LM4781TA/NOPB National Semiconductor, LM4781TA/NOPB Datasheet - Page 17

• Manufacturer Part Number: LM4781TA/NOPB
• Description: IC AMP AUDIO PWR 35W AB TO220-27
• Manufacturer: National Semiconductor
• Series: Overture™r
• Type: Class ABr

Specifications of LM4781TA/NOPB

• Output Type: 3-Channel
• Max Output Power X Channels @ Load: 35W x 3 @ 8 Ohm
• Voltage - Supply: 20 V ~ 70 V, ±10 V ~ 35 V
• Features: Depop, Mute, Short-Circuit and Thermal Protection
• Mounting Type: Through Hole
• Package / Case: TO-220-27 (Bent and Staggered Leads)
• For Use With: LM4781TABD - BOARD EVALUATION LM4781TA
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Other names: *LM4781TA; *LM4781TA/NOPB; LM4781TA

Application Information
sible 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 con-
figuration, the internal power dissipation will increase by a
factor of two over the single ended dissipation. Using Equa-
tion (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
(2) would be 4Ω for each amplifier in the bridge. When using
two of the amplifiers of the LM4781 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 Equa-
tion (2) can be used to calculated the value of P
each amplifier. The P
adding up the power dissipation for each amplifier within the
IC package.
This value of P
heat sink for a bridged amplifier application. Since the inter-
nal dissipation for a given power supply and load is in-
creased by using bridged-mode, the heatsink’s θ
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 appli-
cation.
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 3 , the parallel amplifier configuration consist of de-
signing the amplifiers in the IC to have identical gain, con-
necting the inputs in parallel and then connecting the outputs
in parallel through a small external output resistor. Any num-
ber 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
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 (5) below:
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 dis-
sipation for each amplifier. Total power dissipation (P
R
L(parallel)
DMAX
i
and R
= R
can be used to calculate the correct size
f
DMAX
) for a minimal amount of difference in
L(total)
of the IC package is found by
* Number of amplifiers
(Continued)
L
SA
in Equation
DMAX
will have
DMAX
(5)
for
)
17
within an IC package is found by adding up the power
dissipation for each amplifier in the IC package. Using the
calculated P
mined. 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 LM4781 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 amplifi-
ers in parallel will reduce total IC package power dissipation
reducing the heat sink size requirement.
BI-AMP AND TRI-AMP APPLICATIONS
Bi-amping is the practice of using two different amplifiers to
power the individual drivers in a speaker enclosure. For
example, a two-way speaker enclosure might have a tweeter
and a subwoofer. One amplifier would drive the tweeter and
another would drive the subwoofer. One advantage is that
the gain of each amplifier can be adjusted for the different
driver sensitivities. Another advantage is the crossover can
be designed before the amplifier stages with low cost op
amps instead of large passive components. With the cross-
over before the amplifier stages no power is wasted in the
passive crossover as each individual amplifier provides the
correct frequencies for the driver. Tri-Amping is using three
different amplifier stages in the same way bi-amping is done.
Bi-amping can also be done on a three-way speaker design
by using one amplifier for the subwoofer and another for the
midrange and tweeter.
The LM4781 is perfectly suited for bi-amp or tri-amp appli-
cations with it’s three amplifiers. Two of the amplifiers can be
configured for bridge or parallel mode to drive a subwoofer
with the third amplifier driving the tweeter or tweeter and
midrange. An example would be to use a 4Ω subwoofer and
8Ω tweeter/midrange with the LM4781 in parallel and single-
ended modes. Each amplifier would see an 8Ω load but the
subwoofer would have twice the output power as the
tweeter/midrange. The gain of each amplifier may also be
adjusted for the desired response. Using the LM4781 in a
tri-amp configuration would allow the gain of each amplifier
to be adjusted to achieve the desired speaker response.
SINGLE-SUPPLY AMPLIFIER APPLICATION
The typical application of the LM4781 is a split supply am-
plifier. But as shown in Figure 4, the LM4781 can also be
used in a single power supply configuration. This involves
using some external components to create a half-supply bias
which is used as the reference for the inputs and outputs.
Thus, the signal will swing around half-supply much like it
swings around ground in a split-supply application. Along
with proper circuit biasing, a few other considerations must
be accounted for to take advantage of all of the LM4781
functions, like the mute function.
CLICKS AND POPS
In the typical application of the LM4781 as a split-supply
audio power amplifier, the IC exhibits excellent “click” and
“pop” performance when utilizing the mute mode. In addition,
DMAX
the correct heat sink size can be deter-
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