I

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200482F1

Application Information

DIFFERENTIAL AMPLIFIER EXPLANATION

The LM4915 is a pseudo-differential audio amplifier that

features a fixed gain of 6dB. Internally this is accomplished

by two separate sets of inverting amplifiers, each set to a

gain of 2. The LM4915 features precisely matched internal

gain-setting resistors set to R

Ω and R

Ω, thus

eliminating the need for external resistors and fixing the

differential gain at A

A differential amplifier works in a manner where the differ-

ence between the two input signals is amplified. In most

applications, this would require input signals that are 180˚

out of phase with each other. The LM4915 works in a

pseudo-differential manner, so DC offset normally cancelled

by a fully differential amplifier needs to be blocked by input

coupling capacitors for the LM4915 to amplify the difference

between the inputs.

The LM4915 provides what is known as a ’bridged mode’

output (bridge-tied-load, BTL). This results in output signals

at Vo1 and Vo2 that are 180˚ out of phase with respect to

each other. Bridged mode operation is different from the

single-ended amplifier configuration that connects the load

between the amplifier output and ground. A bridged amplifier

design has distinct advantages over the single-ended con-

figuration: it provides differential drive to the load, thus dou-

bling maximum possible output swing for a specific supply

voltage. Four times the output power is possible compared

with a single-ended amplifier under the same conditions.

This increase in attainable output power assumes that the

amplifier is not current limited or clipped. A bridged configu-

ration, such as the one used in the LM4915, 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. BTL configuration

eliminates the output coupling capacitor required in single-

supply, single-ended amplifier configurations. If an output

coupling capacitor is not used in a single-ended output con-

figuration, the half-supply bias across the load would result

in both increased internal IC power dissipation as well as

permanent loudspeaker damage.

POWER DISSIPATION

Power dissipation is a major concern when designing a

successful amplifer, whether the amplifier is bridged or

single-ended. Equation 1 states the maximum power dissi-

pation point for a single-ended amplifier operating at a given

supply voltage and driving a specified output load.

P

Single-Ended

(1)

However, a direct consequence of the increased power de-

livered to the load by a bridge amplifier is an increase in

internal power dissipation versus a single-ended amplifier

operating at the same conditions.

P

Bridge Mode

(2)

Since the LM4915 has bridged outputs, the maximum inter-

nal power dissipation is 4 times that of a single-ended am-

plifier.

Even with this substantial increase in power dissipation, the

LM4915 does not require additional heatsinking under most

operating conditions and output loading. From Equation 2,

assuming a 5V power supply and an 16

Ω load, the maximum

power dissipation point is 316mW. The maximum power

dissipation point obtained from Equation 2 must not be

greater than the power dissipation results from Equation 3:

P

θ

JA

(3)

The LM4915’s

θ

pending on the ambient temperature, T

surroundings, Equation 3 can be used to find the maximum

internal power dissipation supported by the IC packaging. If

the result of Equation 2 is greater than that of Equation 3,

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