LM4941, LM4941SDBD, LM4941TMBD

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SNAS347C – JUNE 2006 – REVISED MAY 2013

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. Nevertheless, 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 features differential input and output stages. Internally this is

accomplished by two circuits: a differential amplifier and a common mode feedback amplifier that adjusts the

in a differential gain of

(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 External 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 because 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 that are 180° out of phase with 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 assumes that the amplifier is not current limited or clipped. Choose an amplifier's closed-loop gain without

causing excess clipping.

A bridged configuration, such as the one used in the LM4941, also creates a second advantage over single-

ended amplifiers. Since the differential outputs 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

configuration, 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 amplifier, whether the amplifier is bridged or

single-ended. 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 an increase

in internal power dissipation versus a single-ended amplifier operating at the same conditions.

(3)

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Product Folder Links: LM4941 LM4941SDBD LM4941TMBD