LM4992, LM4992SDBD

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SNAS220B – NOVEMBER 2003 – REVISED MAY 2013

APPLICATION INFORMATION

BRIDGE CONFIGURATION EXPLANATION

As shown in Figure 1, the LM4992 has two internal operational amplifiers per channel. The first amplifier's gain is

externally configurable , while the second amplifier is internally fixed in a unity-gain, inverting configuration. The

fixed by the two internal 20k

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

(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 LM4992, 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. The maximum internal power dissipation per channel 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)

and the PC board foil area. By adding copper foil, the thermal resistance of the application can be reduced from

the free air value of

activated. Additional copper foil can be added to any of the leads connected to the LM4992. It is especially

changes must be 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.

EXPOSED-DAP MOUNTING CONSIDERATIONS

The LM4992's exposed-DAP (die attach paddle) packages (NHK) provide a low thermal resistance between the

die and the PCB to which the part is mounted and soldered. This allows rapid heat transfer from the die to the

surrounding PCB copper area heatsink, copper traces, ground plane, and finally, surrounding air. The result is a

low voltage audio power amplifier that produces 1.07W dissipation per channel in an 8

Ω load at ≤ 1% THD+N.

This power is achieved through careful consideration of necessary thermal design. Failing to optimize thermal

design may compromise the LM4992's performance and activate unwanted, though necessary, thermal

shutdown protection.

Copyright © 2003–2013, Texas Instruments Incorporated

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