MUTE

When in C-CUPL mode, the LM4912 also features a mute

function that is independent of load impedance and enables

extremely fast turn-on/turn-off with a minimum of output pop

and click. The mute function leaves the outputs at their bias

level, thus resulting in higher power consumption than shut-

down mode, but also provides much faster turn on/off times.

Mute mode is enabled by providing a logic high signal on the

MUTE pin in the opposite manner as the shutdown function

described above. Threshold voltages and activation tech-

niques match those given for the shutdown function as well.

Additionally, Mute should not be enabled during shutdown or

while entering or returning from shutdown. This is not a valid

operation condition and may result in much higher pop and

click values.

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 LM4912 is tolerant of

external component combinations, consideration to compo-

nent values must be used to maximize overall system qual-

ity.

The LM4912 is unity-gain stable which gives the designer

maximum system flexibility. The LM4912 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

from sources such as audio codecs. Very large values

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

R

Ω. 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 Figures 2 and 3. The input coupling capacitor, C

forms a first order high pass filter which 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

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

coupling capacitor requires more charge to reach its quies-

cent DC voltage. This charge comes from the output via the

feedback Thus, by minimizing the capacitor size based on

necessary low frequency response, turn-on time can be

minimized. A small value of C

0.39µF), is recommended.

AUDIO POWER AMPLIFIER DESIGN

A 25mW/32

Ω Audio Amplifier

Given:

Power Output

25mWrms

Load Impedance

32

Ω

Input Level

1Vrms

Input Impedance

20k

Ω

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.

3V is a standard voltage in most applications, it is chosen for

the supply rail. Extra supply voltage creates headroom that

allows the LM4912 to reproduce peak in excess of 25mW

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 gain can be determined from Equation 2.

(3)

From Equation 4, the minimum A

the desired input impedance is 20k

Ω, and with a A

1, a ratio of 1:1 results from Equation 1 for R

values are chosen with R

Ω and R

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

fied.

f

f

As stated in the External Components section, R

junction with C

C

i

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

The high frequency pole is determined by the product of the

desired frequency pole, f

an A

which is much smaller than the LM4912 GBWP of 10MHz.

This figure displays that is a designer has a need to design

an amplifier with higher differential gain, the LM4912 can still

be used without running into bandwidth limitations.

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