LM4879 datasheet(13/25 Pages) NSC

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Application Information (Continued)

Selection Of Input Capacitor Size

Large input capacitors are both expensive and space hungry

for portable designs. Clearly, a certain sized capacitor is

needed to couple in low frequencies without severe attenu-

ation. But in many cases the speakers used in portable

systems, whether internal or external, have little ability to

reproduce signals below 100 Hz to 150 Hz. Thus, using a

large input capacitor may not increase actual system perfor-

mance.

In addition to system cost and size, click and pop perfor-

mance is effected by the size of the input coupling capacitor,

i. A larger input coupling capacitor requires more charge to

reach its quiescent DC voltage (nominally 1/2 V

DD). This

charge comes from the output via the feedback and is apt to

create pops upon device enable. Thus, by minimizing the

capacitor size based on necessary low frequency response,

turn-on pops can be minimized.

Besides minimizing the input capacitor size, careful consid-

eration should be paid to the bypass capacitor value. Bypass

capacitor, C

B, is the most critical component to minimize

turn-on pops since it determines how fast the LM4879 turns

on. The slower the LM4879’s outputs ramp to their quiescent

DC voltage (nominally 1/2 V

DD), the smaller the turn-on pop.

Choosing C

B equal to 1.0 µF along with a small value of Ci

(in the range of 0.1 µF to 0.39 µF), should produce a virtually

clickless and popless shutdown function. While the device

will function properly, (no oscillations or motorboating), with

B equal to 0.1 µF, the device will be much more susceptible

to turn-on clicks and pops. Thus, a value of C

B equal to

1.0 µF is recommended in all but the most cost sensitive

designs.

AUDIO POWER AMPLIFIER DESIGN

A 1W/8

Ω Audio Amplifier

Given:

Power Output

1 Wrms

Load Impedance

Ω

Input Level

1 Vrms

Input Impedance

20 k

Ω

Bandwidth

100 Hz–20 kHz ± 0.25 dB

A designer must first determine the minimum supply rail to

obtain the specified output power. By extrapolating from the

Output Power vs Supply Voltage graphs in the Typical Per-

formance Characteristics section, the supply rail can be

easily found. A second way to determine the minimum sup-

ply rail is to calculate the required V

opeak using Equation 2

and add the output voltage. Using this method, the minimum

supply voltage would be (V

opeak +(VODTOP +VODBOT)), where

ODBOT and VODTOP are extrapolated from the Dropout Volt-

age vs Supply Voltage curve in the Typical Performance

Characteristics section.

(2)

5V is a standard voltage, in most applications, chosen for the

supply rail. Extra supply voltage creates headroom that al-

lows the LM4879 to reproduce peaks in excess of 1W with-

out producing audible distortion. At this time, the designer

must make sure that the power supply choice along with the

output impedance does not violate the conditions explained

in the Power Dissipation section.

Once the power dissipation equations have been addressed,

the required differential gain can be determined from Equa-

tion 3.

(3)

VD =(Rf/Ri)2

From Equation 3, the minimum A

VD is 2.83; use AVD =3.

Since the desired input impedance was 20 k

Ω, and with a

VD of 3, a ratio of 1.5:1 of Rf to Ri results in an allocation of

i =20k

Ω and R

f =30k

Ω. The final design step is to

address the bandwidth requirements which must be stated

as a pair of −3 dB frequency points. Five times away from a

−3 dB point is 0.17 dB down from passband response which

is better than the required ±0.25 dB specified.

L = 100 Hz/5 = 20 Hz

H =20kHz*5=100 kHz

As stated in the External Components section, R

i in con-

junction with C

i create a highpass filter.

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

The high frequency pole is determined by the product of the

desired frequency pole, f

H, and the differential gain, AVD.

With a A

VD = 3 and fH = 100 kHz, the resulting GBWP =

300 kHz which is much smaller than the LM4879 GBWP of

10 MHz. This figure displays that if a designer has a need to

design an amplifier with a higher differential gain, the

LM4879 can still be used without running into bandwidth

limitations.

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