LM4890 datasheet(19/31 Pages) NSC

19 / 31 page

Application Information (Continued)

SHUTDOWN OUTPUT IMPEDANCE

For R

f = 20k ohms:

OUT1 (between Out1 and GND) = 10k||50k||Rf =6k

Ω

OUT2 (between Out2 and GND) = 10k||(40k+(10k||Rf)) =

8.3k

Ω

OUT1-2 (between Out1 and Out2) = 40k||(10k+(10k||Rf)) =

11.7k

Ω

The -3dB roll off for these measurements is 600kHz

PROPER SELECTION OF EXTERNAL COMPONENTS

Proper selection of external components in applications us-

ing integrated power amplifiers is critical to optimize device

and system performance. While the LM4890 is tolerant of

external component combinations, consideration to compo-

nent values must be used to maximize overall system qual-

ity.

The LM4890 is unity-gain stable which gives the designer

maximum system flexibility. The LM4890 should be used in

low gain configurations to minimize THD+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

IN, 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,

IN. 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

BYPASS, is the most critical component to mini-

mize turn-on pops since it determines how fast the LM4890

turns on. The slower the LM4890’s outputs ramp to their

quiescent DC voltage (nominally 1/2V

DD), the smaller the

turn-on pop. Choosing C

BYPASS equal to 1.0µF along with a

small value of C

IN, (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 C

BYPASS equal to 0.1µF, the device will

be much more susceptible to turn-on clicks and pops. Thus,

a value of C

BYPASS 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

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 which in most applications is cho-

sen for the supply rail. Extra supply voltage creates head-

room that allows the LM4890 to reproduce peaks in excess

of 1W without 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)

f/RIN =AVD/2

From Equation 3, the minimum A

VD is 2.83; use AVD =3.

Since the desired input impedance is 20 k

Ω, and with an A

gain of 3, a ratio of 1.5:1 of R

f to RIN results in an allocation

of R

IN =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 = 100Hz/5 = 20Hz

H = 20kHz*5= 100kHz

LM4890 Datasheet(PDF) 19 Page - National Semiconductor (TI)

LM4890 Datasheet(HTML) 19 Page - National Semiconductor (TI)

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31

Datasheet Download

Link URL