LM4782TABD/NOPB National Semiconductor, LM4782TABD/NOPB Datasheet - Page 18

• Manufacturer Part Number: LM4782TABD/NOPB
• Manufacturer: National Semiconductor

Specifications of LM4782TABD/NOPB

• Lead Free Status / Rohs Status: Compliant

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Application Information
tance, θ
be calculated. This calculation is made using Equation 4
which is derived by solving for θ
Again it must be noted that the value of θ
upon the system designer’s amplifier requirements. If the
ambient temperature that the audio amplifier is to be working
under is higher than 25˚C, then the thermal resistance for the
heat sink, given all other things are equal, will need to be
smaller.
SUPPLY BYPASSING
The LM4782 has excellent power supply rejection and does
not require a regulated supply. However, to improve system
performance as well as eliminate possible oscillations, the
LM4782 should have its supply leads bypassed with low-
inductance capacitors having short leads that are located
close to the package terminals. Inadequate power supply
bypassing will manifest itself by a low frequency oscillation
known as “motorboating” or by high frequency instabilities.
These instabilities can be eliminated through multiple by-
passing utilizing a large tantalum or electrolytic capacitor
(10µF or larger) which is used to absorb low frequency
variations and a small ceramic capacitor (0.1µF) to prevent
any high frequency feedback through the power supply lines.
If adequate bypassing is not provided, the current in the
supply leads which is a rectified component of the load
current may be fed back into internal circuitry. This signal
causes distortion at high frequencies requiring that the sup-
plies be bypassed at the package terminals. It is recom-
mended that a ceramic 0.1µF capacitor and an electrolytic or
tantalum 10µF or larger capacitor be placed as close as
possible to the IC’s supply pins and then an additional elec-
trolytic capacitor of 470µF or more on each supply line.
BRIDGED AMPLIFIER APPLICATION
The LM4782 has three operational amplifiers internally, al-
lowing 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 2.
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 pos-
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 LM4782 in bridge mode, the third
θ
SA
SA
= [(T
, (heat sink to ambient) in ˚C/W for a heat sink can
JMAX
−T
AMB
)−P
DMAX
SA
(θ
in Equation 3.
JC
+θ
CS
(Continued)
SA
)] / P
is dependent
L
in Equation
DMAX
(3)
18
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 (4) 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
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 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
R
L(parallel)
DMAX
DMAX
i
and R
= R
the correct heat sink size can be deter-
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
SA
DMAX
will have
DMAX
(4)
for
)