LM3875 Datasheet, PDF (23/29 Pages) National Semiconductor (TI) – OverturTM Audio Power Amplifier Series High-Performance 56W Audio Power Amplifier

LM3875
www.ti.com
SNAS083D – JUNE 1999 – REVISED APRIL 2013
Output Saturation Threshold (Clipping Point): The output swing limit for a specified input drive beyond that
required for zero output. It is measured with respect to the supply to which the output is swinging.
Output Resistance: The ratio of the change in output voltage to the change in output current with the output
around zero.
Power Dissipation Rating: The power that can be dissipated for a specified time interval without activating the
protection circuitry. For time intervals in excess of 100 ms, dissipation capability is determined by heat sinking of
the IC package rather than by the IC itself.
Thermal Resistance: The peak, junction-temperature rise, per unit of internal power dissipation (units in °C/W),
above the case temperature as measured at the center of the package bottom.
The DC thermal resistance applies when one output transistor is operating continuously. The AC thermal
resistance applies with the output transistors conducting alternately at a high enough frequency that the peak
capability of neither transistor is exceeded.
Power Bandwidth: The power bandwidth of an audio amplifier is the frequency range over which the amplifier
voltage gain does not fall below 0.707 of the flat band voltage gain specified for a given load and output power.
Power bandwidth also can be measured by the frequencies at which a specified level of distortion is obtained
while the amplifier delivers a power output 3 dB below the rated output. For example, an amplifier rated at 60W
with ≤0.25% THD + N, would make its power bandwidth measured as the difference between the upper and
lower frequencies at which 0.25% distortion was obtained while the amplifier was delivering 30W.
Gain-Bandwidth Product: The Gain-Bandwidth Product is a way of predicting the high-frequency usefulness of
an op amp. The Gain-Bandwidth Product is sometimes called the unity-gain frequency or unity-gain cross
frequency because the open-loop gain characteristic passes through or crosses unity gain at this frequency.
Simply, we have the following relationship:
A_CL1 × f₁ = A_CL2 × f₂
(16)
Assuming that at unity-gain
(A_CL1 = 1 or 0 dB) fu = f₁ = GBWP,
(17)
then we have the following:
GBWP = A_CL2 × f₂
(18)
This says that once fu (GBWP) is known for an amplifier, then the open-loop gain can be found at any frequency.
This is also an excellent equation to determine the 3 dB point of a closed-loop gain, assuming that you know the
GBWP of the device. Refer to the diagram below.
Bi-amplification: The technique of splitting the audio frequency spectrum into two sections and using individual
power amplifiers to drive a separate woofer and tweeter. Crossover frequencies for the amplifiers usually vary
between 500 Hz and 1600 Hz. “Biamping” has the advantages of allowing smaller power amps to produce a
given sound pressure level and reducing distortion effects produced by overdrive in one part of the frequency
spectrum affecting the other part.
C.C.I.R./A.R.M.:
Literally: International Radio Consultative Committee Average Responding Meter
This refers to a weighted noise measurement for a Dolby B type noise reduction system. A filter characteristic is
used that gives a closer correlation of the measurement with the subjective annoyance of noise to the ear.
Measurements made with this filter cannot necessarily be related to unweighted noise measurements by some
fixed conversion factor since the answers obtained will depend on the spectrum of the noise source.
S.P.L.: Sound Pressure Level—usually measured with a microphone/meter combination calibrated to a pressure
level of 0.0002 μBars (approximately the threshold hearing level).
S.P.L. = 20 Log 10P/0.0002 dB
(19)
Where P is the R.M.S sound pressure in microbars. (1 Bar = 1 atmosphere = 14.5 lb./in² = 194 dB S.P.L.).
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