Datasheet

LM4782, LM4782TABD
www.ti.com SNAS231B –FEBRUARY 2004–REVISED MARCH 2013
BRIDGED AMPLIFIER APPLICATION
The LM4782 has three operational amplifiers internally, allowing for a few different amplifier configurations. One
of these configurations is referred to as “bridged mode” and involves driving the load differentially through two of
the LM4782's outputs. This configuration is shown in Figure 3. 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 distinct advantage over the single-ended configuration, as it provides differential
drive to the load, thus doubling output swing for a specified supply voltage. Theoretically, 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.
A direct consequence of the increased power delivered to the load by a bridge amplifier is an increase in internal
power dissipation. For each operational amplifier in a bridge configuration, the internal power dissipation will
increase by a factor of two over the single ended dissipation. Using Equation 2 the load impedance should be
divided by a factor of two to find the maximum power dissipation point for each amplifier in a bridge configuration.
In the case of an 8Ω load in a bridge configuration, the value used for R
L
in Equation 2 would be 4Ω for each
amplifier in the bridge. When using two of the amplifiers of the LM4782 in bridge mode, the third amplifier should
have a load impedance equal to or higher than the equivalent impedance seen by each of the bridged amplifiers.
In the example above where the bridge load is 8Ω and each amplifier in the bridge sees a load value of 4Ω then
the third amplifier should also have a 4Ω load impedance or higher. Using a lower load impedance on the third
amplifier will result in higher power dissipation in the third amplifier than the other two amplifiers and may result
in unwanted activation of thermal shut down on the third amplifier. Once the impedance seen by each amplifier is
known then Equation 2 can be used to calculated the value of P
DMAX
for each amplifier. The P
DMAX
of the IC
package is found by adding up the power dissipation for each amplifier within the IC package.
This value of P
DMAX
can be used to calculate the correct size heat sink for a bridged amplifier application. Since
the internal dissipation for a given power supply and load is increased by using bridged-mode, the heatsink's θ
SA
will have to decrease accordingly as shown by Equation 4. Refer to the section, DETERMINING THE CORRECT
HEAT SINK for a more detailed discussion of proper heat sinking for a given application.
PARALLEL AMPLIFIER APPLICATION
Parallel configuration is normally used when higher output current is needed for driving lower impedance loads
(i.e. 4Ω or lower) to obtain higher output power levels. As shown in Figure 4 , the parallel amplifier configuration
consist of designing the amplifiers in the IC to have identical gain, connecting the inputs in parallel and then
connecting the outputs in parallel through a small external output resistor. Any number of amplifiers can be
connected in parallel to obtain the needed output current or to divide the power dissipation across multiple IC
packages. Ideally, each amplifier shares the output current equally. Due to slight differences in gain the current
sharing will not be equal among all channels. If current is not shared equally among all channels then the power
dissipation will also not be equal among all channels. It is recommended that 0.1% tolerance resistors be used to
set the gain (R
i
and R
f
) for a minimal amount of difference in current sharing.
When operating two or more amplifiers in parallel mode the impedance seen by each amplifier is equal to the
total load impedance multiplied by the number of amplifiers driving the load in parallel as shown by Equation 4
below:
R
L(parallel)
= R
L(total)
* Number of amplifiers (4)
Once the impedance seen by each amplifier in the parallel configuration is known then Equation 2 can be used
with this calculated impedance to find the amount of power dissipation for each amplifier. Total power dissipation
(P
DMAX
) within an IC package is found by adding up the power dissipation for each amplifier in the IC package.
Using the calculated P
DMAX
the correct heat sink size can be determined. Refer to the section, DETERMINING
THE CORRECT HEAT SINK, for more information and detailed discussion of proper heat sinking.
If only two amplifiers of the LM4782 are used in parallel mode then the third amplifier should have a load
impedance equal to or higher than the equivalent impedance seen by each of the amplifiers in parallel mode.
Having the same load impedance on all amplifiers means that the power dissipation in each amplifier will be
equal. Using a lower load impedance on the third amplifier will result in higher power dissipation in the third
amplifier than the other two amplifiers and may result in unwanted activation of thermal shut down on the third
amplifier. Having a higher impedance on the third amplifier than the equivalent impedance on the two amplifiers
in parallel will reduce total IC package power dissipation reducing the heat sink size requirement.
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