LM4872
Application Information
BRIDGE CONFIGURATION EXPLANATION
As shown in
the LM4872 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
f
to R
i
while
the second amplifier’s gain is fixed by the two internal 10 kΩ
resistors.
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 by 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 con-
ditions. 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 LM4872,
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
successful amplifier, whether the amplifier is bridged or
single-ended. A direct consequence of the increased power
delivered to the load by a bridge amplifier is an increase in
internal power dissipation. Since the LM4872 has two opera-
tional amplifiers in one package, the maximum internal
power dissipation is 4 times that of a single-ended amplifier.
The maximum power dissipation for a given application can
be derived from the power dissipation graphs or from Equa-
tion 1.
(1)
P
DMAX
= 4*(V
DD
)
2
/(2π
2
R
L
)
It is critical that the maximum junction temperature T
JMAX
of
150˚C is not exceeded. T
JMAX
can be determined from the
power derating curves by using P
DMAX
and the PC board foil
area. By adding additional copper foil, the thermal resistance
of the application can be reduced from a free air value of
150˚C/W, resulting in higher P
DMAX
. Additional copper foil
can be added to any of the leads connected to the LM4872.
It is especially effective when connected to V
DD
, G
ND
, and
the output pins. Refer to the application information on the
LM4872 reference design board for an example of good heat
sinking. If T
JMAX
still exceeds 150˚C, then additional
changes must be made. These changes can include re-
9
duced supply voltage, higher load impedance, or reduced
ambient temperature. The National Reference Design board
using a 5V supply and an 8 ohm load will run in a 110˚C
ambient environement without exceeding T
JMAX
. Internal
power dissipation is a function of output power. Refer to the
Typical Performance Characteristics
curves for power dis-
sipation 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 appli-
cations employ a 5V regulator with 10 µF Tantalum or elec-
trolytic capacitor and a 0.1 µF bypass capacitor which aid in
supply stability. This does not eliminate the need for bypass-
ing the supply nodes of the LM4872. The selection of bypass
capacitor, especially C
B
, is dependent upon PSRR require-
ments, click and pop performance as explained in the sec-
tion,
Proper Selection of External Components,
system
cost, and size constraints.
SHUTDOWN FUNCTION
In order to reduce power consumption while not in use, the
LM4872 contains a shutdown pin to externally turn off the
amplifier’s bias circuitry. This shutdown feature turns the
amplifier 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 LM4872 supply current
draw will be minimized in idle mode. While the device will be
disabled with shutdown pin voltages less than V
DD
, the idle
current may be greater than the typical value of 0.01 µA. In
either case, the shutdown pin should be tied to a stable
voltage 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 so-
lution is to use a single-pole, single-throw switch in conjunc-
tion with an external pull-up resistor. When the switch is
closed, the shutdown pin is connected to ground and en-
ables the amplifier. If the switch is open, then the external
pull-up resistor will disable the LM4872. This scheme guar-
antees 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 LM4872 is tolerant of
external component combinations, consideration to compo-
nent values must be used to maximize overall system qual-
ity.
The LM4872 is unity-gain stable which gives a designer
maximum system flexibility. The LM4872 should be used in
low gain configurations to minimize THD+N values, and
maximize the signal to noise ratio. Low gain configurations
require large input signals to obtain a given output power.
Input signals equal to or greater than 1 Vrms are available
from sources such as audio codecs. Please refer to the
section,
Audio Power Amplifier Design,
for a more com-
plete explanation of proper gain selection.
Besides gain, one of the major considerations is the closed-
loop bandwidth of the amplifier. To a large extent, the band-
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