lm4894itlx National Semiconductor Corporation, lm4894itlx Datasheet - Page 12

• Manufacturer Part Number: lm4894itlx
• Description: 1 Watt Fully Differential Audio Power Amplifier With Shutdown Select
• Manufacturer: National Semiconductor Corporation

www.national.com
Application Information
other. Bridged mode operation is different from the single-
ended amplifier configuration that connects the load be-
tween the amplifier output and ground. A bridged amplifier
design has distinct advantages over the single-ended con-
figuration: it provides differential drive to the load, thus dou-
bling 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 clip-
ping, please refer to the Audio Power Amplifier Design
section.
A bridged configuration, such as the one used in the
LM4894, 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 LM4894 include increased power supply rejection
ratio, common-mode noise reduction, and click and pop
reduction.
EXPOSED-DAP PACKAGE PCB MOUNTING
CONSIDERATIONS
The LM4894’s exposed-DAP (die attach paddle) package
(LD) 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. The result is a low voltage audio power amplifier that
produces 1.4W at ≤ 1% THD with a 4Ω load. This high power
is achieved through careful consideration of necessary ther-
mal design. Failing to optimize thermal design may compro-
mise the LM4894’s high power performance and activate
unwanted, though necessary, thermal shutdown protection.
The LD 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 4 (2x2) vias.
The via diameter should be 0.012in - 0.013in with a 0.050in
pitch. 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. If the heatsink and amplifier
share the same PCB layer, a nominal 2.5in
necessary for 5V operation with a 4Ω load. Heatsink areas
not placed on the same PCB layer as the LM4894 should be
5in
The last two area recommendations apply for 25˚C ambient
2
(min) for the same supply voltage and load resistance.
(Continued)
2
(min) area is
o1
and V
o2
,
12
temperature. In all circumstances and conditions, the junc-
tion temperature must be held below 150˚C to prevent acti-
vating the LM4894’s thermal shutdown protection. The
LM4894’s power de-rating curve in the Typical Performance
Characteristics shows the maximum power dissipation ver-
sus temperature. Example PCB layouts for the exposed-
DAP TSSOP and LLP packages are shown in the Demon-
stration Board Layout section. Further detailed and specific
information concerning PCB layout, fabrication, and mount-
ing an LLP package is available from National Semiconduc-
tor’s package Engineering Group under application note
AN1187.
PCB LAYOUT AND SUPPLY REGULATION
CONSIDERATIONS FOR DRIVING 3Ω AND 4Ω LOADS
Power dissipated by a load is a function of the voltage swing
across the load and the load’s impedance. As load imped-
ance decreases, load dissipation becomes increasingly de-
pendent on the interconnect (PCB trace and wire) resistance
between the amplifier output pins and the load’s connec-
tions. Residual trace resistance causes a voltage drop,
which results in power dissipated in the trace and not in the
load as desired. For example, 0.1Ω trace resistance reduces
the output power dissipated by a 4Ω load from 1.4W to
1.37W. This problem of decreased load dissipation is exac-
erbated as load impedance decreases. Therefore, to main-
tain the highest load dissipation and widest output voltage
swing, PCB traces that connect the output pins to a load
must be as wide as possible.
Poor power supply regulation adversely affects maximum
output power. A poorly regulated supply’s output voltage
decreases with increasing load current. Reduced supply
voltage causes decreased headroom, output signal clipping,
and reduced output power. Even with tightly regulated sup-
plies, trace resistance creates the same effects as poor
sup-ply regulation. Therefore, making the power supply
traces as wide as possible helps maintain full output voltage
swing.
POWER DISSIPATION
Power dissipation is a major concern when designing a
successful amplifer, whether the amplifier is bridged or
single-ended. Equation 2 states the maximum power dissi-
pation point for a single-ended amplifier operating at a given
supply voltage and driving a specified output load.
However, a direct consequence of the increased power de-
livered to the load by a bridge amplifier is an increase in
internal power dissipation versus a single-ended amplifier
operating at the same conditions.
Since the LM4894 has bridged outputs, the maximum inter-
nal power dissipation is 4 times that of a single-ended am-
plifier. Even with this substantial increase in power dissipa-
tion, the LM4894 does not require additional heatsinking
under most operating conditions and output loading. From
Equation 3, assuming a 5V power supply and an 8Ω load,
the maximum power dissipation point is 625mW. The maxi-
P
P
DMAX
DMAX
= 4*(V
=(V
DD
DD
)
2
/(2π
)
2
/(2π
2
R
2
L
R
) Single-Ended
L
) Bridge Mode
(2)
(3)