Application Information
BRIDGE CONFIGURATION EXPLANATION
As shown in
Figure 1
, the LM4861 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 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 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. 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 Boomer Au-
dio Power Amplifiers, also creates a second advantage over
single-ended amplifiers. 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 cou-
pling capacitor which is required in a single supply, single-
ended amplifier configuration. Without an output coupling ca-
pacitor in a single supply, single-ended amplifier, the half-
supply bias across the load would result in both increased
internal IC power dissipation and also permanent loud-
speaker damage. An output coupling capacitor forms a high
pass filter with the load requiring that a large value such as
470 µF be used with an 8Ω load to preserve low frequency
response. This combination does not produce a flat re-
sponse down to 20 Hz, but does offer a compromise be-
tween printed circuit board size and system cost, versus low
frequency response.
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 LM4861 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 LM4861 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 the LM4861 surface mount package,
θ
JA
= 140˚C/W and
T
JMAX
= 150˚C. Depending on the ambient temperature, T
A
,
of the system surroundings, Equation 2 can be used to find
the maximum internal power 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 de-
creased or the load impedance increased. For the typical ap-
plication of a 5V power supply, with an 8Ω load, the maxi-
mum ambient temperature possible without violating the
maximum junction temperature is approximately 62.5˚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 low frequency THD + N 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 sup-
ply nodes of the LM4861. The selection of bypass capaci-
tors, especially C
B
, is thus dependant upon desired low fre-
quency THD + N, system cost, and size constraints.
SHUTDOWN FUNCTION
In order to reduce power consumption while not in use, the
LM4861 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.
Upon going into shutdown, the output is immediately discon-
nected from the speaker. A typical quiescent current of
0.6 µA results when the supply voltage is applied to the shut-
down pin. In many applications, a microcontroller or micro-
processor output is used to control the shutdown circuitry
which provides a quick, smooth transition into shutdown. An-
other solution is to use a single-pole, single-throw switch that
when closed, is connected to ground and enables the ampli-
fier. If the switch is open, then a soft pull-up resistor of 47 kΩ
will disable the LM4861. There are no soft pull-down resis-
tors inside the LM4861, so a definite shutdown pin voltage
must be applied externally, or the internal logic gate will be
left floating which could disable the amplifier unexpectedly.
HIGHER GAIN AUDIO AMPLIFIER
The LM4861 is unity-gain stable and requires no external
components besides gain-setting resistors, an input coupling
capacitor, and proper supply bypassing in the typical appli-
cation. However, if a closed-loop differential gain of greater
than 10 is required, a feedback capacitor may be needed, as
shown in
Figure 2,
to bandwidth limit the amplifier. This feed-
back capacitor creates a low pass filter that eliminates pos-
sible high frequency oscillations. Care should be taken when
calculating the −3 dB frequency in that an incorrect combina-
6
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