Datasheet

LM4860
www.ti.com SNAS096C –AUGUST 1994–REVISED MAY 2013
APPLICATION INFORMATION
BRIDGE CONFIGURATION EXPLANATION
As shown in Figure 1, the LM4860 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 40 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 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 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 Boomer Audio 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 coupling capacitor which is required
in a single supply, single-ended amplifier configuration. Without an output coupling capacitor 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 loudspeaker 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 response down to 20 Hz, but does offer a compromise
between printed circuit board size and system cost, versus low frequency response.
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 LM4860 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 LM4860 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 the LM4860 surface mount package, θ
JA
= 100°C/W and T
JMAX
= 150°C. 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 or the load impedance increased. 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 88°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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