MAX9750CEUI datasheet(19/30 Pages) MAXIM

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2.6W Stereo Audio Power Amplifiers and

DirectDrive Headphone Amplifiers

______________________________________________________________________________________

Applications Information

BTL Speaker Amplifiers

The MAX9750/MAX9751/MAX9755 feature speaker

amplifiers designed to drive a load differentially, a con-

figuration referred to as bridge-tied load (BTL). The BTL

configuration (Figure 9) offers advantages over the sin-

gle-ended configuration, where one side of the load is

connected to ground. Driving the load differentially

doubles the output voltage compared to a single-

ended amplifier under similar conditions. Thus, the

device’s differential gain is twice the closed-loop gain

of the input amplifier. The effective gain is given by:

Substituting 2 X VOUT(P-P) into the following equation

yields four times the output power due to double the

output voltage:

Since the differential outputs are biased at midsupply,

there is no net DC voltage across the load. This elimi-

nates the need for DC-blocking capacitors required for

single-ended amplifiers. These capacitors can be large

and expensive, can consume board space, and can

degrade low-frequency performance.

Power Dissipation and Heat Sinking

Under normal operating conditions, the MAX9750/

MAX9751/MAX9755 can dissipate a significant amount

of power. The maximum power dissipation for each

package is given in the Absolute Maximum Ratings

under Continuous Power Dissipation, or can be calcu-

lated by the following equation:

where TJ(MAX) is +150°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

+42°C/W. For optimum power dissipation, the exposed

paddle of the package should be connected to the

ground plane (see the Layout and Grounding section).

Output Power (Speaker Amplifier)

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:

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 heatsinking to the device. Large

output, supply, and ground PC board traces improve the

maximum power dissipation in the package.

DISS MAX

() =

DISSPKG MAX

J MAX

()

−

RMS

OUT P P

OUT

RMS

−

()

=×

VOUT(P-P)

2 x VOUT(P-P)

VOUT(P-P)

-1

Figure 9. Bridge-Tied Load Configuration

OUTPUT POWER (mW)

125

100

0.01

0.1

100

1000

0.001

150

VDD = 5V

RL = 16

Ω

AV = 3dB

OUTPUTS IN PHASE

OUTPUTS 180

° OUT OF PHASE

Figure 10. Total Harmonic Distortion Plus Noise vs. Output Power

with Inputs In/Out of Phase (Headphone Mode)

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