LM4701T National Semiconductor, LM4701T Datasheet - Page 12

• Manufacturer Part Number: LM4701T
• Description: LM4701 Overture Audio Power Amplifier Series 30W Audio Power Amplifier with Mute and Standby Modes
• Manufacturer: National Semiconductor

www.national.com
Application Information
the fault condition is temporary, but a sustained fault will
cause the device to cycle in a Schmitt Trigger fashion be-
tween the thermal shutdown temperature limits of 165˚C and
155˚C. This greatly reduces the stress imposed on the IC by
thermal cycling, which in turn improves its reliability under
sustained fault conditions.
Since the die temperature is directly dependent upon the
heat sink used, the heat sink should be chosen such that
thermal shutdown will not be reached during normal opera-
tion. Using the best heat sink possible within the cost and
space constraints of the system will improve the long-term
reliability of any power semiconductor device, as discussed
in the Determining the Correct Heat Sink Section.
DETERMINING MAXIMUM POWER DISSIPATION
Power dissipation within the integrated circuit package is a
very important parameter requiring a thorough understand-
ing if optimum power output is to be obtained. An incorrect
maximum power dissipation calculation may result in inad-
equate heat sinking causing thermal shutdown and thus lim-
iting the output power.
Equation (1) exemplifies the theoretical maximum power dis-
sipation point of each amplifier where V
voltage.
Thus by knowing the total supply voltage and rated output
load, the maximum power dissipation point can be calcu-
lated. Refer to the graphs of Power Dissipation vs Output
Power in the Typical Performance Characteristics section
which show the actual full range of power dissipation not just
the maximum theoretical point that results from equation (1).
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 does not operate under
normal circumstances.
The thermal resistance from the die (junction) to the outside
air (ambient) is a combination of three thermal resistances,
case), of the LM4701 is 2˚C/W. Using Thermalloy Therma-
cote thermal compound, the thermal resistance,
sink), is about 0.2˚C/W. Since convection heat flow (power
dissipation) is analogous to current flow, thermal resistance
is analogous to electrical resistance, and temperature drops
are analogous to voltage drops, the power dissipation out of
the LM4701 is equal to the following:
where T
ture and
Once the maximum package power dissipation has been
calculated using equation (1), the maximum thermal resis-
tance,
calculation is made using equation (3) which is derived by
solving for
Again it must be noted that the value of
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.
JC
,
CS
JMAX
SA
and
JA
, (in ˚C/W) for a heat sink can be calculated. This
SA
SA
=
= 150˚C, T
= [(T
SA
in equation (2).
JC
. The thermal resistance,
JMAX
P
+
DMAX
P
CS
−T
DMAX
AMB
AMB
+
= (T
is the system ambient tempera-
SA
)−P
= V
JMAX
.
DMAX
CC
− T
2
/2
(
CC
AMB
JC
2
(Continued)
R
is the total supply
+
L
)/
SA
CS
JC
JA
is dependent
)]/P
(junction to
CS
DMAX
(case to
(1)
(2)
(3)
12
SUPPLY BYPASSING
The LM4701 has excellent power supply rejection and does
not require a regulated supply. However, to improve system
performance as well as eliminate possible oscillations, the
LM4701 should have its supply leads bypassed with
low-inductance capacitors having short leads that are lo-
cated close to the package terminals. Inadequate power
supply bypassing will manifest itself by a low frequency oscil-
lation known as “motorboating” or by high frequency insta-
bilities. These instabilities can be eliminated through multiple
bypassing utilizing a large tantalum or electrolytic capacitor
(10 µF or larger) which is used to absorb low frequency
variations and a small ceramic capacitor (0.1 µF) to prevent
any high frequency feedback 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
distortion at high frequencies requiring that the supplies be
bypassed at the package terminals with an electrolytic ca-
pacitor of 470 µF or more.
BRIDGED AMPLIFIER APPLICATION
One common power amplifier configuration is shown in Fig-
ure 2 and is referred to as “bridged mode” operation. Bridged
mode operation is different from the classical single-ended
amplifier configuration where one side of the output 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. Consequently, 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 con-
figuration, the internal power dissipation will increase by a
factor of two over the single ended dissipation. Since there
are two amplifiers used in a bridge configuration, the maxi-
mum system power dissipation point will increase by a factor
of four over the figure obtained by equation (1).
This value of P
heat sink for a bridged amplifier application, assuming that
both IC’s are mounted on the same heatsink. Since the inter-
nal dissipation for a given power supply and load is in-
creased by using bridged-mode, the heatsink’s
to decrease accordingly as shown by equation (3). Refer to
the section, Determining the Correct Heat Sink, for a more
detailed discussion of proper heat sinking for a given appli-
cation.
SINGLE-SUPPLY AMPLIFIER APPLICATION
The typical application of the LM4701 is a split supply ampli-
fier. But as shown in Figure 3 , the LM4701 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 LM4701
functions.
DMAX
can be used to calculate the correct size
SA
will have