LM2876 datasheet(16/19 Pages) NSC

16 / 19 page

Definition of Terms (Continued)

ciently short that the amplifier power supply does not sag

during the measurement, or when high quality external

power supplies are used. This measurement (an IHF stan-

dard) assumes that with normal music program material the

amplifier power supplies will sag insignificantly.

Peak Power: Most commonly referred to as the power out-

put capability of an amplifier that can be delivered to the

load; specified by the part’s maximum voltage swing.

Headroom: The margin between an actual signal operating

level (usually the power rating of the amplifier with particular

supply voltages, a rated load value, and a rated THD + N fig-

ure) and the level just before clipping distortion occurs, ex-

pressed in decibels.

Large Signal Voltage Gain: The ratio of the output voltage

swing to the differential input voltage required to drive the

output from zero to either swing limit. The output swing limit

is the supply voltage less a specified quasi-saturation volt-

age. A pulse of short enough duration to minimize thermal ef-

fects is used as a measurement signal.

Output-Current Limit: The output current with a fixed out-

put voltage and a large input overdrive. The limiting current

drops with time once SPiKe protection circuitry is activated.

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 volt-

age to the change in output current with the output around

zero.

Power Dissipation Rating: The power that can be dissi-

pated for a specified time interval without activating the pro-

tection circuitry. For time intervals in excess of 100 ms, dis-

sipation 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 pack-

age bottom.

The DC thermal resistance applies when one output transis-

tor is operating continuously. The AC thermal resistance ap-

plies 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 ampli-

fier 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 out-

put. 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 deliv-

ering 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 re-

lationship: A

CL1 xf1 = ACL2 xf2

Assuming that at unity-gain (A

CL1 = 1 or (0 dB)) fu = fi =

GBWP, then we have the following: GBWP = A

CL2 xf2

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 on the following page.

Biamplification: The technique of splitting the audio fre-

quency spectrum into two sections and using individual

power amplifiers to drive a separate woofer and tweeter.

Crossover frequencies for the amplifiers usually vary be-

tween 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 prodused 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 un-

weighted noise measurements by some fixed conversion

factor since the answers obtained will depend on the spec-

trum 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

where P is the R.M.S. sound pressure in microbars.

(1 Bar = 1 atmosphere = 14.5 lb/in

2 = 194 dB S.P.L.).

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