LM4863
Non-MTE Specific Characteristics
Power Supply
Rejection Ratio
(Continued)
Supply Current vs
Supply Voltage
Open Loop
Frequency Response
DS012881-21
DS012881-22
DS012881-23
Application Information
EXPOSED-DAP MOUNTING CONSIDERATIONS
The exposed-DAP must be connected to ground. The
exposed-DAP package of the LM4863MTE requires special
attention to thermal design. If thermal design issues are not
properly addressed, an LM4863MTE driving 4Ω will go into
thermal shutdown.
The exposed-DAP on the bottom of the LM4863MTE should
be soldered down to a copper pad on the circuit board. Heat
is conducted away from the exposed-DAP by a copper
plane. If the copper plane is not on the top surface of the cir-
cuit board, 8 to 10 vias of 0.013 inches or smaller in diameter
should be used to thermally couple the exposed-DAP to the
plane. For good thermal conduction, the vias must be
plated-through and solder-filled.
The copper plane used to conduct heat away from the
exposed-DAP should be as large as pratical. If the plane is
on the same side of the circuit board as the exposed-DAP,
2.5in
2
is the minimum for 5V operation into 4Ω. If the heat
sink plane is buried or not on the same side as the exposed-
DAP, 5in
2
is the minimum for 5V operation into 4Ω. If the am-
bient temperature is higher than 25˚C, a larger copper plane
or forced-air cooling will be required to keep the
LM4863MTE junction temperature below the thermal shut-
down temperature (150˚C). See the power derating curve for
the LM4863MTE for derating information.
The LM4863MTE requires forced-air cooling when operating
into 3Ω. With the part attached to 2.5in
2
of exposed copper,
with a 3Ω load, and with an ambient temperature of 25˚C,
450 linear-feet per minute kept the part out of thermal shut-
down. In higher ambient temperatures, higher airflow rates
and/or larger copper areas will be required to keep the part
out of thermal shutdown.
See
DEMOBOARD CIRCUIT LAYOUT
for an example of an
exposed-DAP TSSOP circuit board layout.
3Ω & 4Ω LAYOUT CONSIDERATIONS
With low impedance loads, the output power at the loads is
heavily dependent on trace resistance from the output pins
of the LM4863. Traces from the output of the LM4863MTE to
the load or load connectors should be as wide as practical.
Any resistance in the output traces will reduce the power de-
livered to the load. For example, with a 4Ω load and 0.1Ω of
trace resistance in each output, output power at the load
drops from 2.2W to 2.0W
Output power is also dependent on supply regulation. To
keep the supply voltage from sagging under full output
power conditions, the supply traces should be as wide as
practical.
BRIDGE CONFIGURATION EXPLANATION
As shown in
Figure 1,
the LM4863 has two pairs of opera-
tional amplifiers internally, allowing for a few different ampli-
fier configurations. The first amplifier’s gain is externally con-
figurable, 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
f
to R
i
while
the second amplifier’s gain is fixed by the two internal 20 kΩ
resistors.
Figure 1
shows that the output of amplifier one
serves as the input to amplifier two which results in both am-
plifiers producing signals identical in magnitude, but out of
phase 180˚. Consequently, the differential gain for each
channel of the IC is
A
VD
= 2
*
(R
f
/R
i
)
By driving the load differentially through outputs +OutA and
−OutA or +OutB and −OutB, an amplifier configuration com-
monly 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 con-
nected 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 the output swing for a speci-
fied supply voltage. Four times the output power is possible
as compared to a single-ended amplifier under the same
conditions. This increase in attainable output power as-
sumes that the amplifier is not current limited or clipped. In
order to choose an amplifier’s closed-loop gain without caus-
ing excessive clipping, please refer to the
Audio Power Am-
plifier Design
section.
A bridge configuration, such as the one used in LM4863,
also creates a second advantage over single-ended amplifi-
ers. Since the differential outputs, +OutA, −OutA, +OutB,
and −OutB, are biased at half-supply, no net DC voltage ex-
ists across the load. This eliminates the need for an output
coupling capacitor which is required in a single supply,
single-ended amplifier configuration. If an output coupling
capacitor is not used in a single-ended configuration, the
half-supply bias across the load would result in both in-
creased internal IC power dissipation as well as permanent
loudspeaker damage.
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