LM4875

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SNAS042C – JANUARY 2002 – REVISED MAY 2013

APPLICATION INFORMATION

BRIDGE CONFIGURATION EXPLANATION

As shown in Figure 1, the LM4875 consists of two operational amplifiers internally. An external DC voltage sets

the closed-loop gain of the first amplifier, whereas two internal 20k

Ω resistors set the second amplifier's gain at -

1. The LM4875 can be used to drive a speaker connected between the two amplifier outputs or a monaural

Figure 1 shows that the output of Amp1 serves as the input to Amp2. This results in both amplifiers producing

signals that are identical in magnitude, but 180° out of phase.

referred to as “bridge mode“ ). This mode is different from single-ended driven loads that are connected between

a single amplifier's output and ground.

Bridge mode has a distinct advantage over the single-ended configuration: its differential drive to the load

doubles the output swing for a specified supply voltage. This results in four times the output power when

compared to a single-ended amplifier under the same conditions. This increase in attainable output assumes that

the amplifier is not current limited or the output signal is not clipped.

Another advantage of the differential bridge output is no net DC voltage across 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 1

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

driving a specified output load.

Single-Ended

(1)

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

in internal power dissipation point for a bridge amplifier operating at the same given conditions.

Bridge Mode

(2)

The LM4875 has two operational amplifiers in one package and the maximum internal power dissipation is 4

times that of a single-ended amplifier. However, even with this substantial increase in power dissipation, the

LM4875 does not require heatsinking. From Equation 2, assuming a 5V power supply and an 8

Ω load, the

maximum power dissipation point is 633 mW. The maximum power dissipation point obtained from Equation 2

must not be greater than the power dissipation that results from Equation 3:

(3)

For the SOIC package,

dissipation supported by the IC packaging. If the result of Equation 2 is greater than that of Equation 3, then

either decrease the supply voltage, increase the load impedance, or reduce the ambient temperature. For a

typical application using the SOIC packaged LM4875, a 5V power supply, and an 8

Ω load, the maximum ambient

temperature that does not violate the maximum junction temperature is approximately 55°C. The maximum

ambient temperature for the VSSOP package with the same conditions is approximately 30°C. These results

further assume that a device is a surface mount part operating around the maximum power dissipation point.

Since internal power dissipation is a function of output power, higher ambient temperatures are allowed as output

power decreases. Refer to the TYPICAL PERFORMANCE CHARACTERISTICS curves for power dissipation

information at lower output power levels.

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