LM4928

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SNAS330D – DECEMBER 2005 – REVISED FEBRUARY 2006

APPLICATION INFORMATION

DIFFERENTIAL AMPLIFIER EXPLANATION

The LM4928 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

This results 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, this would require input signals that are 180° out of phase with each other. The

LM4928 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 LM4928 simply amplifies the

difference between them.

All of these applications provide what is known as a "bridged mode" output (bridge-tied-load, BTL). This results in

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. In order to choose

an amplifier's closed-loop gain without causing excess clipping, please refer to the Audio Power Amplifier

Design section.

A bridged configuration, such as the one used in the LM4928, also creates a second advantage over single-

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 LM4928 include increased power supply rejection ratio,

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

EXPOSED-DAP PACKAGE PCB MOUNTING CONSIDERATIONS

The LM4928's exposed-DAP (die attach paddle) package (WSON) 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 LM4928's high power performance and activate unwanted, though necessary, thermal

shutdown protection. The WSON 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 at least 4 vias thermal via. The via diameter should be 0.012in - 0.013in.

Ensure efficient thermal conductivity by plating-through and solder-filling the vias.

Best thermal performance is achieved with the largest practical copper heat sink area. In all circumstances and

conditions, the junction temperature must be held below 150°C to prevent activating the LM4928's thermal

shutdown protection. The LM4928's power de-rating curve in the Typical Performance Characteristics shows the

maximum power dissipation versus temperature. Example PCB layouts are shown in the Demonstration Board

Layout section. Further detailed and specific information concerning PCB layout, fabrication, and mounting an

WSON package is available from Texas Instruments' package Engineering Group under application note AN1187

(SNOA401).

Copyright © 2005–2006, Texas Instruments Incorporated

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