Output Power vs Load Resistance

Supply Current Shutdown Voltage

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Application Information

DIFFERENTIAL AMPLIFIER EXPLANATION

The LM4923 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, this

would require input signals that are 180˚ out of phase with

each other. The LM4923 can be used, however, as a single

ended input amplifier while still retaining its fully differential

benefits. In fact, completely unrelated signals may be placed

on the input pins. The LM4923 simply amplifies the differ-

ence between them.

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

LM4923, 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 LM4923 include increased power supply rejection

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

reduction.

EXPOSED-DAP PACKAGE PCB MOUNTING

CONSIDERATIONS

The LM4923’s exposed-DAP (die attach paddle) package

(LLP) provide a low thermal resistance between the die and

the PCB to which the part is mounted and soldered. This

allows rapid heat transfer from the die to the surrounding

PCB copper traces, ground plane and, finally, surrounding

air. Failing to optimize thermal design may compromise the

LM4923’s high power performance and activate unwanted,

though necessary, thermal shutdown protection. The LLP

package must have its DAP soldered to a copper pad on the

PCB. The DAP’s PCB copper pad is connected to a large

plane of continuous unbroken copper. This plane forms a

thermal mass and heat sink and radiation area. Place the

heat sink area on either outside plane in the case of a

two-sided PCB, or on an inner layer of a board with more

than two layers. Connect the DAP copper pad to the inner

layer or backside copper heat sink area with a thermal via.

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