
Application Information (Continued)
Proper mounting of the IC is required to minimize the thermal
drop between the package and the heat sink. The heat sink
must also have enough metal under the package to conduct
heat from the center of the package bottom to the fins with-
out excessive temperature drop.
A thermal grease such as Wakefield type 120 or Thermalloy
Thermacote should be used when mounting the package to
the heat sink. Without this compound, the thermal resistance
will be no better than 0.5C/W, and probably much worse.
With the compound, thermal resistance will be 0.2C/W or
less, assuming under 0.005 inch combined flatness runout
for the package and heat sink. Proper torquing of the mount-
ing bolts is important and can be determined from heat sink
manufacturer’s specification sheets.
Should it be necessary to isolate V
from the heat sink, an in-
sulating washer is required. Hard washers like berylum ox-
ide, anodized aluminum and mica require the use of thermal
compound on both faces. Two-mil mica washers are most
common, giving about 0.4C/W interface resistance with the
compound.
Silicone-rubber washers are also available. A 0.5C/W ther-
mal resistance is claimed without thermal compound. Expe-
rience has shown that these rubber washers deteriorate and
must be replaced should the IC be dismounted.
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 (P
D) calculation may result in in-
adequate heatsinking, causing thermal shutdown circuitry to
operate and limit the output power.
The following equations can be used to accurately calculate
the maximum and average integrated circuit power dissipa-
tion for your amplifier design, given the supply voltage, rated
load, and output power. These equations can be directly ap-
plied to the Power Dissipation vs Output Power curves in the
Typical Performance Characteristics section.
Equation (1) exemplifies the maximum power dissipation of
the IC and
Equations (2), (3) exemplify the average IC power
dissipation expressed in different forms.
P
DMAX = VCC
2/2
π2 R
L
(1)
where V
CC is the total supply voltage
P
DAVE = (VOpk/RL)[VCC/π VOpk/2]
(2)
where V
CC is the total supply voltage and VOpk = VCC/π
P
DAVE = VCC VOpk/π RL VOpk
2/2 R
L
(3)
where V
CC is the total supply voltage.
Determining the Correct Heat Sink
Once the maximum IC power dissipation is known for a
given supply voltage, rated load, and the desired rated out-
put power the maximum thermal resistance (in C/W) of a
heat sink can be calculated. This calculation is made using
Equation (4) and is based on the fact that thermal heat flow
parameters are analogous to electrical current flow proper-
ties.
It is also known that typically the thermal resistance,
θ
JC
(junction to case), of the LM3875 is 1C/W and that using
Thermalloy Thermacote thermal compound provides a ther-
mal resistance,
θ
CS (case to heat sink), of about 0.2C/W as
explained in the Heat Sinking section.
Referring to the figure below, it is seen that the thermal resis-
tance from the die (junction) to the outside air (ambient) is a
combination of three thermal resistances, two of which are
known,
θ
JC and θCS. Since convection heat flow (power dis-
sipation) 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 LM3875 is equal to the following:
P
DMAX = (TJmax TAmb)/θJA
where
θ
JA = θJC + θCS + θSA
But since we know P
DMAX, θJC, and θSC for the application
and we are looking for
θ
SA, we have the following:
θ
SA = [(TJmax TAmb)PDMAX (θJC + θCS)]/PDMAX
(4)
Again it must be noted that the value of
θ
SA is dependent
upon the system designer’s amplifier application and its cor-
responding parameters as described previously. If the ambi-
ent temperature that the audio amplifier is to be working un-
der is higher than the normal 25C, then the thermal
resistance for the heat sink, given all other things are equal,
will need to be smaller.
Equations (1), (4) are the only equations needed in the de-
termination of the maximum heat sink thermal resistance.
This is, of course, given that the system designer knows the
required supply voltages to drive his rated load at a particular
power output level and the parameters provided by the semi-
conductor manufacturer. These parameters are the junction
to case thermal resistance,
θ
JC,TJmax = 150C, and the rec-
ommended Thermalloy Thermacote thermal compound re-
sistance,
θ
CS.
SIGNAL-TO-NOISE RATIO
In the measurement of the signal-to-noise ratio, misinterpre-
tations of the numbers actually measured are common. One
amplifier may sound much quieter than another, but due to
improper testing techniques, they appear equal in measure-
ments. This is often the case when comparing integrated cir-
cuit designs to discrete amplifier designs. Discrete transistor
amps often “run out of gain” at high frequencies and there-
fore have small bandwidths to noise as indicated below.
Integrated circuits have additional open loop gain allowing
additional feedback loop gain in order to lower harmonic dis-
tortion and improve frequency response. It is this additional
bandwidth that can lead to erroneous signal-to-noise mea-
surements if not considered during the measurement pro-
DS011449-10
DS011449-11
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