LM2876 datasheet(18/22 Pages) NSC

18 / 22 page

Application Information (Continued)

spond quickly enough to nonlinearities. This decreased abil-

ity to respond to nonlinearities increases the THD + N speci-

fication.

The desired input impedance is set by R

IN. Very high values

can cause board layout problems and DC offsets at the

output. The value for the feedback resistance, R

f1, should be

chosen to be a relatively large value (10 k

Ω–100 kΩ), and

the other feedback resistance, Ri, is calculated using stan-

dard op amp configuration gain equations. Most audio am-

plifiers are designed from the non-inverting amplifier configu-

ration.

DESIGN A 25W/8

Ω AUDIO AMPLIFIER

Given:

Power Output

25W

Load Impedance

Ω

Input Level

1V(max)

Input Impedance

100 k

Ω

Bandwidth

20 Hz–20 kHz ± 0.25 dB

Equation (5) and Equation (6) give:

25W/8

Ω

opeak = 20.0V

opeak = 2.5A

Therefore the supply required is: ±24.0V @ 2.5A

With 15% regulation and high line the final supply voltage is

±30.36V using Equation (7). At this point it is a good idea to

check the Power Output vs Supply Voltage to ensure that the

required output power is obtainable from the device while

maintaining low THD + N. It is also good to check the Power

Dissipation vs Supply Voltage to ensure that the device can

handle the internal power dissipation. At the same time

designing in a relatively practical sized heat sink with a low

thermal resistance is also important. Refer to Typical Per-

formance Characteristics graphs and the Thermal Con-

siderations section for more information.

The minimum gain from Equation (8) is: A

≥ 14

We select a gain of 15 (Non-Inverting Amplifier); resulting in

a sensitivity of 942.8 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/(15 − 1) = 7.1 k

Ω; use 6.8 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 LM2876 is important.

GBWP

≥ A

V xf3dB=15x100 kHz=1.5 MHz

GBWP = 2.0 MHz (min) for the LM2876

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

≥ 1/(2π Ri f

L) = 5.9 µF; use 10 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 LM2876 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.

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