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參數(shù)資料
| 型號： | LM2876MWC |
| 廠商： | NATIONAL SEMICONDUCTOR CORP |
| 元件分類： | 音頻／視頻放大 |
| 英文描述： | 40 W, 1 CHANNEL, AUDIO AMPLIFIER, UUC7 |
| 封裝： | WAFER |
| 文件頁數(shù)： | 4/22頁 |
| 文件大小： | 611K |
| 代理商： | LM2876MWC |
Application Information (Continued)
audio amplifier design where
θ
CS may be a different value. It
should be noted that the idea behind dissipating the maxi-
mum power within the IC is to provide the device with a low
resistance to convection heat transfer such as a heat sink.
Therefore, it is necessary for the system designer to be con-
servative in his heat sink calculations. As a rule, the lower
the thermal resistance of the heat sink the higher the amount
of power that may be dissipated. This is of course guided by
the cost and size requirements of the system. Convection
cooling heat sinks are available commercially, and their
manufacturers should be consulted for ratings.
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, 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 mounting
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 beryluum 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 heat sinking, causing thermal shutdown circuitry to
operate and limit the output power.
The following equations can be used to acccurately 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 =VCC2/2π
2R
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/πRL VOpk
2/2R
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 LM2876 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 LM2876 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.
Equation (1) and Equation (4) are the only equations needed
in the determination of the maximum heat sink thermal resis-
tance. 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 semiconductor manufacturer. These parameters are
the junction to case thermal resistance,
θ
JC,TJmax = 150C,
and the recommended Thermalloy Thermacote thermal
compound resistance,
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.
DS011775-12
LM2876
www.national.com
12
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