LM4861

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SNAS095C – MAY 1997 – REVISED MAY 2013

APPLICATION INFORMATION

BRIDGE CONFIGURATION EXPLANATION

As shown in Figure 2 , the LM4861 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. Consequently, 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 which will damage high frequency transducers

used in loudspeaker systems, please refer to AUDIO POWER AMPLIFIER DESIGN.

A bridge configuration, such as the one used in Boomer Audio Power Amplifiers, also creates a second

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 configuration. Without an output coupling capacitor in a single supply,

single-ended amplifier, the half-supply bias across the load would result in both increased internal IC power

dissipation and also permanent loudspeaker damage. An output coupling capacitor forms a high pass filter with

the load requiring that a large value such as 470

μF be used with an 8Ω load to preserve low frequency response.

This combination does not produce a flat response down to 20Hz, but does offer a compromise between printed

circuit board size and system cost, versus low frequency response.

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. Equation 3 states the maximum power dissipation point for a bridge

amplifier operating at a given supply voltage and driving a specified output load.

(2)

Since the LM4861 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 LM4861 does

not require heatsinking. From Equation 3, assuming a 5V power supply and an 8

Ω load, the maximum power

dissipation point is 625mW.The maximum power dissipation point obtained from Equation 3 must not be greater

than the power dissipation that results from Equation 3:

(3)

For the LM4861 surface mount package,

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

either the supply voltage must be decreased or the load impedance increased. 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 62.5°C provided that device operation is around the maximum power

dissipation point. Power dissipation is a function of output power and thus, if typical operation is not around the

maximum power dissipation point, the ambient temperature can be increased. Refer to the Typical Performance

Characteristics curves for power dissipation information for lower output powers.

Copyright © 1997–2013, Texas Instruments Incorporated

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