LM4871
Application Information
BRIDGE CONFIGURATION EXPLANATION
As shown in
Figure 1,
the LM4871 has two operational am-
plifiers internally, allowing for a few different amplifier con-
figurations. 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
f
to R
i
while
the second amplifier’s gain is fixed by the two internal 40 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 the IC is
A
VD
= 2 *(R
f
/R
i
)
By driving the load differentially through outputs Vo1 and
Vo2, an amplifier configuration commonly referred to as
“bridged mode” is established. Bridged mode operation is
different from the classical single-ended amplifier configura-
tion 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 condi-
tions. 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 ex-
cessive clipping, please refer to the
Audio Power Amplifier
Design
section.
A bridge configuration, such as the one used in LM4871,
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
eliminates the need for an output coupling capacitor which is
required in a single supply, single-ended amplifier configura-
tion. Without an output coupling capacitor, the half-supply
bias across the load would result in both increased internal
IC power dissipation and also possible loudspeaker damage.
POWER DISSIPATION
Power dissipation is a major concern when designing a suc-
cessful amplifier, whether the amplifier is bridged or single-
ended. A direct consequence of the increased power deliv-
ered to the load by a bridge amplifier is an increase in
internal power dissipation. Equation 1 states the maximum
power dissipation point for a bridge amplifier operating at a
given supply voltage and driving a specified output load.
P
DMAX
= 4*(V
DD
)
2
/(2π
2
R
L
)
(1)
Since the LM4871 has two operational amplifiers in one
package, the maximum internal power dissipation is 4 times
that of a single-ended ampifier. Even with this substantial in-
crease in power dissipation, the LM4871 does not require
heatsinking under most operating conditions and output
loading. From Equation 1, assuming a 5V power supply and
an 8Ω load, the maximum power dissipation point is
625 mW. The maximum power dissipation point obtained
from Equation 1 must not be greater than the power dissipa-
tion that results from Equation 2:
P
DMAX
= (T
JMAX
–T
A
)/θ
JA
(2)
For package M08A,
θ
JA
= 140˚C/W, and for package N08E,
θ
JA
= 107˚C/W assuming free air operation. T
JMAX
= 150˚C
for the LM4871. The
θ
JA
can be decreased by using some
form of heat sinking. The resultant
θ
JA
will be the summation
of the
θ
JC
,
θ
CS
, and
θ
SA
.
θ
JC
is the junction to case of the
5
package,
θ
CS
is the case to heat sink thermal resistance and
θ
SA
is the heat sink to ambient thermal resistance. By adding
additional copper area around the LM4871, the
θ
JA
can be
reduced from its free air value of 140˚C/W for package
M08A. Depending on the ambient temperature, T
A
, and the
θ
JA
, Equation 2 can be used to find the maximum internal
power dissipation supported by the IC packaging. If the re-
sult of Equation 1 is greater than that of Equation 2, then ei-
ther the supply voltage must be decreased, the load imped-
ance increased, the
θ
JA
decreased, or the ambient
temperature reduced. For the typical application of a 5V
power supply, with an 8Ω load, and no additional heatsink-
ing, the maximum ambient temperature possible without vio-
lating the maximum junction temperature is approximately
61˚C provided that device operation is around the maximum
power dissipation point and assuming surface mount pack-
aging. Internal power dissipation 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 Performance Characteristics
curves
for power dissipation information for different output powers
and output loading.
POWER SUPPLY BYPASSING
As with any amplifier, proper supply bypassing is critical for
low noise performance and high power supply rejection. The
capacitor location on both the bypass and power supply pins
should be as close to the device as possible. Typical applica-
tions employ a 5V regulator with 10 µF and a 0.1 µF bypass
capacitors which aid in supply stability. This does not elimi-
nate the need for bypassing the supply nodes of the
LM4871. The selection of bypass capacitors, especially C
B
,
is dependent upon PSRR requirements, click and pop per-
formance as explained in the section,
Proper Selection of
External Components,
system cost, and size constraints.
SHUTDOWN FUNCTION
In order to reduce power consumption while not in use, the
LM4871 contains a shutdown pin to externally turn off the
amplifier’s bias circuitry. This shutdown feature turns the am-
plifier off when a logic high is placed on the shutdown pin.
The trigger point between a logic low and logic high level is
typically half- supply. It is best to switch between ground and
supply to provide maximum device performance. By switch-
ing the shutdown pin to V
DD
, the LM4871 supply current
draw will be minimized in idle mode. While the device will be
disabled with shutdown pin voltages less then V
DD
, the idle
current may be greater than the typical value of 0.6 µA. In ei-
ther case, the shutdown pin should be tied to a definite volt-
age to avoid unwanted state changes.
In many applications, a microcontroller or microprocessor
output is used to control the shutdown circuitry which pro-
vides a quick, smooth transition into shutdown. Another solu-
tion is to use a single-pole, single-throw switch in conjunction
with an external pull-up resistor. When the switch is closed,
the shutdown pin is connected to ground and enables the
amplifier. If the switch is open, then the external pull-up re-
sistor will disable the LM4871. This scheme guarantees that
the shutdown pin will not float thus preventing unwanted
state changes.
PROPER SELECTION OF EXTERNAL COMPONENTS
Proper selection of external components in applications us-
ing integrated power amplifiers is critical to optimize device
and system performance. While the LM4871 is tolerant of
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