LM4990MM datasheet(14/19 Pages) NSC

14 / 19 page

Application Information (Continued)

external component combinations, consideration to compo-

nent values must be used to maximize overall system qual-

ity.

The LM4990 is unity-gain stable which gives the designer

maximum system flexibility. The LM4990 should be used in

low gain configurations to minimize THD+N+N values, and

maximize the signal to noise ratio. Low gain configurations

require large input signals to obtain a given output power.

Input signals equal to or greater than 1Vrms are available

from sources such as audio codecs. Please refer to the

section, Audio Power Amplifier Design, for a more com-

plete explanation of proper gain selection.

Besides gain, one of the major considerations is the closed-

loop bandwidth of the amplifier. To a large extent, the band-

width is dictated by the choice of external components

shown in Figure 1. The input coupling capacitor, C

i, forms a

first order high pass filter which limits low frequency re-

sponse. This value should be chosen based on needed

frequency response for a few distinct reasons.

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 100Hz to 150Hz. 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 LM4990 turns

on. The slower the LM4990’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 along with a small value of Ci (in

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

clickless and popless shutdown function. While the device

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

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

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

B equal to

1.0µF is recommended in all but the most cost sensitive

designs.

AUDIO POWER AMPLIFIER DESIGN

A 1W/8

Ω Audio Amplifier

Given:

Power Output

1Wrms

Load Impedance

Ω

Input Level

1Vrms

Input Impedance

20k

Ω

Bandwidth

100Hz–20kHz ± 0.25dB

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.

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

the supply rail. Extra supply voltage creates headroom that

allows the LM4990 to reproduce peaks in excess of 1W

without producing audible distortion. At this time, the de-

signer 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 2.

(2)

f/Ri =AVD/2

From Equation 2, the minimum A

VD is 2.83; use AVD =3.

Since the desired input impedance was 20k

Ω, and with a

VD impedance of 2, a ratio of 1.5:1 of Rf to Ri results in an

allocation of R

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 −3dB frequency points. Five times away

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

which is better than the required ±0.25dB specified.

L = 100Hz/5 = 20Hz

H = 20kHz*5= 100kHz

As stated in the External Components section, R

i in con-

junction with C

i create a highpass filter.

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

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 = 100kHz, the resulting GBWP =

300kHz which is much smaller than the LM4990 GBWP of

2.5MHz. This figure displays that if a designer has a need to

design an amplifier with a higher differential gain, the

LM4990 can still be used without running into bandwidth

limitations.

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