Application Information

BRIDGE CONFIGURATION EXPLANATION

As shown in Figure 1, the LM4878 has two operational

amplifiers internally, allowing for a few different amplifier

configurations. 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 10 k

Ω

resistors. Figure 1 shows that the output of amplifier one

serves as the input to amplifier two which results in both

amplifiers producing signals identical in magnitude, but out

of phase by 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 con-

ditions. 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 LM4878,

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

successful amplifier, whether the amplifier is bridged or

single-ended. A direct consequence of the increased power

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

internal power dissipation. Since the LM4878 has two opera-

tional amplifiers in one package, the maximum internal

power dissipation is 4 times that of a single-ended amplifier.

The maximum power dissipation for a given application can

be derived from the power dissipation graphs or from Equa-

tion 1.

P

(1)

It is critical that the maximum junction temperature T

150˚C is not exceeded. T

power derating curves by using P

area. By adding additional copper foil, the thermal resistance

of the application can be reduced from a free air value of

150˚C/W, resulting in higher P

can be added to any of the leads connected to the LM4878.

It is especially effective when connected to V

the output pins. Refer to the application information on the

LM4878 reference design board for an example of good heat

sinking. If T

JMAX

still exceeds 150˚C, then additional

changes must be made. These changes can include re-

duced supply voltage, higher load impedance, or reduced

ambient temperature. The National Reference Design board

using a 5V supply and an 8 ohm load will run in a 110˚C

ambient environement without exceeding T

power dissipation is a function of output power. Refer to the

Typical Performance Characteristics curves for power dis-

sipation information for different output 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 appli-

cations employ a 5V regulator with 10 µF Tantalum or elec-

trolytic capacitor and a 0.1 µF bypass capacitor which aid in

supply stability. This does not eliminate the need for bypass-

ing the supply nodes of the LM4878. The selection of a

bypass capacitor, especially C

requirements, click and pop performance as explained in the

section Proper Selection of External Components, sys-

tem cost, and size constraints.

SHUTDOWN FUNCTION

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

LM4878 contains a shutdown pin to externally turn off the

amplifier’s bias circuitry. This shutdown feature turns the

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

The shutdown pin on the LM4878 has an internal 54K resis-

tor connected to ground that enables the shutdown feature

even if the shutdown pin is not connected to ground. By

switching the shutdown pin to ground, the LM4878 supply

current draw will be minimized in idle mode. While the device

will be disabled with shutdown pin voltages less than

0.4V

value of 0.01 µA.

In many applications, a microcontroller or microprocessor

output is used to control the shutdown circuitry which pro-

vides a quick, smooth transition into shutdown. Another so-

lution is to use a single-pole, single-throw switch to V

When the switch is closed, the shutdown pin is connected to

V

that the shutdown pin will not float thus preventing unwanted

state changes. J1 operates the shutdown function as shown

in the Applications Circuit Figure 4. J1 must be installed to

operate the part. A switch may be installed in place of J1 for

easier evaluation of the shutdown function.

PROPER SELECTION OF EXTERNAL COMPONENTS

Proper selection of external components in applications us-

ing integrated power amplifiers is critical to optimize device

and system performance. While the LM4878 is tolerant of

external component combinations, consideration to compo-

nent values must be used to maximize overall system qual-

ity.

The LM4878 is unity-gain stable which gives a designer

maximum system flexibility. The LM4878 should be used in

low gain configurations to minimize THD+N values, and

maximize the signal to noise ratio. Low gain configurations

require large input signals to obtain a given output power.

Input signals equal to or greater than 1 Vrms are available

from sources such as audio codecs. Please refer to the

section, Audio Power Amplifier Design, for a more com-

plete explanation of proper gain selection.

Besides gain, one of the major considerations is the closed-

loop bandwidth of the amplifier. To a large extent, the band-

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