LM4805 datasheet(11/18 Pages) NSC

11 / 18 page

Typical Performance Characteristics (Continued)

Maximum Duty Cycle

vs Temperature - ”X”

DS (ON)

vs Temperature

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DS (ON)

vs V

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Application Information

BRIDGE CONFIGURATION EXPLANATION

The Audio Amplifier portion of the LM4805 has two internal

amplifiers allowing different amplifier configurations. The first

amplifier’s gain is externally configurable, whereas the sec-

ond 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 Rf to Ri while the second amplifier’s

gain is fixed by the two internal 20k

Ω resistors. Figure 1

shows that the output of amplifier one serves as the input to

amplifier two. This results in both amplifiers producing sig-

nals identical in magnitude, but out of phase by 180˚. Con-

sequently, the differential gain for the Audio Amplifier is

VD = 2 *(Rf/Ri)

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 classic single-ended amplifier configura-

tion where one side of the load is connected to ground.

A bridge amplifier design has a few distinct advantages over

the single-ended configuration. It provides differential drive

to the load, thus doubling the output swing for a specified

supply voltage. Four times the output power is possible as

compared to a single-ended amplifier under the same con-

ditions. 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 ex-

cessive clipping, please refer to the Audio Power Amplifier

Design section.

The bridge configuration 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 cou-

pling capacitor, the half-supply bias across the load would

result in both increased internal IC power dissipation and

also possible loudspeaker damage.

AMPLIFIER 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 amplifier portion of the

LM4805 has two operational amplifiers, the maximum inter-

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