LM4780TA National Semiconductor, LM4780TA Datasheet - Page 17

• Manufacturer Part Number: LM4780TA
• Description: Audio Power Amplifier IC
• Manufacturer: National Semiconductor

Specifications of LM4780TA

• Amplifier Case Style: TO-220
• Peak Reflow Compatible (260 C): No
• Termination Type: Through Hole
• Leaded Process Compatible: No
• Package / Case: 27-TO-220
• Operational Class: Class-AB
• Audio Amplifier Output Configuration: 1-Channel Mono/2-Channel Stereo
• Output Power (typ): 120x1/60x2@8OhmW
• Audio Amplifier Function: Speaker
• Input Offset Voltage: 10mV
• Input Bias Current: 1uA
• Total Harmonic Distortion: 0.03@4Ohm@30W%
• Single Supply Voltage (typ): 18V
• Dual Supply Voltage (typ): ±12/±15/±18/±24/±28V
• Power Supply Requirement: Single/Dual
• Power Dissipation: 125W
• Unity Gain Bandwidth Product (typ): 8MHz
• Rail/rail I/o Type: No
• Power Supply Rejection Ratio: 120dB
• Single Supply Voltage (min): 20V
• Single Supply Voltage (max): 84V
• Dual Supply Voltage (min): ±10V
• Dual Supply Voltage (max): ±42V
• Operating Temp Range: -20C to 85C
• Operating Temperature Classification: Commercial
• Mounting: Through Hole
• Pin Count: 27 +Tab
• Package Type: TO-220
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

DETERMINING THE CORRECT HEAT SINK
The choice of a heat sink for a high-power audio amplifier is
made entirely to keep the die temperature at a level such that
the thermal protection circuitry is not activated under normal
circumstances.
The thermal resistance from the die to the outside air, θ
(junction to ambient), is a combination of three thermal resis-
tances, θ
to ambient). The thermal resistance, θ
the LM4780T is 0.8°C/W. Using Thermalloy Thermacote ther-
mal compound, the thermal resistance, θ
about 0.2°C/W. Since convection heat flow (power dissipa-
tion) is analogous to current flow, thermal resistance is anal-
ogous to electrical resistance, and temperature drops are
analogous to voltage drops, the power dissipation out of the
LM4780 is equal to the following:
where T
ture and θ
Once the maximum package power dissipation has been cal-
culated using
θ
calculated. This calculation is made using
is derived by solving for θ
Again it must be noted that the value of θ
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-in-
ductance capacitors having short leads that are located close
to the package terminals. Inadequate power supply bypass-
ing will manifest itself by a low frequency oscillation known as
“motorboating” or by high frequency instabilities. These in-
stabilities can be eliminated through multiple bypassing uti-
lizing 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 feed-
back through the power supply lines.
If adequate bypassing is not provided, the current in the sup-
ply leads which is a rectified component of the load current
may be fed back into internal circuitry. This signal causes dis-
tortion at high frequencies requiring that the supplies be by-
passed at the package terminals with an electrolytic capacitor
of 470μF or more.
SA
, (heat sink to ambient) in °C/W for a heat sink can be
θ
SA
= [(T
JMAX
JC
JA
(junction to case), θ
JMAX
= θ
= 150°C, T
P
Equation
DMAX
JC
−T
+ θ
AMB
= (T
CS
)−P
2, the maximum thermal resistance,
AMB
JMAX
+ θ
SA
DMAX
SA
in
is the system ambient tempera-
−T
.
CS
Equation
AMB
(θ
(case to sink), and θ
JC
) / θ
+θ
JC
CS
20058652
JA
SA
(junction to case), of
3.
CS
)] / P
is dependent upon
Equation 4
(case to sink), is
DMAX
SA
which
(sink
(3)
(4)
JA
17
BRIDGED AMPLIFIER APPLICATION
The LM4780 has two operational amplifiers internally, allow-
ing 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
ation 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 con-
ditions. 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 dis-
sipation. For each operational amplifier in a bridge configu-
ration, 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 opera-
tional amplifiers in one package, the package dissipation will
increase by a factor of four. To calculate the LM4780's max-
imum power dissipation point for a bridged load, multiply
Equation 2
This value of P
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
accordingly as shown by
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 3
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 dis-
sipation will also not be equal among all channels. It is rec-
ommended that 0.1% tolerance resistors be used to set the
gain (R
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
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
IC package is found by adding up the power dissipation for
each amplifier in the IC package. Using the calculated
R
i
L(parallel)
and R
, the parallel amplifier configuration consist of de-
by a factor of four.
f
) for a minimal amount of difference in current
DMAX
= R
L(total)
can be used to calculate the correct size
* Number of amplifiers
Equation
Equation 5
Figure
4. Refer to the section,
SA
2. Bridged mode oper-
will have to decrease
below:
DMAX
www.national.com
) within an
(5)