LM2876 datasheet(15/22 Pages) NSC

15 / 22 page

Application Information (Continued)

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 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

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 =VCC2/2

π2R

(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/

πR

L −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

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 LM2876 is equal to the following:

DMAX =(TJmax −TAmb)/

where

JA =

JC +

CS +

01177512

But since we know P

DMAX,

JC, and

SC for the application

and we are looking for

SA, we have the following:

SA = [(TJmax −TAmb)−PDMAX (

JC +

CS)]/PDMAX

(4)

Again it must be noted that the value of

SA is dependent

upon the system designer’s amplifier application and its

corresponding parameters as described previously. If the

ambient temperature that the audio amplifier is to be working

under 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 re-

sistance. 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

circuit designs to discrete amplifier designs. Discrete transis-

tor amps often “run out of gain” at high frequencies and

therefore have small bandwidths to noise as indicated below.

01177513

Integrated circuits have additional open loop gain allowing

additional feedback loop gain in order to lower harmonic

distortion and improve frequency response. It is this addi-

tional bandwidth that can lead to erroneous signal-to-noise

measurements if not considered during the measurement

process. In the typical example above, the difference in

bandwidth appears small on a log scale but the factor of 10

in bandwidth, (200 kHz to 2 MHz) can result in a 10 dB

theoretical difference in the signal-to-noise ratio (white noise

is proportional to the square root of the bandwidth in a

system).

In comparing audio amplifiers it is necessary to measure the

magnitude of noise in the audible bandwidth by using a

“weighting” filter.(Note 16) A “weighting” filter alters the fre-

quency response in order to compensate for the average

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