Datasheet

LM4923, LM4923LQBD
www.ti.com SNAS211E –JULY 2004–REVISED MAY 2013
APPLICATION INFORMATION
DIFFERENTIAL AMPLIFIER EXPLANATION
The LM4923 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
output voltages so that the average value remains V
DD
/ 2. When setting the differential gain, the amplifier can be
considered to have "halves". Each half uses an input and feedback resistor (R
i1
and R
F1
) to set its respective
closed-loop gain (see Figure 1). With R
i1
= R
i2
and R
F1
= R
F2
, the gain is set at -R
F
/ R
i
for each half. This results
in a differential gain of
A
VD
= -R
F
/R
i
(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
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
difference 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
o1
and V
o2
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. 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 LM4923, also creates a second advantage over single-
ended amplifiers. Since the differential outputs, V
o1
and V
o2
, 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 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 (WQFN) 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 WQFN 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. The via diameter should be 0.012in - 0.013in. Ensure
efficient thermal conductivity by plating-through and solder-filling the vias.
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