Applications Information

BTL Amplifier

The MAX9716/MAX9717 are designed to drive a load

differentially, a configuration referred to as bridge-tied

load or BTL. The BTL configuration (Figure 1) offers

advantages over the single-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 differential gain of the device is twice the

closed-loop gain of the input amplifier. The effective

gain is given by:

equations yields four times the output power due to

doubling of the output voltage:

There is no net DC voltage across the load because the

differential outputs are each biased at midsupply. This

eliminates the need for DC-blocking capacitors

required for single-ended amplifiers. These capacitors

can be large and expensive, consume board space,

and degrade low-frequency performance.

Power Dissipation and Heat Sinking

Under normal operating conditions, the MAX9716/

MAX9717 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:

and

specified in the Absolute Maximum Ratings section. For

example,

The increase in power delivered by the BTL configuration

directly results in an increase in internal power dissipation

over the single-ended configuration. The maximum power

following equation:

If the power dissipation for a given application exceeds

the maximum allowed for a given package, reduce

power dissipation by increasing the ground plane heat-

sinking capability and the size of the traces to the device

(see the Layout and Grounding section). Other methods

increase load impedance, decrease ambient tempera-

ture, reduce gain, or reduce input signal.

Thermal-overload protection limits total power dissipation

in the MAX9716/MAX9717. Thermal protection circuitry

disables the amplifier output stage when the junction

temperature exceeds +160°C. The amplifiers are

enabled once the junction temperature cools by 15°C. A

pulsing output under continuous thermal-overload condi-

tions results as the device heats and cools.

Fixed Gain

The MAX9717B, MAX9717C, and MAX9717D feature

internally fixed gains of 6dB, 9dB, and 12dB, respec-

tively (see the Selector Guide). Fixed gain simplifies

designs, reduces pin count, decreases required foot-

print size, and eliminates external gain-setting resistors.

Typical Operating Circuit are used to achieve each

fixed gain.

P

V

R

DISS MAX

CC

L

()

=

2

2

2

π

P

TT

DISSPKG MAX

J MAX

A

JA

()

()

=

−

θ

P

V

R

OUT

RMS

L

=

2

V

V

RMS

OUT P P

()

=

−

22

A

R

R

V

F

IN

=×

2

Low-Cost, Mono, 1.4W BTL Audio Power

Amplifiers

10

______________________________________________________________________________________

+1

-1

Figure 1. Bridge-Tied Load Configuration