LM4879, LM4879MMBD, LM4879SDBD

www.ti.com

SNAS142F – MAY 2004 – REVISED OCTOBER 2004

APPLICATION INFORMATION

BRIDGE CONFIGURATION EXPLANATION

As shown in Figure 1, the LM4879 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 by 180°. Consequently, the differential gain for the IC is

(1)

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

configuration where one side of the 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 the AUDIO POWER AMPLIFIER DESIGN section.

A bridge configuration, such as the one used in LM4879, also creates a second advantage over single-ended

amplifiers. 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 configuration. 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 LM4879 has two operational 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 Equation 2.

(2)

Additional copper foil can be added to any of the leads connected to the LM4879. It is especially effective when

made. These changes can include reduced supply voltage, higher load impedance, or reduced ambient

temperature. Internal power dissipation is a function of output power. Refer to the TYPICAL PERFORMANCE

CHARACTERISTICS curves for power dissipation 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 applications employ a 5V regulator with 10 µF tantalum or electrolytic capacitor and a ceramic

bypass capacitor which aid in supply stability. This does not eliminate the need for bypassing the supply nodes of

and pop performance (as explained in the section, PROPER SELECTION OF EXTERNAL COMPONENTS),

system cost, and size constraints.

Copyright © 2004, Texas Instruments Incorporated

Submit Documentation Feedback

11

Product Folder Links: LM4879 LM4879MMBD LM4879SDBD