LM4929

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SNAS293B – DECEMBER 2004 – REVISED APRIL 2013

APPLICATION INFORMATION

AMPLIFIER CONFIGURATION EXPLANATION

As shown in Figure 1, the LM4929 has three operational amplifiers internally. Two of the amplifier's have

externally configurable gain while the other amplifier is internally fixed at the bias point acting as a unity-gain

Consequently, the gain for each channel of the IC is

(1)

not require output coupling capacitors. The classical single-ended amplifier configuration where one side of the

load is connected to ground requires large, expensive output coupling capacitors.

A configuration such as the one used in the LM4929 has a major advantage over single supply, single-ended

load. This eliminates the need for output coupling capacitors which are required in a single-supply, single-ended

amplifier configuration. Without output coupling capacitors in a typical single-supply, single-ended amplifier, the

bias voltage is placed across the load resulting in both increased internal IC power dissipation and possible

loudspeaker damage.

The LM4929 eliminates these output coupling capacitors by running in OCL mode. Unless shorted to ground,

matches the bias voltage present on VoA and VoB outputs that drive the headphones. The headphones operate

in a manner similar to a bridge-tied load (BTL). Because the same DC voltage is applied to both headphone

speaker terminals this results in no net DC current flow through the speaker. AC current flows through a

headphone speaker as an audio signal's output amplitude increases on the speaker's terminal.

The headphone jack's sleeve is not connected to circuit ground when used in OCL mode. Using the headphone

presents no difficulty when the external equipment uses capacitively coupled inputs. For the very small minority

of equipment that is DC coupled, the LM4929 monitors the current supplied by the amplifier that drives the

headphone jack's sleeve. If this current exceeds 500mAPK, the amplifier is shutdown, protecting the LM4929 and

the external equipment.

POWER DISSIPATION

Power dissipation is a major concern when using any power amplifier and must be thoroughly understood to

ensure a successful design. When operating in capacitor-coupled mode, 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)

Since the LM4929 has three operational amplifiers in one package, the maximum power dissipation increases

due to the use of the third amplifier as a buffer and is given in Equation 3:

(3)

The maximum power dissipation point obtained from Equation 3 must not be greater than the power dissipation

that results from Equation 4:

(4)

For package DGS,

of the system surroundings, Equation 4 can be used to find the maximum internal power dissipation supported by

the IC packaging. If the result of Equation 3 is greater than that of Equation 4, then either the supply voltage

supply, with a 32

Ω load, the maximum ambient temperature possible without violating the maximum junction

temperature is approximately 144°C provided that device operation is around the maximum power dissipation

point. Thus, for typical applications, power dissipation is not an issue. Power dissipation is a function of output

power and thus, if typical operation is not around the maximum power dissipation point, the ambient temperature

may be increased accordingly. Refer to the Typical Performance Characteristics for power dissipation information

for lower output powers.

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