Application Information

Demo Board Schematic

BRIDGE CONFIGURATION EXPLANATION

As shown in

Figure 1, the LM4876 has two operational am-

plifiers internally, allowing for a few different amplifier con-

figurations. The first amplifier’s gain is externally config-

urable, while the second amplifier is internally fixed in a

unity-gain, inverting configuration. The closed-loop gain of

the first amplifier is set by selecting the ratio of R

the second amplifier’s gain is fixed by the two internal 40 k

Ω

resistors.

Figure 1 shows that the output of amplifier one

serves as the input to amplifier two which results in both am-

plifiers producing signals identical in magnitude, but out of

phase 180˚. Consequently, the differential gain for the IC is

A

By driving the load differentially through outputs Vo1 and

Vo2, an amplifier configuration commonly referred to as

“bridged mode” is established. Bridged mode operation is

different from the classical single-ended amplifier configura-

tion where one side of its load is connected to ground.

A bridge amplifier design has a few distinct advantages over

the single-ended configuration, as it provides differential

drive to the load, thus doubling output swing for a specified

supply voltage. Four times the output power is possible as

compared to a single-ended amplifier under the same condi-

tions. This increase in attainable output power assumes that

the amplifier is not current limited or clipped. In order to

choose an amplifier’s closed-loop gain without causing ex-

cessive clipping, please refer to the Audio Power Amplifier

Design section.

A bridge configuration, such as the one used in LM4876,

also creates a second advantage over single-ended amplifi-

ers. Since the differential outputs, Vo1 and Vo2, are biased

at half-supply, no net DC voltage exists across the load. This

eliminates the need for an output coupling capacitor which is

required in a single supply, single-ended amplifier configura-

tion. Without an output coupling capacitor, the half-supply

bias across the load would result in both increased internal

IC power dissipation and also possible loudspeaker damage.

POWER DISSIPATION

Power dissipation is a major concern when designing a suc-

cessful amplifier, whether the amplifier is bridged or single-

ended. A direct consequence of the increased power deliv-

ered to the load by a bridge amplifier is an increase in

internal power dissipation. Equation 1 states the maximum

power dissipation point for a bridge amplifier operating at a

given supply voltage and driving a specified output load.

P

(1)

Since the LM4876 has two operational amplifiers in one

package, the maximum internal power dissipation is 4 times

that of a single-ended ampifier. Even with this substantial in-

crease in power dissipation, the LM4876 does not require

heatsinking under most operating conditions and output

loading. From Equation 1, assuming a 5V power supply and

an 8

Ω load, the maximum power dissipation point is

625 mW. The maximum power dissipation point obtained

from Equation 1 must not be greater than the power dissipa-

tion that results from Equation 2:

P

(2)

For package M08A,

θ

tion. T

creased by using some form of heat sinking. The resultant

θ

junction to case of the package,

θ

thermal resistance and

θ

mal resistance. By adding additional copper area around the

LM4876, the

θ

140˚C/W for package M08A. Depending on the ambient tem-

perature, T

maximum internal power dissipation supported by the IC

packaging. If the result of Equation 1 is greater than that of

Equation 2, then either the supply voltage must be de-

creased, the load impedance increased, the

θ

or the ambient temperature reduced. For the typical applica-

tion of a 5V power supply, with an 8

Ω load, and no additional

heatsinking, the maximum ambient temperature possible

without violating the maximum junction temperature is ap-

proximately 61˚C provided that device operation is around

the maximum power dissipation point and assuming surface

mount packaging. Internal power dissipation is a function of

output power. If typical operation is not around the maximum

power dissipation point, the ambient temperature can be in-

creased. Refer to the Typical Performance Characteris-

tics curves for power dissipation information for different out-

put powers and output loading.

POWER SUPPLY BYPASSING

As with any amplifier, proper supply bypassing is critical for

low noise performance and high power supply rejection. The

capacitor location on both the bypass and power supply pins

should be as close to the device as possible. Typical applica-

tions employ a 5V regulator with 10 µF and a 0.1 µF bypass

capacitors which aid in supply stability. This does not elimi-

nate the need for bypassing the supply nodes of the

LM4876. The selection of bypass capacitors, especially C

is dependent upon PSRR requirements, click and pop per-

formance as explained in the section, Proper Selection of

External Components, system cost, and size constraints.

SHUTDOWN FUNCTION

In order to reduce power consumption while not in use, the

LM4876 contains a shutdown pin to externally turn off the

amplifier’s bias circuitry. This shutdown feature turns the am-

plifier off when a logic low is placed on the shutdown pin. By

switching the shutdown pin to ground, the LM4876 supply

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