LM2876TF datasheet(12/19 Pages) NSC

12 / 19 page

Application Information (Continued)

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 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 0.5˚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 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 0.4˚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 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.

DMAX = VCC2/2π

(1)

where V

CC is the total supply voltage

DAVE = (VOpk/RL)[VCC/π −VOpk/2]

(2)

where V

CC is the total supply voltage and VOpk = VCC/π

DAVE = VCC VOpk/πRL −VOpk

2/2R

(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 LM2876 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 resis-

tance from the die (junction) to the outside air (ambient) is a

combination of three thermal resistances, two of which are

known,

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:

DMAX = (TJmax −TAmb)/θJA

where

JA = θJC + θCS + θSA

But since we know P

DMAX, θJC, and θSC for the application

and we are looking for

SA, we have the following:

SA = [(TJmax −TAmb)−PDMAX (θJC + θCS)]/PDMAX

(4)

Again it must be noted that the value of

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 25˚C, 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,

JC,TJmax = 150˚C,

and the recommended Thermalloy Thermacote thermal

compound resistance,

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

LM2876TF Datasheet(PDF) 12 Page - National Semiconductor (TI)

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