Application Information
BRIDGE CONFIGURATION EXPLANATION
As shown in
Figure 1,
the LM4864 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 10 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 V
o1
and V
o2
,
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 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 LM4864,
also creates a second advantage over single-ended amplifi-
ers. Since the differential outputs, V
o1
and V
o2
, 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. 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 dissipa-
tion as well as permanent loudspeaker damage.
POWER DISSIPATION
Power dissipation is a major concern when designing a suc-
cessful 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.
P
DMAX
= (V
DD
)
2
/(2π
2
R
L
)
Single-Ended (1)
However, a direct consequence of the increased power de-
livered to the load by a bridge amplifier is an increase in in-
ternal power dissipation point for a bridge amplifier operating
at the same conditions.
P
DMAX
= 4(V
DD
)
2
/(π
2
R
L
)
Bridge Mode (2)
Since the LM4864 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 in-
crease in power dissipation, the LM4864 does not require
heatsinking. 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 2 must not be greater than the power dissipa-
tion that results from Equation 3:
P
DMAX
= (T
JMAX
− T
A
)/θ
JA
(3)
For package MUA08A,
θ
JA
= 210˚C/W, for package M08A,
θ
JA
= 170˚C/W and for package N08E,
θ
JA
= 107˚C/W.
T
JMAX
= 150˚C for the LM4864. Depending on the ambient
temperature, T
A
, of the system surroundings, Equation 3 can
be used to find the maximum internal power dissipation sup-
ported by the IC packaging. If the result of Equation 2 is
greater than that of Equation 3, then either the supply volt-
age must be decreased, the load impedance increased, the
ambient temperature reduced, or the
θ
JA
reduced with heat-
sinking. In many cases larger traces near the output, V
DD
,
and Gnd pins can be used to lower the
θ
JA
. The larger areas
of copper provide a form of heatsinking allowing a 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 44˚C provided that device operation is
around the maximum power dissipation point and assuming
surface mount packaging. 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 Char-
acteristics
curves for power dissipation information for
lower output powers.
POWER SUPPLY BYPASSING
As with any power amplifier, proper supply bypassing is criti-
cal for low noise performance and high power supply rejec-
tion. The capacitor location on both the bypass and power
supply pins should be as close to the device as possible. The
effect of a larger half supply bypass capacitor is improved
PSRR due to increased half-supply stability. Typical applica-
tions employ a 5V regulator with 10 µF and a 0.1 µF bypass
capacitors which aid in supply stability, but do not eliminate
the need for bypassing the supply nodes of the LM4864. The
selection of bypass capacitors, especially C
B
, is thus depen-
dent upon desired PSRR requirements, click and pop perfor-
mance as explained in the section,
Proper Selection of Ex-
ternal Components,
system cost, and size constraints.
SHUTDOWN FUNCTION
In order to reduce power consumption while not in use, the
LM4864 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 LM4864 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 0.7 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 LM4864. This scheme guarantees that
the shutdown pin will not float, thus preventing unwanted
state changes.
7
www.national.com
NSC [ NATIONAL SEMICONDUCTOR ]
