LM3875 Datasheet, PDF (17/29 Pages) National Semiconductor (TI) – OverturTM Audio Power Amplifier Series High-Performance 56W Audio Power Amplifier

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P_DMAX = (T_Jmax − T_Amb)/θ_JA
where
• where θ_JA = θ_JC + θ_CS + θ_SA
LM3875
SNAS083D – JUNE 1999 – REVISED APRIL 2013
(4)
Figure 39.
But since we know P_DMAX, θ_JC, and θ_SC for the application and we are looking for θ_SA, we have the following:
θ_SA = [(T_Jmax − T_Amb) − P_DMAX (θ_JC + θ_CS)]/P_DMAX
(5)
Again it must be noted that the value of θ_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 5 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, θ_JC, T_Jmax = 150°C, and the recommended
Thermalloy Thermacote thermal compound resistance, θ_CS.
SIGNAL-TO-NOISE RATIO
In the measurement of the signal-to-noise ratio, misinterpretations 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 measurements. This is often the case when comparing integrated circuit designs to discrete
amplifier designs. Discrete transistor amps often “run out of gain” at high frequencies and therefore have small
bandwidths to noise as indicated below.
Figure 40.
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 additional 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 (see Note below). A “weighting” filter alters the frequency response in order to
compensate for the average human ear's sensitivity to the frequency spectra. The weighting filters at the same
time provide the bandwidth limiting as discussed in the previous paragraph.
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