LM4950TS datasheet(18/25 Pages) NSC

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Application Information (Continued)

AUDIO POWER AMPLIFIER DESIGN

Audio Amplifier Design: Driving 4W into an 8

Ω BTL

The following are the desired operational parameters:

Power Output

RMS

Load Impedance

Ω

Input Level

0.3V

RMS (max)

Input Impedance

20k

Ω

Bandwidth

50Hz–20kHz ± 0.25dB

The design begins by specifying the minimum supply voltage

necessary to obtain the specified output power. One way to

find the minimum supply voltage is to use the Output Power

vs Power Supply Voltage curve in the Typical Performance

Characteristics section. Another way, using Equation (8), is

to calculate the peak output voltage necessary to achieve

the desired output power for a given load impedance. To

account for the amplifier’s dropout voltage, two additional

voltages, based on the Clipping Dropout Voltage vs Power

Supply Voltage in the Typical Performance Characteris-

tics curves, must be added to the result obtained by Equa-

tion (8). The result is Equation (9).

(8)

DD =VOUTPEAK +VODTOP +VODBOT

(9)

The Output Power vs. Power Supply Voltage graph for an 8

Ω

load indicates a minimum supply voltage of 10.2V. The com-

monly used 12V supply voltage easily meets this. The addi-

tional voltage creates the benefit of headroom, allowing the

LM4950 to produce peak output power in excess of 4W

without clipping or other audible distortion. The choice of

supply voltage must also not create a situation that violates

of maximum power dissipation as explained above in the

Power Dissipation section. After satisfying the LM4950’s

power dissipation requirements, the minimum differential

gain needed to achieve 4W dissipation in an 8

Ω BTL load is

found using Equation (10).

(10)

Thus, a minimum gain of 18.9 allows the LM4950’s to reach

full output swing and maintain low noise and THD+N perfor-

mance. For this example, let A

V-BTL = 19. The amplifier’s

overall BTL gain is set using the input (RIN

A) and feedback

(R) resistors of the first amplifier in the series BTL configu-

ration. Additionaly, A

V-BTL is twice the gain set by the first

amplifier’s R

IN and Rf. With the desired input impedance set

at 20k

Ω, the feedback resistor is found using Equation (11).

f / RIN =AV-BTL / 2(11)

The value of R

f is 190k

Ω (choose 191kΩ, the closest value).

The nominal output power is 4W.

The last step in this design example is setting the amplifier’s

-3dB frequency bandwidth. To achieve the desired ±0.25dB

pass band magnitude variation limit, the low frequency re-

sponse must extend to at least one-fifth the lower bandwidth

limit and the high frequency response must extend to at least

five times the upper bandwidth limit. The gain variation for

both response limits is 0.17dB, well within the ±0.25dB-

desired limit. The results are an

L =50Hz/5= 10Hz

(12)

20047079

FIGURE 4. Piezo-electric Transducer Capacitance

≤ 200nF

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