Datasheet

LM4780, LM4780TABD
SNAS193B –JULY 2003–REVISED APRIL 2013 www.ti.com
Once the maximum package power dissipation has been calculated using Equation 3, the maximum thermal
resistance, θ
SA
, (heat sink to ambient) in °C/W for a heat sink can be calculated. This calculation is made using
Equation 5 which is derived by solving for θ
SA
in Equation 4.
θ
SA
= [(T
JMAX
−T
AMB
)−P
DMAX
(θ
JC
+θ
CS
)] / P
DMAX
(5)
Again it must be noted that the value of θ
SA
is dependent 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 LM4780 has excellent power supply rejection and does not require a regulated supply. However, to improve
system performance as well as eliminate possible oscillations, the LM4780 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 bypassing 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
supplies be bypassed at the package terminals with an electrolytic capacitor of 470μF or more.
BRIDGED AMPLIFIER APPLICATION
The LM4780 has two 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 the
LM4780'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. Thus, for an audio power amplifier such as the
LM4780, which has two operational amplifiers in one package, the package dissipation will increase by a factor
of four. To calculate the LM4780's maximum power dissipation point for a bridged load, multiply Equation 3 by a
factor of four.
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 5. 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 6
below:
R
L(parallel)
= R
L(total)
* Number of amplifiers (6)
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