LM4819, LM4819MBD

SNAS133D – FEBRUARY 2001 – REVISED MARCH 2013

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APPLICATION INFORMATION

BRIDGE CONFIGURATION EXPLANATION

closed-loop gain of the first amplifier (and the amplifier overall), whereas two internal 20k

Ω resistors set the

second amplifier's gain at -1. The LM4819 is typically used to drive a speaker connected between the two

amplifier outputs.

Figure 1 shows that the output of Amp1 servers as the input to Amp2, 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 assumes that the amplifier is not current limited or 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

Example 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 load.

(2)

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

in the internal power dissipation point for a bridge amplifier operating at the same given conditions. Equation 3

states the maximum power dissipation point for a bridged amplifier operating at a given supply voltage and

driving a specified load.

(3)

The LM4819 has two operational amplifiers in one package and the maximum internal power dissipation is four

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

Lm4819 does not require heatsinking. From Equation 3, assuming a 5V power supply and an 8

Ω load, the

maximum power dissipation point is 633mW. The maximum power dissipation point obtained from Equation 3

must not exceed the power dissipation predicted by Equation 4:

(4)

For the micro DGK0008A package,

power dissipation supported by the IC packaging. If the result of Equation 3 is greater than the result of

Equation 4, then decrease the supply voltage, increase the load impedance, or reduce the ambient temperature.

For a typical application using the D0008A packaged LM4819 with a 5V power supply and an 8

Ω load, the

maximum ambient temperature that does not violate the maximum junction temperature is approximately 42°C. If

a DGK0008A packaged part is used instead with the same supply voltage and load, the maximum ambient

temperature is 17°C. In both cases, it is assumed that a device is a surface mount part operating around the

maximum power dissipation point. The assumption that the device is operating around the maximum power

dissipation point is incorrect for an 8

Ω load. The maximum power dissipation point occurs when the output power

is equal to the maximum power dissipation or 50% efficiency. The LM4819 is not capable of the output power

level (633mW) required to operate at the maximum power dissipation point for an 8

Ω load. To find the maximum

power dissipation, the graph Figure 22 must be used. From the graph, the maximum power dissipation for an 8

Ω

10

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