LM4919 datasheet(13/15 Pages) NSC

13 / 15 page

Application Information (Continued)

during the shutdown transitions or any other time the part is

in normal operation. Failure to operate mute correctly may

result in much higher click and pop values or failure of the

device to mute at all.

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 LM4919 is tolerant of

external component combinations, consideration to compo-

nent values must be used to maximize overall system qual-

ity. The LM4919 is unity-gain stable that gives the designer

maximum system flexibility. The LM4919 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 1V

rms are available

from sources such as audio codecs. Very large values

should not be used for the gain-setting resistors. Values for

i and Rf should be less than 1M

Ω. Please refer to the

section, Audio Power Amplifier Design, for a more complete

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 bandwidth is dictated by the

choice of external components shown in Figures 2 and 3.

The input coupling capacitor, C

i, forms a first order high pass

filter that limits low frequency response. This value should be

chosen based on needed frequency response and turn-on

time.

SELECTION OF INPUT CAPACITOR SIZE

Amplifying the lowest audio frequencies requires a high

value input coupling capacitor, C

i. A high value capacitor can

be expensive and may compromise space efficiency in por-

table designs. In many cases, however, the headphones

used in portable systems have little ability to reproduce

signals below 60Hz. Applications using headphones with this

limited frequency response reap little improvement by using

a high value input capacitor. In addition to system cost and

size, turn on time is affected by the size of the input coupling

capacitor C

i. A larger input coupling capacitor requires more

charge to reach its quiescent DC voltage. This charge

comes from the output via the feedback. Thus, by minimizing

the capacitor size based on necessary low frequency re-

sponse, turn-on time can be minimized. A small value of C

(in the range of 0.1µF to 0.47µF), is recommended.

Bypass Capacitor Value Selection

Besides minimizing the input capacitor size, careful consid-

eration should be paid to value of C

B, the capacitor con-

nected to the BYPASS pin. Since C

B determines how fast

the LM4919 settles to quiescent operation, its value is critical

when minimizing turn-on pops. The slower the LM4919’s

outputs ramp to their quiescent DC voltage (nominally V

DD/

2), the smaller the turn-on pop. Choosing C

B equal to 4.7µF

along with a small value of C

i (in the range of 0.1µF to

0.47µF), produces a click-less and pop-less shutdown func-

tion. As discussed above, choosing C

i no larger than neces-

sary for the desired bandwidth helps minimize clicks and

pops. This ensures that output transients are eliminated

when power is first applied or the LM4919 resumes opera-

tion after shutdown.

OPTIMIZING CLICK AND POP REDUCTION

PERFORMANCE

The LM4919 contains circuitry that eliminates turn-on and

shutdown transients ("clicks and pops"). For this discussion,

turn-on refers to either applying the power supply voltage or

when the micro-power shutdown mode is deactivated.

As the V

DD/2 voltage present at the BYPASS pin ramps to its

final value, the LM4919’s internal amplifiers are configured

as unity gain buffers. An internal current source charges the

capacitor connected between the BYPASS pin and GND in a

controlled, linear manner. Ideally, the input and outputs track

the voltage applied to the BYPASS pin. The gain of the

internal amplifiers remains unity until the voltage on the

bypass pin reaches V

DD/2. As soon as the voltage on the

bypass pin is stable, the device becomes fully operational

and the amplifier outputs are reconnected to their respective

output pins. Although the BYPASS pin current cannot be

modified, changing the size of C

B alters the device’s turn-on

time. There is a linear relationship between the size of C

and the turn-on time. Here are some typical turn-on times for

various values of C

Single-Ended

B(µF)

0.1

117ms

0.22

179ms

0.47

310ms

1.0

552ms

2.2

1.14s

4.7

2.4s

BTL

B(µF)

ON (ms)

0.1

0.22

0.47

1.0

112

2.2

163

4.7

283

In order to eliminate "clicks and pops", all capacitors must be

discharged before turn-on. Rapidly switching V

DD may not

allow the capacitors to fully discharge, which may cause

"clicks and pops".

AUDIO POWER AMPLIFIER DESIGN

A 25mW/32

Ω Audio Amplifier

Given:

Power Output

10mWrms

Load Impedance

Ω

Input Level

0.4Vrms

Input Impedance

20k

Ω

A designer must first choose a mode of operation (SE or

BTL) and determine the minimum supply rail to obtain the

specified output power. By extrapolating from the Output

Power vs. Supply Voltage graphs in the Typical Performance

Characteristics section, the supply rail can be easily found.

1.5V is a standard voltage in most applications, it is chosen

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