LM4876

SNAS054E – FEBRUARY 2000 – REVISED MAY 2013

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APPLICATION INFORMATION

BRIDGE CONFIGURATION EXPLANATION

closed-loop gain of Amp1, whereas two internal 40k

Ω resistors set Amp2's gain at -1. The LM4876 drives a load,

Figure 1 shows that the Amp1 output serves as the Amp2 input, which results in both amplifiers producing

signals identical in magnitude, but 180° out of phase. Taking advantage of this phase difference, a load is placed

differential gain of

(1)

Bridge mode is different from single-ended amplifiers that drive loads connected between a single amplifier's

output and ground. For a given supply voltage, bridge mode has a distinct advantage over the single-ended

configuration: its differential output doubles the voltage swing across the load. This results in four times the

output power when 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 that the output signal is not clipped. To ensure

minimum output signal clipping when choosing an amplifier's closed-loop gain, refer to the AUDIO POWER

AMPLIFIER DESIGN section.

Another advantage of the differential bridge output is no net DC voltage across the load. This results from biasing

require. Eliminating an output coupling capacitor in a single-ended configuration forces a single-supply amplifier's

half-supply bias voltage across the load. The current flow created by the half-supply bias voltage increases

internal IC power dissipation and may permanently damage loads such as speakers.

POWER DISSIPATION

Power dissipation is a major concern when designing a successful bridged or single-ended amplifier. Equation 2

states the maximum power dissipation point for a single-ended amplifier operating at a given supply voltage and

driving a specified output load.

(2)

However, a direct consequence of the increased power delivered to the load by a bridge amplifier is higher

internal power dissipation for the same conditions.

The LM4876 has two operational amplifiers in one package and the maximum internal power dissipation is four

times that of a single-ended amplifier. Equation 3 states the maximum power dissipation for a bridge amplifier.

However, even with this substantial increase in power dissipation, the LM4876 does not require heatsinking.

From Equation 3, assuming a 5V power supply and an 8

Ω load, the maximum power dissipation point is 633mW.

(3)

The maximum power dissipation point given by Equation 3 must not exceed the power dissipation given by

Equation 4:

(4)

Rearranging Equation 4 results in Equation 5. This equation gives the maximum ambient temperature that still

allows maximum power dissipation without violating the LM4876's maximum junction temperature.

(5)

For a typical application with a 5V power supply and an 8W load, the maximum ambient temperature that allows

maximum power dissipation without exceeding the maximum junction temperature is approximately 61°C.

(6)

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