livered to the load. For example, with a 4

Ω load and 0.1Ω of

trace resistance in each output, output power at the load

drops from 2.1W to 2.0W

Output power is also dependent on supply regulation. To

keep the supply voltage from sagging under full output

power conditions, the supply traces should be as wide as

practical.

BRIDGE CONFIGURATION EXPLANATION

As shown in

Figure 1, the LM4873 has two pairs of opera-

tional amplifiers internally, allowing for a few different ampli-

fier configurations. The first amplifier’s gain is externally con-

figurable, 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 20 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 each

channel of the IC is

A

By driving the load differentially through outputs +OutA and

−OutA or +OutB and −OutB, an amplifier configuration com-

monly referred to as “bridged mode” is established. Bridged

mode operation is different from the classical single-ended

amplifier configuration where one side of its load is con-

nected 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 the output swing for a speci-

fied supply voltage. Four times the output power is possible

as compared to a single-ended amplifier under the same

conditions. This increase in attainable output power as-

sumes that the amplifier is not current limited or clipped. In

order to choose an amplifier’s closed-loop gain without caus-

ing excessive clipping, please refer to the Audio Power Am-

plifier Design section.

A bridge configuration, such as the one used in LM4873,

also creates a second advantage over single-ended amplifi-

ers. Since the differential outputs, +OutA, −OutA, +OutB,

and −OutB, are biased at half-supply, no net DC voltage ex-

ists across the load. This eliminates the need for an output

coupling capacitor which is required in a single supply,

single-ended amplifier configuration. If an output coupling

capacitor is not used in a single-ended configuration, the

half-supply bias across the load would result in both in-

creased internal IC power dissipation as well as permanent

loudspeaker damage.

POWER DISSIPATION

Whether the power amplifier is bridged or single-ended,

power dissipation is a major concern when designing the

amplifier. Equation 1 states the maximum power dissipation

point for a single-ended amplifier operating at a given supply

voltage and driving a specified load.

P

Single-Ended

(1)

However, a direct consequence of the increased power de-

livered to the load by a bridge amplifier is an increase in in-

ternal power dissipation. Equation 2 states the maximum

power dissipation point for a bridge amplifier operating at the

same given conditions.

P

Bridge Mode

(2)

Since the LM4873 is a dual channel power amplifier, the

maximum internal power dissipation is 2 times that of Equa-

tion 1 or Equation 2 depending on the mode of operation.

Even with this substantial increase in power dissipation, the

LM4873 does not require heatsinking. The power dissipation

from Equation 2, assuming a 5V power supply and an 8

Ω

load, must not be greater than the power dissipation that re-

sults from Equation 3:

P

(3)

For packages M16A and MTC20,

θ

package N16A,

θ

JA

= 63˚C/W. T

JMAX

= 150˚C for the

LM4873. Depending on the ambient temperature, T

system surroundings, Equation 3 can be used to find the

maximum internal power dissipation supported by the IC

packaging. If the result of Equation 2 is greater than that of

Equation 3, then either the supply voltage must be de-

creased, the load impedance increased, or the ambient tem-

perature reduced. For the typical application of a 5V power

supply, with an 8

Ω bridged load, the maximum ambient tem-

perature possible without violating the maximum junction

temperature is approximately 48˚C provided that device op-

eration is around the maximum power dissipation point and

assuming surface mount packaging. Internal power dissipa-

tion is a function of output power. If typical operation is not

around the maximum power dissipation point, the ambient

temperature can be increased. Refer to the Typical Perfor-

mance Characteristics curves for power dissipation infor-

mation for different output powers.

POWER SUPPLY BYPASSING

As with any power amplifier, proper supply bypassing is criti-

cal for low noise performance and high power supply rejec-

tion. The capacitor location on both the bypass and power

supply pins should be as close to the device as possible. The

effect of a larger half supply bypass capacitor is improved

PSRR due to increased half-supply stability. Typical applica-

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

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

nate the need for bypassing the supply nodes of the

LM4873. The selection of bypass capacitors, especially C

is thus dependent upon desired PSRR requirements, click

and pop performance 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

LM4873 contains a shutdown pin to externally turn off the

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

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

The trigger point between a logic low and logic high level is

typically half supply. It is best to switch between ground and

the supply V

switching the shutdown pin to V

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

will be disabled with shutdown pin voltages less than V

the idle current may be greater than the typical value of

0.7 µA. In either case, the shutdown pin should be tied to a

definite voltage to avoid unwanted state changes.

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 solu-

tion is to use a single-pole, single-throw switch in conjunction

with an external pull-up resistor. When the switch is closed,

the shutdown pin is connected to ground and enables the

amplifier. If the switch is open, then the external pull-up re-

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