LM3876TF datasheet(16/22 Pages) NSC

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Application Information (Continued)

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 LM3876 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 LM3876 is equal to the following:

DMAX =(TJmax −TAmb)/

where

JA =

JC +

CS +

01183212

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.

Equations (1), (4) are the only equations needed in the

determination 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

semiconductor manufacturer. These parameters are the

junction to case thermal resistance,

JC,TJmax = 150˚C, and

the recommended Thermalloy Thermacote thermal com-

pound 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.

01183213

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

frequency response in order to compensate for the average

human ear’s sensitivity to the frequency spectra. The weight-

ing filters at the same time provide the bandwidth limiting as

discussed in the previous paragraph.

Note 16: CCIR/ARM: A Practical Noise Measurement Method; by Ray

Dolby, David Robinson and Kenneth Gundry, AES Preprint No. 1353 (F-3).

In addition to noise filtering, differing meter types give differ-

ent noise readings. Meter responses include:

RMS reading,

average responding,

peak reading, and

quasi peak reading.

Although theoretical noise analysis is derived using true

RMS based calculations, most actual measurements are

taken with ARM (Average Responding Meter) test equip-

ment.

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