Application Information

AMPLIFIER CONFIGURATION

As shown in Figure 1, the LM4980 consists of a stereo pair

of audio amplifiers. These amplifiers operate on a single

supply and have single-ended inputs and outputs. The qui-

escent operating point of each amplifier input and output is

equal to the voltage applied to the V

C

Careful consideration should be paid to value of C

the capacitor connected between the MIDCAP pin and

ground. The value of C

LM4980 settles to quiescent operation and determines the

amount of output transient suppression. Choosing C

MIDCAP

equal to 4.7µF along with a small value of C

of 0.1µF to 1.0µF), produces shutdown function that is es-

sentially output-transient free. Choosing C

necessary for the desired bandwidth helps minimize clicks

and pops. This ensures that output transients are minimized

when power is first applied or the LM4980 resumes opera-

tion after shutdown. The MIDCAP offers the following ben-

efits: better linearity for reduced THD+N, reduced channel-

to-channel crosstalk, and less susceptibility to ground noise.

For the ultimate suppression of output transient when power

is applied or removed, ensure that the voltage applied to the

SHDN pin is a logic low. This will activate the micro-power

shutdown.

OPTIMIZING OUTPUT-GROUND NOISE REDUCTION

In addition to the output-ground noise reduction afforded by

C

of a ferrite bead. The ferrite bead (FB) is placed between

ground and common connection between the C

the headphone ground connection. This is shown in Figure

3. The ferrite bead is beneficial in environments where the

headphone and C

circuitry (such as video) that may inject noise on a common

ground.

OPTIMIZING OUTPUT TRANSIENT SUPPRESSION

The LM4980 contains circuitry that eliminates turn-on and

shutdown output transients ("clicks and pops"). For this dis-

cussion, turn-on refers to either applying the power supply

voltage or when the micro-power shutdown mode is deacti-

vated. The turn-on time delay is the time duration that occurs

between the application of the power supply voltage or de-

activating shutdown and when the applied input signal ap-

pears at the amplifier outputs.

C

output transients. The amount of suppression increases as

C

C

relationship between the value of C

time. Here are some typical turn-on times for various values

of C

TABLE 1. Typical turn-on time versus C

C

Turn-On Time (ms)

4.7

250

6.8

360

10.0

530

STAND-ALONE V

The LM4980 is designed to take advantage of audio DACs

(digital-to-analog converters) and other signal sources that,

in addition to generating an analog signal, also create an AC

ground potential. This AC ground potential is typically V

This V

Using two external resistors allows the LM4980 to be easily

used in applications where the V

generated and supplied to the LM4980 by other circuits.

Figure 4 shows this configuration.

SELECTING THE OUTPUT COUPLING CAPACITOR

VALUE

To ensure that no performance degrading DC current flows

through the load (something with which speakers would just

as soon not have to tolerate), coupling capacitors are nec-

essary between the amplifier output pins and the load. Be-

sides blocking DC current, the output coupling capacitor

value, together with the load resistance, produces a low

frequency amplitude rolloff, whose cutoff frequency is found

using Equation 1.

(1)

When driving 32

Ω headphones, the 220µF C

pacitors shown in Figure 2 produce a cutoff frequency equal

to 23Hz.

The output coupling capacitors also influence the output

transient behavior at power-up and when activating or deac-

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FIGURE 3. Adding a ferrite bead improves

ground-noise suppression

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FIGURE 4. Simple circuit generates LM4980’s V

MID

voltage

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