LM3886_03 datasheet(21/24 Pages) NSC

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Application Information (Continued)

The minimum gain from Equation (8) is:

≥ 12.6

We select a gain of 13 (Non-Inverting Amplifier); resulting in

a sensitivity of 973 mV.

Letting R

IN equal 100 k

Ω gives the required input imped-

ance, however, this would eliminate the “volume control”

unless an additional input impedance was placed in series

with the 10 k

Ω potentiometer that is depicted in Figure 1.

Adding the additional 100 k

Ω resistor would ensure the

minumum required input impedance.

For low DC offsets at the output we let R

f1 = 100 k

Ω. Solving

for Ri (Non-Inverting Amplifier) gives the following:

Ri=R

f1/(AV − 1) = 100k/(13 − 1) = 8.3 k

Ω; use 8.2 kΩ

The bandwidth requirement must be stated as a pole, i.e.,

the 3 dB frequency. Five times away from a pole gives

0.17 dB down, which is better than the required 0.25 dB.

Therefore:

L =20Hz/5=4Hz

H =20kHzx5=100 kHz

At this point, it is a good idea to ensure that the Gain-

Bandwidth Product for the part will provide the designed gain

out to the upper 3 dB point of 100 kHz. This is why the

minimum GBWP of the LM3886 is important.

GBWP

≥ A

V xf3dB=13x100 kHz=1.3 MHz

GBWP = 2.0 MHz (min) for the LM3886

Solving for the low frequency roll-off capacitor, Ci, we have:

≥ 1/(2π Ri f

L) = 4.85 µF; use 4.7 µF.

Definition of Terms

Input Offset Voltage: The absolute value of the voltage

which must be applied between the input terminals through

two equal resistances to obtain zero output voltage and

current.

Input Bias Current: The absolute value of the average of

the two input currents with the output voltage and current at

zero.

Input Offset Current: The absolute value of the difference

in the two input currents with the output voltage and current

at zero.

Input Common-Mode Voltage Range (or Input Voltage

Range): The range of voltages on the input terminals for

which the amplifier is operational. Note that the specifica-

tions are not guaranteed over the full common-mode voltage

range unless specifically stated.

Common-Mode Rejection: The ratio of the input common-

mode voltage range to the peak-to-peak change in input

offset voltage over this range.

Power Supply Rejection: The ratio of the change in input

offset voltage to the change in power supply voltages pro-

ducing it.

Quiescent Supply Current: The current required from the

power supply to operate the amplifier with no load and the

output voltage and current at zero.

Slew Rate: The internally limited rate of change in output

voltage with a large amplitude step function applied to the

input.

Class B Amplifier: The most common type of audio power

amplifier that consists of two output devices each of which

conducts for 180˚ of the input cycle. The LM3886 is a

Quasi-AB type amplifier.

Crossover Distortion: Distortion caused in the output stage

of a class B amplifier. It can result from inadequate bias

current providing a dead zone where the output does not

respond to the input as the input cycle goes through its zero

crossing point. Also for ICs an inadequate frequency re-

sponse of the output PNP device can cause a turn-on delay

giving crossover distortion on the negative going transition

through zero crossing at the higher audio frequencies.

THD+N: Total Harmonic Distortion plus Noise refers to the

measurement technique in which the fundamental compo-

nent is removed by a bandreject (notch) filter and all remain-

ing energy is measured including harmonics and noise.

Signal-to-Noise Ratio: The ratio of a system’s output signal

level to the system’s output noise level obtained in the

absence of a signal. The output reference signal is either

specified or measured at a specified distortion level.

Continuous Average Output Power: The minimum sine

wave continuous average power output in watts (or dBW)

that can be delivered into the rated load, over the rated

bandwidth, at the rated maximum total harmonic distortion.

Music Power: A measurement of the peak output power

capability of an amplifier with either a signal duration suffi-

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

figure) and the level just before clipping distortion occurs,

expressed 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

effects 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

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 transis-

tor is operating continuously. The AC thermal resistance

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