LM4908MM datasheet(14/21 Pages) NSC

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Application Information (Continued)

Also, careful consideration must be taken in selecting a

certain type of capacitor to be used in the system. Different

types of capacitors (tantalum, electrolytic, ceramic) have

unique performance characteristics and may affect overall

system performance.

Bypass Capacitor Value

Besides minimizing the input capacitor size, careful consid-

eration should be paid to the value of the bypass capacitor,

B. Since CB determines how fast the LM4908 settles to

quiescent operation, its value is critical when minimizing

turn-on pops. The slower the LM4908’s outputs ramp to their

quiescent DC voltage (nominally 1/2 V

DD), the smaller the

turn-on pop. Choosing C

B equal to 1.0µF or larger, will

minimize turn-on pops. As discussed above, choosing C

i no

larger than necessary for the desired bandwith helps mini-

mize clicks and pops.

AUDIO POWER AMPLIFIER DESIGN

Design a Dual 70mW/32

Ω Audio Amplifier

Given:

Power Output

70mW

Load Impedance

Ω

Input Level

1Vrms (max)

Input Impedance

20k

Ω

Bandwidth

100Hz–20kHz ± 0.50dB

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 Supply Voltage curve in the Typical Performance Char-

acteristics section. Another way, using Equation (5), is to

calculate the peak output voltage necessary to achieve the

desired output power for a given load impedance. To ac-

count for the amplifier’s dropout voltage, two additional volt-

ages, based on the Dropout Voltage vs Supply Voltage in the

Typical Performance Characteristics curves, must be

added to the result obtained by Equation (5). For a single-

ended application, the result is Equation (6).

(5)

≥ (2V

OPEAK +(VODTOP +VODBOT))

(6)

The Output Power vs Supply Voltage graph for a 32

Ω load

indicates a minimum supply voltage of 4.8V. This is easily

met by the commonly used 5V supply voltage. The additional

voltage creates the benefit of headroom, allowing the

LM4908 to produce peak output power in excess of 70mW

without clipping or other audible distortion. The choice of

supply voltage must also not create a situation that violates

maximum power dissipation as explained above in the

Power Dissipation section. Remember that the maximum

power dissipation point from Equation (1) must be multiplied

by two since there are two independent amplifiers inside the

package. Once the power dissipation equations have been

addressed, the required gain can be determined from Equa-

tion (7).

(7)

Thus, a minimum gain of 1.497 allows the LM4908 to reach

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

mance. For this example, let A

V = 1.5.

The amplifiers overall gain is set using the input (R

i ) and

feedback (R

f ) resistors. With the desired input impedance

set at 20k

Ω, the feedback resistor is found using Equation

(8).

V =Rf/Ri

(8)

The value of R

f is 30k

Ω.

The last step in this design is setting the amplifier’s −3db

frequency bandwidth. To achieve the desired ±0.25dB pass

band magnitude variation limit, the low frequency response

must extend to at lease 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 = 100Hz/5 = 20Hz

(9)

and a

H = 20kHz

5 = 100kHz

(10)

As stated in the External Components section, both R

i in

conjunction with C

i, and Co with RL, create first order high-

pass filters. Thus to obtain the desired low frequency re-

sponse of 100Hz within ±0.5dB, both poles must be taken

into consideration. The combination of two single order filters

at the same frequency forms a second order response. This

results in a signal which is down 0.34dB at five times away

from the single order filter −3dB point. Thus, a frequency of

20Hz is used in the following equations to ensure that the

response is better than 0.5dB down at 100Hz.

≥ 1/(2π *20kΩ * 20 Hz) = 0.397µF; use 0.39µF.

≥ 1/(2π *32Ω * 20 Hz) = 249µF; use 330µF.

The high frequency pole is determined by the product of the

desired high frequency pole, f

H, and the closed-loop gain,

V. With a closed-loop gain of 1.5 and fH = 100kHz, the

resulting GBWP = 150kHz which is much smaller than the

LM4908’s GBWP of 3MHz. This figure displays that if a

designer has a need to design an amplifier with a higher

gain, the LM4908 can still be used without running into

bandwidth limitations.

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