Datasheet

LM4862
SNAS102F –MAY 1997–REVISED MAY 2013 www.ti.com
APPLICATION INFORMATION
BRIDGE CONFIGURATION EXPLANATION
As shown in Figure 1, the LM4862 has two operational amplifiers internally, allowing for a few different amplifier
configurations. The first amplifier's gain is externally configurable, 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 amplifiers 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
) (1)
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 causing excessive clipping which will damage high frequency transducers
used in loudspeaker systems, please refer to AUDIO POWER AMPLIFIER DESIGN.
A bridge configuration, such as the one used in LM4862, 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 coupling capacitor which is required in a single supply, single-
ended amplifier configuration. 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 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. Equation 2 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
) (2)
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 increase in power dissipation, the LM4862 does
not require heatsinking. From Equation 2, 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 dissipation that results from Equation 3:
P
DMAX
= (T
JMAX
–T
A
)/θ
JA
(3)
For package D0008A, θ
JA
= 170°C/W and for package P0008E, θ
JA
= 107°C/W. T
JMAX
= 150°C for the LM4862.
Depending on the ambient temperature, T
A
, of the system surroundings, Equation 3 can be used to find the
maximum internal power dissipation supported by the IC packaging. If the result of Equation 2 is greater than
that of Equation 3, then either the supply voltage must be decreased, the load impedance increased, or the
ambient temperature reduced. 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. Power dissipation is a function of
output power and thus, if typical operation is not around the maximum power dissipation point, the ambient
temperature can be increased. Refer to Typical Performance Characteristics for power dissipation information for
lower output powers.
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