LM3875TF datasheet(13/20 Pages) NSC

13 / 20 page

Application Information (Continued)

THERMAL CONSIDERATIONS

Heat Sinking

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 heat sink should be chosen to

dissipate the maximum IC power for a given supply voltage

and rated load.

With high-power pulses of longer duration than 100 ms, the

case temperature will heat up drastically without the use of a

heat sink. Therefore the case temperature, as measured at

the center of the package bottom, is entirely dependent on

heat sink design and the mounting of the IC to the heat sink.

For the design of a heat sink for your audio amplifier appli-

cation refer to the Determining the Correct Heat Sink

section.

Since a semiconductor manufacturer has no control over

which heat sink is used in a particular amplifier design, we

can only inform the system designer of the parameters and

the method needed in the determination of a heat sink. With

this in mind, the system designer must choose his supply

voltages, a rated load, a desired output power level, and

know the ambient temperature surrounding the device.

These parameters are in addition to knowing the maximum

junction temperature and the thermal resistance of the IC,

both of which are provided by National Semiconductor.

As a benefit to the system designer we have provided Maxi-

mum Power Dissipation vs Supply Voltages curves for vari-

ous loads in the Typical Performance Characteristics sec-

tion, giving an accurate figure for the maximum thermal

resistance required for a particular amplifier design. This

data was based on

JC = 1˚C/W and

CS = 0.2˚C/W. We also

provide a section regarding heat sink determination for any

audio amplifier design where

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

conservative 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

without 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 0.5˚C/W, and probably much worse.

With the compound, thermal resistance will be 0.2˚C/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

insulating washer is required. Hard washers like berylum

oxide, anodized aluminum and mica require the use of ther-

mal compound on both faces. Two-mil mica washers are

most common, giving about 0.4˚C/W interface resistance

with the compound.

Silicone-rubber washers are also available. A 0.5˚C/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

inadequate 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

applied 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.

DMAX =VCC

2/2

π2 R

(1)

where V

CC is the total supply voltage

DAVE =(VOpk/RL)[VCC/

π −V

Opk/2]

(2)

where V

CC is the total supply voltage and VOpk =VCC/

DAVE =VCC VOpk/

π R

L −VOpk

2/2 R

(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,

(junction to case), of the LM3875 is 1˚C/W and that using

Thermalloy Thermacote thermal compound provides a ther-

mal resistance,

CS (case to heat sink), of about 0.2˚C/W as

explained in the Heat Sinking section.

Referring to the figure below, it is seen that the thermal

resistance from the die (junction) to the outside air (ambient)

is a combination of three thermal resistances, two of which

are known,

JC and

CS. 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 LM3875 is equal to the following:

DMAX =(TJmax −TAmb)/

where

JA =

JC +

CS +

01144910

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