LM4916MM datasheet(15/18 Pages) NSC

15 / 18 page

Application Information (Continued)

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 LM4916 settles to quiescent operation, its value is critical

when minimizing turn-on pops. The slower the LM4916’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 LM4916 resumes opera-

tion after shutdown.

Minimizing External Components

Operating the LM4916 at higher gain settings can minimize

the use of external components. For instance, a BTL con-

figuration with a gain setting greater than 8V/V (A

V > 8)

makes the output capacitor C

O unnecessary. For the Single

Ended configuration, a gain setting greater than 4V/V (A

V >

4) eliminates the need for output capacitor C

O2 and output

resistor R

O, on each output channel.

If the LM4916 is operating with a lower gain setting (A

V < 4),

external components can be further minimized only in Single

Ended mode. For each channel, output capacitor (C

O2 ) and

output resistor (R

O) can be eliminated. These components

need to be compensated for by adding a 7.5k

Ω resistor (R

between the input pin and ground pin on each channel

(between Pin 1 and GND, and between Pin 5 and GND).

OPTIMIZING CLICK AND POP REDUCTION

PERFORMANCE

The LM4916 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 LM4916’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

for the supply rail. Extra supply voltage creates headroom

that allows the LM4916 to reproduce peak in excess of

10mW 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

gain can be determined from Equation 2.

(4)

From Equation 4, the minimum AV is 1; use A

V = 1. Since the

desired input impedance is 20k, and with a A

V gain of 1, a

ratio of 1:1 results from Equation 1 for R

f to R. The values

are chosen with R

i = 20k and Rf = 20k. 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.

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