LM3875 datasheet(16/20 Pages) NSC

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Application Information (Continued)

resistance, the square wave response will exhibit ringing if

the capacitance is greater than about 0.2 µF. If highly ca-

pacitive loads are expected due to long speaker cables, a

method commonly employed to protect amplifiers from low

impedances at high frequencies is to couple to the load

through a 10

Ω resistor in parallel with a 0.7 µH inductor. The

inductor-resistor combination as shown in the Typical Ap-

plication Circuit isolates the feedback amplifier from the

load by providing high output impedance at high frequencies

thus allowing the 10

Ω resistor to decouple the capacitive

load and reduce the Q of the series resonant circuit. The LR

combination also provides low output impedance at low

frequencies thus shorting out the 10

Ω resistor and allowing

the amplifier to drive the series RC load (large capacitive

load due to long speaker cables) directly.

GENERALIZED AUDIO POWER AMPLIFIER DESIGN

The system designer usually knows some of the following

parameters when starting an audio amplifier design:

Desired Power Output

Input Level

Input Impedance

Load Impedance

Maximum Supply Voltage

Bandwidth

The power output and load impedance determine the power

supply requirements, however, depending upon the applica-

tion some system designers may be limited to certain maxi-

mum supply voltages. If the designer does have a power

supply limitation, he should choose a practical load imped-

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

parameters must be considered. Add the dropout voltage

(5 volts for LM3875) to the peak output swing, V

opeak,toget

the supply rail value, (i.e. + V

opeak + Vod) at a current of

opeak). 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 maxi-

mum 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 as quickly to nonlinearities. This decreased ability to

respond to nonlinearities increases the THD + N specifica-

tion.

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 40W/8

Ω AUDIO AMPLIFIER

Given:

Power Output

40W

Load Impedance

Ω

Input Level

(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

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

≥ 18

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

a sensitivity of 894 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

minimum 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/(21 − 1)=5k

Ω; 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 give

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 LM3875 is important.

GBWP = A

V xf3dB=21x100 kHz=2.1 MHz

GBWP = 2.0 MHz (min) for LM3875

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

Ci > 1/(2

π Ri f

L) = 7.8 µF; use 10 µF.

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