LM4836

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SNAS045F – JUNE 1999 – REVISED MAY 2013

Figure 3 shows that Amp1A's output serves as Amp2A's input. This results in both amplifiers producing signals

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

between

−OUTA and +OUTA and driven differentially (commonly referred to as “bridge mode”). This results in a

differential gain of

(1)

Bridge mode amplifiers are 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 produces 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 is accomplished by

biasing channel A's and channel B's outputs at half-supply. This eliminates the coupling capacitor that single

supply, single-ended amplifiers require. Eliminating an output coupling capacitor in a single-ended configuration

forces a single-supply amplifier's half-supply bias voltage across the load. This 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 single-ended or bridged 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 LM4836 has two operational amplifiers per channel. The maximum internal power dissipation per channel

operating in the bridge mode is four times that of a single-ended amplifier. From Equation 3, assuming a 5V

power supply and a 4

Ω load, the maximum single channel power dissipation is 1.27W or 2.54W for stereo

operation.

(3)

The LM4836's power dissipation is twice that given by Equation 2 or Equation 3 when operating in the single-

ended mode or bridge mode, respectively. Twice the maximum power dissipation point given by Equation 3 must

not exceed the power dissipation given by Equation 4:

(4)

2

on a PCB, the LM4836's

the maximum internal power dissipation supported by the IC packaging. Rearranging Equation 4and substituting

maximum stereo power dissipation without violating the LM4836's maximum junction temperature.

(5)

For a typical application with a 5V power supply and an 4

Ω load, the maximum ambient temperature that allows

maximum stereo power dissipation without exceeding the maximum junction temperature is approximately 99°C

for the LQ package and 45°C for the MTE package.

(6)

maximum junction temperature by reducing the power supply voltage or increasing the load resistance. Further

allowance should be made for increased ambient temperatures.

The above examples 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 or duty cycle decreases.

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