LM4923LQBD National Semiconductor, LM4923LQBD Datasheet - Page 9

• Manufacturer Part Number: LM4923LQBD
• Description: BOARD EVALUATION LM4923LQ
• Manufacturer: National Semiconductor
• Series: Boomer®r

Specifications of LM4923LQBD

• Amplifier Type: Class AB
• Output Type: 1-Channel (Mono)
• Max Output Power X Channels @ Load: 1.1W x 1 @ 8 Ohm
• Voltage - Supply: 2.3 V ~ 5.5 V
• Operating Temperature: -40°C ~ 85°C
• Board Type: Fully Populated
• Utilized Ic / Part: LM4923
• Lead Free Status / RoHS Status: Not applicable / Not applicable

Application Information
DIFFERENTIAL AMPLIFIER EXPLANATION
The LM4923 is a fully differential audio amplifier that features
differential input and output stages. Internally this is accom-
plished by two circuits: a differential amplifier and a common
mode feedback amplifier that adjusts the output voltages so
that the average value remains V
ferential gain, the amplifier can be considered to have
"halves". Each half uses an input and feedback resistor (R
and R
With R
half. This results in a differential gain of
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 oth-
er. 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 be-
tween 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
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 config-
uration: 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 sin-
gle-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 re-
fer 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 ampli-
F1
i1
) to set its respective closed-loop gain (see Figure 1).
= R
o1
i2
and R
Output Power vs Load Resistance
and V
F1
o2
A
= R
that are 180° out of phase with respect
VD
F2
= -R
, the gain is set at -R
F
/R
DD
i
/ 2. When setting the dif-
F
/ R
20071327
i
for each
(1)
i1
9
fiers. Since the differential outputs, V
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 cou-
pling capacitor required in single-supply, single-ended ampli-
fier 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 dam-
age. Further advantages of bridged mode operation specific
to fully differential amplifiers like the LM4923 include in-
creased 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 al-
lows rapid heat transfer from the die to the surrounding PCB
copper traces, ground plane and, finally, surrounding air. Fail-
ing to optimize thermal design may compromise the LM4923's
high power performance and activate unwanted, though nec-
essary, 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. 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 prac-
tical copper heat sink area. In all circumstances and condi-
tions, the junction temperature must be held below 150°C to
prevent activating the LM4923's thermal shutdown protection.
The LM4923's power de-rating curve in the Typical Perfor-
mance Characteristics shows the maximum power dissipa-
tion 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 LLP package is available from National
Supply Current Shutdown Voltage
o1
and V
o2
20071328
, are biased at
www.national.com