MAX9711 Просмотр технического описания (PDF) - Maxim Integrated

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MAX9711

3W Mono/Stereo BTL Audio Power Amplifiers with Shutdown

Maxim Integrated 

3W Mono/Stereo BTL Audio Power Amplifiers

with Shutdown

Power Dissipation and Heat Sinking

Under normal operating conditions, the MAX9710/

MAX9711 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) −

θJA

TA

where TJ(MAX) is +150°C, TA is the ambient temperature,

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 20-pin thin QFN package is

48.1°C/W.

The increase in power delivered by the BTL configura-

tion directly results in an increase in internal power dis-

sipation over the single-ended configuration. The

maximum power dissipation for a given VDD and load is

given by the following equation:

PDISS(MAX)

=

2VDD2

π 2RL

If the power dissipation for a given application exceeds

the maximum allowed for a given package, either reduce

VDD, increase load impedance, decrease the ambient

temperature, or add heat sinking to the device (see

Layout and Grounding section). Large output, supply,

and ground PC board traces improve the maximum

power dissipation in the package.

Thermal-overload protection limits total power dissipa-

tion in the MAX9710/MAX9711. When the junction

temperature exceeds +160°C, the thermal protection

circuitry disables the amplifier output stage. The

amplifiers are enabled once the junction temperature

cools by 15°C. A pulsing output under continuous

thermal-overload conditions results as the device heats

and cools.

Component Selection

Gain-Setting Resistors

External feedback components set the gain of both

devices. Resistors RF and RIN (Functional Diagram) set

the gain of the amplifier as follows:

AVD

=

2

×

RF

RIN

Input Filter

The input capacitor (CIN), in conjunction with RIN, forms

a highpass filter that removes the DC bias from an

incoming signal. The AC-coupling capacitor allows the

amplifier to bias the signal to an optimum DC level.

Assuming zero-source impedance, the -3dB point of

the highpass filter is given by:

f

−3dB

=

1

2πRINCIN

For optimum click-and-pop suppression, choose:

RIN x CIN < RBIAS x CBIAS

where RBIAS = 50kΩ.

Setting f-3dB too high affects the low-frequency

response of the amplifier. Use capacitors with

dielectrics that have low-voltage coefficients, such as

tantalum or aluminum electrolytic. Capacitors with high-

voltage coefficients, such as ceramics, may result in an

increase of distortion at low frequencies.

BIAS Capacitor

BIAS is the output of the internally generated 2.5VDC

bias voltage. The BIAS bypass capacitor, CBIAS,

improves PSRR and THD+N by reducing power supply

and other noise sources at the common-mode bias

node, and also generates the clickless/popless startup

DC bias waveform for the speaker amplifiers. Bypass

BIAS with a 1µF capacitor to GND. Smaller values of

CBIAS produce faster tON/tOFF times but may result in

increased click/pop levels.

Supply Bypassing

Proper power-supply bypassing ensures low-noise,

low-distortion performance. Place a 0.1µF ceramic

capacitor from VDD to PGND. Add additional bulk

capacitance as required by the application. Locate the

bypass capacitor as close to the device as possible.

Piezoelectric Speaker Driver

Low-profile piezoelectric speakers can provide quality

sound for portable electronics. However, piezoelectric

speakers typically require large voltage swings

(>8VP-P) across the speaker element to produce

audible sound pressure levels. The MAX9711 can be

configured to drive a piezoelectric speaker with up to

10VP-P while operating from a single 5V supply.

Figure 2 shows the THD+N of the MAX9711 driving a

piezoelectric speaker. Note that as frequency increas-

es, the THD+N increases. This is due to the capacitive

nature of the piezoelectric speaker; as frequency

increases, the speaker impedance decreases, resulting

in a larger current draw from the amplifier.

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