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LM4734 Datasheet(PDF) 15 Page - National Semiconductor (TI)

[Old version datasheet] Texas Instruments acquired National semiconductor.
Part No. LM4734
Description  3 Channel 20W Audio Power Amplifier with Mute and Standby
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Manufacturer  NSC [National Semiconductor (TI)]
Direct Link  http://www.national.com
Logo NSC - National Semiconductor (TI)

LM4734 Datasheet(HTML) 15 Page - National Semiconductor (TI)

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Application Information (Continued)
P
DMAX =(VCC)
2 /2
π2R
L
(1)
Thus by knowing the total supply voltage and rated output
load, the maximum power dissipation point can be calcu-
lated. The package dissipation is three times the number
which results from Equation (1) since there are three ampli-
fiers in each LM4734. Refer to the graphs of Power Dissipa-
tion versus Output Power in the Typical Performance Char-
acteristics section which show the actual full range of power
dissipation not just the maximum theoretical point that re-
sults 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 is not activated under
normal circumstances.
The thermal resistance from the die to the outside air,
θ
JA
(junction to ambient), is a combination of three thermal re-
sistances,
θ
JC (junction to case),
θ
CS (case to sink), and
θ
SA
(sink to ambient). The thermal resistance,
θ
JC (junction to
case), of the LM4734TA is 1.0˚C/W. Using Thermalloy Ther-
macote thermal compound, the thermal resistance,
θ
CS
(case to 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 tem-
perature drops are analogous to voltage drops, the power
dissipation out of the LM4734 is equal to the following:
P
DMAX =(TJMAX−TAMB)/
θ
JA
(2)
where T
JMAX = 150˚C, TAMB is the system ambient tempera-
ture and
θ
JA =
θ
JC +
θ
CS +
θ
SA.
200898B8
Once the maximum package power dissipation has been
calculated using Equation 2, the maximum thermal resis-
tance,
θ
SA, (heat sink to ambient) in ˚C/W for a heat sink can
be calculated. This calculation is made using Equation 4
which is derived by solving for
θ
SA in Equation 3.
θ
SA = [(TJMAX−TAMB)−PDMAX(
θ
JC +
θ
CS)]/PDMAX
(3)
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 LM4734 has excellent power supply rejection and does
not require a regulated supply. However, to improve system
performance as well as eliminate possible oscillations, the
LM4734 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 LM4734 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
LM4734’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 8
Ω load
in a bridge configuration, the value used for R
L in Equation
(2) would be 4
Ω for each amplifier in the bridge. When using
two of the amplifiers of the LM4734 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 8
and each amplifier in the bridge sees a load value of 4
Ω then
the third amplifier should also have a 4
Ω 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
DMAX for
each amplifier. The P
DMAX of the IC package is found by
adding up the power dissipation for each amplifier within the
IC package.
This value of P
DMAX can be used to calculate the correct size
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
SA will have
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.
www.national.com
15


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