LM4960

SNAS221C – OCTOBER 2004 – REVISED MAY 2013

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APPLICATION INFORMATION

BRIDGE CONFIGURATION EXPLANATION

The Audio Amplifier portion of the LM4960 has two internal amplifiers allowing different amplifier configurations.

The first amplifier’s gain is externally configurable, whereas the second 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 2 shows that the output of amplifier one

serves as the input to amplifier two. This results in both amplifiers producing signals identical in magnitude, but

out of phase by 180°. Consequently, the differential gain for the Audio Amplifier 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 classic 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. 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 conditions. This increase in attainable

output power assumes that the amplifier is not current limited or clipped.

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 coupling 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 LM4960 has two operational amplifiers,

the maximum internal power dissipation is 4 times that of a single-ended amplifier. The maximum power

dissipation for a given BTL application can be derived from Equation (2).

where

•

(2)

BOOST CONVERTER POWER DISSIPATION

At higher duty cycles, the increased ON-time of the switch FET means the maximum output current will be

determined by power dissipation within the LM2731 FET switch. The switch power dissipation from ON-time

conduction is calculated by Equation (3).

where

•

DC is the duty cycle

(3)

There will be some switching losses as well, so some derating needs to be applied when calculating IC power

dissipation.

TOTAL POWER DISSIPATION

The total power dissipation for the LM4960 can be calculated by adding Equation (2) and Equation (3) together

to establish Equation (4):

(4)

The result from Equation (4) must not be greater than the power dissipation that results from Equation (5):

(5)

8

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