowed for a particular package, either reduce

SVDD, increase load impedance, decrease the ambient

temperature, or add heatsinking to the device. Large

output, supply, and ground traces improve the maxi-

mum power dissipation in the package.

Thermal-overload protection limits total power dissipa-

tion in the MAX9722A/MAX9722B. When the junction

temperature exceeds +145°C, the thermal-protection

circuitry disables the amplifier output stage. The ampli-

fiers are enabled once the junction temperature cools

by 5°C. This results in a pulsing output under continu-

ous thermal-overload conditions.

Output Power

The device has been specified for the worst-case sce-

nario—when both inputs are in-phase. Under this con-

dition, the amplifiers simultaneously draw current from

the charge pump, leading to a slight loss in SVSS head-

room. In typical stereo audio applications, the left and

right signals have differences in both magnitude and

phase, subsequently leading to an increase in the max-

OUTPUT POWER vs. SUPPLY VOLTAGE

160

fIN = 1kHz

140 RL = 32Ω

THD+N = 10%

INPUTS 180°

OUT OF PHASE

120

100

80

60

INPUTS

IN PHASE

40

20

0

2.4 2.6 2.8 3.0 3.2 3.4 3.6 3.8 4.0

SUPPLY VOLTAGE (V)

Figure 6. Distortion Contributed by DC-Blocking Capacitors

imum attainable output power. Figure 6 shows the two

extreme cases for in- and out-of-phase. In reality, the

available power lies between these extremes.

Powering Other Circuits

from a Negative Supply

An additional benefit of the MAX9722A/MAX9722B is

the internally generated, negative supply voltage

(PVSS). This voltage provides the ground-referenced

output level. PVSS can, however, be used to power

other devices within a design limit current drawn from

PVSS to 5mA; exceeding this affects the headphone

amplifier operation. A typical application is a negative

supply to adjust the contrast of LCD modules.

PVSS is roughly proportional to PVDD and is not a regulat-

ed voltage. The charge-pump output impedance must be

taken into account when powering other devices from

PVSS. The charge-pump output impedance plot appears

in the Typical Operating Characteristics. For best results,

use 1µF charge-pump capacitors.

UVLO

The MAX9722A/MAX9722B feature an UVLO function

that prevents the device from operating if the supply

voltage is less than 2.2V (typ). This feature ensures

proper operation during brownout conditions and pre-

vents deep battery discharge. Once the supply voltage

reaches the UVLO threshold, the MAX9722A/

MAX9722B charge pump is turned on and the ampli-

fiers are powered.

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