lm4898mmx National Semiconductor Corporation, lm4898mmx Datasheet - Page 12

• Manufacturer Part Number: lm4898mmx
• Description: 1 Watt Fully Differential Audio Power Amplifier With Shutdown Select
• Manufacturer: National Semiconductor Corporation

www.national.com
Application Information
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 sec-
tion.
A bridged configuration, such as the one used in theLM4898,
also creates a second advantage over single-ended amplifi-
ers. Since the differential outputs, Vo1 and Vo2,are biased at
half-supply, no net DC voltage exists across the load. This
assumes that the input resistor pair and the feedback resis-
tor pair are properly matched (see Proper Selection of Ex-
ternal 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 LM4898 include
increased power supply rejection ratio, common-mode noise
reduction, and click and pop reduction.
EXPOSED-DAP PACKAGE PCB MOUNTING
CONSIDERATIONS
The LM4898’s exposed-DAP (die attach paddle) package
(LD) provides 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 LM4898’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 LM4898 should be
5in
The last two area recommendations apply for 25˚C ambient
temperature. In all circumstances and conditions, the junc-
tion temperature must be held below 150˚C to prevent acti-
vating the LM4898’s thermal shutdown protection. The
LM4898’s power derating curve in the Typical Performance
Characteristics shows the maximum power dissipation ver-
sus temperature. Further detailed and specific information
concerning PCB layout, fabrication, and mounting an LLP
package is available from National Semiconductor’s pack-
age Engineering Group under application note AN-1187.
2
(min) for the same supply voltage and load resistance.
(Continued)
2
(min) area is
12
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
to1.37W. This problem of decreased load dissipation is ex-
acerbated as load impedance decreases. Therefore, to
maintain the highest load dissipation and widest output volt-
age 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
supply 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 amplifier, 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 LM4898 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 LM4898 does not require additional heatsinking
under most operating conditions and output loading. From
Equation 3, assuming a 5V power supply and an 8. load,the
maximum power dissipation point is 625mW. The maximum
power dissipation point obtained from Equation 3 must not
be greater than the power dissipation results from Equa-
tion4:
The LM4898’s θ
Depending on the ambient temperature, T
surroundings, Equation 4 can be used to find the maximum
internal power dissipation supported by the IC packaging. If
the result of Equation 3 is greater than that of Equation 4,
then either the supply voltage must be decreased, the load
impedance increased, the ambient temperature reduced, or
theθ
traces near the output, V
lower the θ
heatsinking allowing higher power dissipation. For the typical
application of a 5V power supply, with an 8Ω load, the
maximum ambient temperature possible without violating the
maximum junction temperature is approximately 30˚C pro-
vided that device operation is around the maximum power
dissipation point. Recall that internal power dissipation is a
JA
reduced with heatsinking. In many cases, larger
P
P
JA
DMAX
DMAX
. The larger areas of copper provide a form of
P
= 4*(V
JA
=(V
DMAX
in an MUA10A package is 190˚C/W.
DD
)
DD
= (T
2
DD
/(2π
)
2
, and GND pins can be used to
/(2π
JMAX
2
R
2
L
R
) Single-Ended
- T
L
) Bridge Mode
A
)/θ
JA
A
, of the system
(2)
(3)
(4)