LM4900 National Semiconductor, LM4900 Datasheet - Page 13

• Manufacturer Part Number: LM4900
• Description: 265mW at 3.3V Supply Audio Power Amplifier with Shutdown Mode
• Manufacturer: National Semiconductor

Application Information
EXPOSED-DAP PACKAGE PCB MOUNTING CONSIDERATION
The LM4900’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 from the die to the surrounding PCB cop-
per traces, ground plane, and surrounding air. This allows
the LM4900LD to operate at higher output power levels in
higher ambient temperatures than the MM package. Failing
to optimize thermal design may compromise the 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, 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 2 vias. The via
diameter should be 0.012in - 0.013in with a 1.27mm pitch.
Ensure efficient thermal conductivity by plating through the
vias.
Best thermal performance is achieved with the largest prac-
tical heat sink area. The power derating curve in the Typical
Performance Characteristics shows the maximum power
dissipation versus temperature for several different areas of
heat sink area. Placing the majority of the heat sink area on
another plane is preferred as heat is best dissipated through
the bottom of the chip. Further detailed and specific informa-
tion concerning PCB layout, fabrication, and mounting an LD
(LLP) package is available from National Semiconductor’s
Package Engineering Group under application note AN1187.
BRIDGE CONFIGURATION EXPLANATION
As shown in Figure 1 , the LM4900 has two operational
amplifiers internally, allowing for a few different amplifier
configurations. The first amplifier’s gain is externally config-
urable, while the second amplifier is internally fixed in a
unity-gain, inverting configuration. The closed-loop gain of
the first amplifier is set by selecting the ratio of R
the second amplifier’s gain is fixed by the two internal 10 k
resistors. Figure 1 shows that the output of amplifier one
serves as the input to amplifier two which results in both
amplifiers producing signals identical in magnitude, but out
of phase 180˚. Consequently, the differential gain for the IC
is
By driving the load differentially through outputs V
an amplifier configuration commonly referred to as “bridged
mode” is established. Bridged mode operation is different
from the classical single-ended amplifier configuration where
one side of its load is connected to ground.
A bridge amplifier design has a few distinct advantages over
the single-ended configuration, as it provides differential
drive to the load, thus doubling output swing for a specified
supply voltage. Four times the output power is possible as
compared to a single-ended amplifier under the same con-
ditions. This increase in attainable output power assumes
that the amplifier is not current limited or clipped. In order to
A
VD
= 2
*
(R
F
/R
i
)
F
o1
to R
and V
i
while
o2
,
13
choose an amplifier’s closed-loop gain without causing ex-
cessive clipping, please refer to the Audio Power Amplifier
Design section.
A bridge configuration, such as the one used in LM4900,
also creates a second advantage over single-ended amplifi-
ers. Since the differential outputs, V
half-supply, no net DC voltage exists across the load. This
eliminates the need for an output coupling capacitor which is
required in a single supply, single-ended amplifier configura-
tion. If an output coupling capacitor is not used in a
single-ended configuration, the half-supply bias across the
load would result in both increased internal lC power dissi-
pation as well as permanent loudspeaker damage.
POWER DISSIPATION
Power dissipation is a major concern when designing a
successful amplifier, whether the amplifier is bridged or
single-ended. Equation 1 states the maximum power dissi-
pation point for a bridge 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 point for a bridge amplifier oper-
ating at the same conditions.
Since the LM4900 has two operational amplifiers in one
package, the maximum internal power dissipation is 4 times
that of a single-ended amplifier. Even with this substantial
increase in power dissipation, the LM4900 does not require
heatsinking. From Equation 1, assuming a 5V power supply
and an 8
625 mW. The maximum power dissipation point obtained
from Equation 2 must not be greater than the power dissi-
pation that results from Equation 3:
For package MUA08A,
the LM4900. Depending on the ambient temperature, T
the system surroundings, Equation 3 can be used to find the
maximum internal power dissipation supported by the IC
packaging. If the result of Equation 2 is greater than that of
Equation 3, then either the supply voltage must be de-
creased, the load impedance increased, the ambient tem-
perature reduced, or the
many cases larger traces near the output, V
pins can be used to lower the
provide a form of heatsinking allowing a higher power dissi-
pation. For the typical application of a 5V power supply, with
an 8
without violating the maximum junction temperature is ap-
proximately 30˚C provided that device operation is around
the maximum power dissipation point. Internal power dissi-
pation is a function of output power. If typical operation is not
around the maximum power dissipation point, the ambient
temperature can be increased. Refer to the Typical Perfor-
mance Characteristics curves for power dissipation infor-
mation for lower output powers.
P
P
load, the maximum ambient temperature possible
DMAX
DMAX
load, the maximum power dissipation point is
P
= (V
= 4(V
DMAX
DD
DD
= (T
)
2
)
/(2
2
/(2
JMAX
JA
2
JA
R
2
= 190˚C/W. T
R
L
)
− T
L
JA
reduced with heatsinking. In
)
. The larger areas of copper
A
)/
o1
Single-Ended (1)
Bridge Mode (2)
JA
and V
JMAX
o2
(3)
, are biased at
DD
= 150˚C for
www.national.com
, and Gnd
A
, of