Application Information
BRIDGE CONFIGURATION EXPLANATION
As shown in
Figure 1,
the LM4862 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 the 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. Consequently, four times the output power is
possible as compared to a single-ended amplifier under the
same conditions. 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 caus-
ing excessive clipping which will damage high frequency
transducers used in loudspeaker systems, please refer to
the
Audio Power Amplifier Design
section.
A bridge configuration, such as the one used in LM4862,
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. Without an output coupling capacitor, the half-supply
bias across the load would result in both increased internal
lC power dissipation and also permanent loudspeaker dam-
age.
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
delivered 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 LM4862 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 LM4862 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 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
= 170˚C/W and for package N08E,
θ
JA
= 107˚C/W. T
JMAX
= 150˚C for the LM4862. Depending
on the ambient temperature, T
A
, of the system surroundings,
Equation 2 can be used to find the maximum internal power
www.national.com
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dissipation supported by the IC packaging. If the result of
Equation 1 is greater than that of equation 2, then either the
supply voltage must be decreased, the load impedance in-
creased, or the ambient temperature reduced. For the typical
application of a 5V power supply, with an 8Ω load, the maxi-
mum ambient temperature possible without violating the
maximum junction temperature is approximately 44˚C pro-
vided that device operation is around the maximum power
dissipation point. Power dissipation is a function of output
power and thus, if typical operation is not around the maxi-
mum power dissipation point, the ambient temperature can
be increased. Refer to the
Typical Performance Character-
istics
curves for power dissipation information for lower out-
put 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. As
displayed in the
Typical Performance Characteristics
sec-
tion, the effect of a larger half supply bypass capacitor is im-
proved PSSR due to increased half-supply stability. Typical
applications 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
LM4862. The selection of bypass capacitors, especially C
B
,
is thus dependant upon desired PSSR requirements, click
and pop performance 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
LM4862 contains a shutdown pin to externally turn off the
amplifier’s bias circuitry. The 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 LM4862 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 because leaving the pin floating may result in an un-
wanted shutdown condition.
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 that when
closed, is connected to ground and enables the amplifier. If
the switch is open, then a soft pull-up resistor of 47 kΩ will
disable the LM4862. There are no soft pull-down resistors in-
side the LM4862, so a definite shutdown pin voltage must be
applied externally, or the internal logic gate will be left float-
ing which could disable the amplifier unexpectedly.
AUTOMATIC SWITCHING CIRCUIT
As shown in
Figure 2,
the LM4862 and the LM4880 can be
set up to automatically switch on and off depending on
whether headphones are plugged in. The LM4880 is used to
drive a stereo single ended load, while the LM4862 drives a
bridged internal speaker.
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