LM4864

SNAS109F – SEPTEMBER 1999 – REVISED MAY 2013

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APPLICATION INFORMATION

BRIDGE CONFIGURATION EXPLANATION

As shown in Figure 1, the LM4864 has two operational amplifiers internally, allowing for a few different amplifier

configurations. The first amplifier's gain is externally configurable, while the second amplifier is internally fixed in

amplifier one serves as the input to amplifier two which results in both amplifiers producing signals identical in

magnitude, but out of phase 180°. Consequently, the differential gain for the IC is

(1)

“bridged mode” is established. Bridged mode operation is different from the classical single-ended amplifier

configuration 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 conditions. 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 excessive clipping, please refer to AUDIO POWER AMPLIFIER DESIGN section.

A bridge configuration, such as the one used in LM4864, also creates a second advantage over single-ended

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 increased internal lC power dissipation as well as permanent

loudspeaker damage.

POWER DISSIPATION

Power dissipation is a major concern when designing a successful amplifier, whether the amplifier is bridged or

single-ended. Equation 2 states the maximum power dissipation point for a bridge amplifier operating at a given

supply voltage and driving a specified output load.

Single-Ended (1)

(2)

However, a direct consequence of the increased power delivered to the load by a bridge amplifier is an increase

in internal power dissipation point for a bridge amplifier operating at the same conditions.

Bridge Mode (2)

(3)

Since the LM4864 has two operational amplifiers in one package, the maximum internal power dissipation is 4

times that of a single-ended amplifier. Even with this substantial increase in power dissipation, the LM4864 does

not require heatsinking. From Equation 2, assuming a 5V power supply and an 8

Ω load, the maximum power

dissipation point is 633 mW. The maximum power dissipation point obtained from Equation 3 must not be greater

than the power dissipation that results from Equation 4:

(3)

(4)

For package DGK0008A,

107°C/W, and for package NGY0010A,

dissipation supported by the IC packaging. If the result of Equation 3 is greater than that of Equation 4, then

either the supply voltage must be decreased, the load impedance increased, the ambient temperature reduced,

or the

used to lower the

dissipation. For the typical application of a 5V power supply, with an 8

Ω load, the maximum ambient temperature

possible without violating the maximum junction temperature is approximately 44°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 increased. Refer to Typical Performance Characteristics for power

dissipation information for lower output powers.

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