Application Information

OPTIMIZING RF IMMUNITY

The internal circuitry of the LM4941 suppresses the amount

of RF signal that is coupled into the chip. However, certain

external factors, such as output trace length, output trace

orientation, distance between the chip and the antenna,

antenna strength, speaker type, and type of RF signal, may

affect the RF immunity of the LM4941. In general, the RF

immunity of the LM4941 is application specific. Neverthe-

less, optimal RF immunity can be achieved by using short

output traces and increasing the distance between the

LM4941 and the antenna.

DIFFERENTIAL AMPLIFIER EXPLANATION

The LM4941 is a fully differential audio amplifier that fea-

tures differential input and output stages. Internally this is

accomplished by two circuits: a differential amplifier and a

common mode feedback amplifier that adjusts the output

voltages so that the average value remains V

setting the differential gain, the amplifier can be considered

to have "halves". Each half uses an input and feedback

resistor (R

(see Figure 1). With R

at -R

A

(1)

It is extremely important to match the input resistors to each

other, as well as the feedback resistors to each other for best

amplifier performance. See the Proper Selection of Exter-

nal Components section for more information. A differential

amplifier works in a manner where the difference between

the two input signals is amplified. In most applications, input

signals will be 180˚ out of phase with each other. The

LM4941 can be used, however, as a single ended input

amplifier while still retaining its fully differential benefits be-

cause it simply amplifies the difference between the inputs.

All of these applications provide what is known as a "bridged

mode" output (bridge-tied-load, BTL). This results in output

signals at V

respect to each other. Bridged mode operation is different

from the single-ended amplifier configuration that connects

the load between the amplifier output and ground. A bridged

amplifier design has distinct advantages over the single-

ended configuration: it provides differential drive to the load,

thus doubling maximum possible output swing for a specific

supply voltage. Four times the output power is possible

compared with a single-ended amplifier under the same

conditions. This increase in attainable output power as-

sumes that the amplifier is not current limited or clipped. In

order to choose an amplifier’s closed-loop gain without caus-

ing excess clipping, please refer to the Audio Power Am-

plifier Design section.

A bridged configuration, such as the one used in the

LM4941, also creates a second advantage over single-

ended amplifiers. Since the differential outputs, V

are biased at half-supply, no net DC voltage exists across

the load. This assumes that the input resistor pair and the

feedback resistor pair are properly matched (see Proper

Selection of External Components). BTL configuration

eliminates the output coupling capacitor required in single-

supply, single-ended amplifier configurations. If an output

coupling capacitor is not used in a single-ended output con-

figuration, the half-supply bias across the load would result

in both increased internal IC power dissipation as well as

permanent loudspeaker damage. Further advantages of

bridged mode operation specific to fully differential amplifiers

like the LM4941 include increased power supply rejection

ratio, common-mode noise reduction, and click and pop

reduction.

POWER DISSIPATION

Power dissipation is a major concern when designing a

successful amplifer, whether the amplifier is bridged or

single-ended. Equation 2 states the maximum power dissi-

pation point for a single-ended amplifier operating at a given

supply voltage and driving a specified output load.

P

(2)

However, a direct consequence of the increased power de-

livered to the load by a bridge amplifier is an increase in

internal power dissipation versus a single-ended amplifier

operating at the same conditions.

P

(3)

Since the LM4941 has bridged outputs, the maximum inter-

nal power dissipation is 4 times that of a single-ended am-

plifier. Even with this substantial increase in power dissipa-

tion, the LM4941 does not require additional heatsinking

under most operating conditions and output loading. From

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

Ω load,

the maximum power dissipation point is 625mW. The maxi-

mum power dissipation point obtained from Equation 3 must

not be greater than the power dissipation results from Equa-

tion 4:

P

θ

JA

(4)

The LM4941’s

θ

Depending on the ambient temperature, T

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 must be decreased, the load

impedance increased, the ambient temperature reduced, or

the

θ

traces near the output, V

lower the

θ

heatsinking allowing higher power dissipation. For the typical

application of a 5V power supply, with an 8

Ω load, the

maximum ambient temperature possible without violating the

maximum junction temperature is approximately 110˚C pro-

vided that device operation is around the maximum power

dissipation point. Recall that internal power dissipation is a

function of output power. If typical operation is not around the

maximum power dissipation point, the LM4941 can operate

at higher ambient temperatures. Refer to the Typical Per-

formance Characteristics curves for power dissipation in-

formation.

POWER SUPPLY BYPASSING

As with any power amplifier, proper supply bypassing is

critical for low noise performance and high power supply

rejection ratio (PSRR). The capacitor location on both the

bypass and power supply pins should be as close to the

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