LM3876TF datasheet(15/19 Pages) NSC

15 / 19 page

Application Information (Continued)

ance which would allow the amplifier to provide the desired

output power, keeping in mind the current limiting capabili-

ties of the device. In any case, the output signal swing and

current are found from (where P

O is the average output

power):

(5)

(6)

To determine the maximum supply voltage the following pa-

rameters must be considered. Add the dropout voltage (5V

for LM3876) to the peak output swing, V

opeak, to get the sup-

ply rail value (i.e. ± (V

opeak + Vod) at a current of Iopeak). The

regulation of the supply determines the unloaded voltage,

usually about 15% higher. Supply voltage will also rise 10%

during high line conditions. Therefore, the maximum supply

voltage is obtained from the following equation:

Max. supplies

≈ ± (V

opeak + Vod)(1 + regulation)(1.1) (7)

The input sensitivity and the output power specs determine

the minimum required gain as depicted below:

(8)

Normally the gain is set between 20 and 200; for a 40W, 8

Ω

audio amplifier this results in a sensitivity of 894 mV and

89 mV, respectively. Although higher gain amplifiers provide

greater output power and dynamic headroom capabilities,

there are certain shortcomings that go along with the so

called “gain.” The input referred noise floor is increased and

hence the SNR is worse. With the increase in gain, there is

also a reduction of the power bandwidth which results in a

decrease in feedback thus not allowing the amplifier to re-

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 out-

put. 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 ampli-

fiers are designed from the non-inverting amplifier configura-

tion.

DESIGN A 40W/8

Ω AUDIO AMPLIFIER

Given:

Power Output

40W

Load Impedance

Ω

Input Level

1V(max)

Input Impedance

100 k

Ω

Bandwidth

20 Hz–20 kHz ± 0.25 dB

Equations (5), (6) give:

40W/8

Ω

opeak = 25.3V

opeak = 3.16A

Therefore the supply required is: ±30.3V @ 3.16A

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

±38.3V 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 de-

signing 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:

V ≥ 18

We select a gain of 21 (Non-Inverting Amplifier); resulting in

a sensitivity of 894 mV.

Letting R

IN equal 100 kΩ gives the required input imped-

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

less 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 kΩ. Solving

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

Ri = R

f1/(AV −1) = 100k/(21 − 1) = 5kΩ; use 5.1 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 = 20 Hz/5 = 4Hz

H = 20 kHz x 5 = 100 kHz

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

Gain-Bandwidth Product for the part will provide the de-

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

why the minimum GBWP of the LM3876 is important.

GBWP

≥ A

V xf3dB = 21 x 100 kHz = 2.1 MHz

GBWP = 2.0 MHz (min) for the LM3876

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

≥ 1/(2π Ri f

L) = 7.8 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 cur-

rent.

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

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 in-

put.

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