LM4811G-SM2-R datasheet(7/9 Pages) UTC

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LM4811

Preliminary

CMOS IC

UNISONICTECHNOLOGIESCO.,LTD

7 of 9

www.unisonic.com.tw

QW-R502-560.b



APPLICATION INFORMATION(Cont.)

PROPER SELECTION OF EXTERNAL COMPONENTS

In applications, selection of external components when using integrated power amplifiers is critical for optimum

device and system performance. Moreover, the vales of external components are also important to consider as they

directly affect the overall system quality. The UTC LM4811 is tolerant to a variety external component combinations.

The UTC LM4811 is unity-gain stable, providing designers maximize system performance. Low gain settings

maximize signal-to-noise performance and minimizes THD+N. Low gain configurations require large input signals to

obtain a given output power. Input signals reach 1Vrms or higher are available from sources such as audio codecs.

SELECTION OF INPUT AND OUTPUT CAPACITOR SIZE

Another consideration is the closed loop bandwidth besides gain. To a large extent, the bandwidth is dicated by

the choice of external components shown in Figure 1. Both the CI (input coupling capacitor) and the CO (output

coupling capacitor) form first order high pass filters which limit low frequency response. The values of these

capacitors should be based on the desired frequency response weighed against the following:

Large value input and output capacitors are both expensive and space consuming for portable designs. Clearly

a certain sized capacitor is needed to couple in low frequencies without severe attenuation. But in many cases the

speakers used in portable systems, whether internal or external, have little ability to reproduce signals below 150Hz.

Thus large input and output capacitors may not increase system performance.

Moreover, for the system cost and size, click and pop performance is affected by the size of the CI (input

coupling capacitor). A larger input coupling capacitor requires more charge to reach its quiescent DC voltage

(nominally 1/2 VDD ). This charge comes from the output via the feedback and the pops are apt to be generated upon

device enable. Reducing the value of input coupling capacitor can minimize the turn on pops, which is based on

necessary low frequency response.

The value of bypass capacitor should be taken into consideration besides minimizing the input and output

capacitor values. For minimizing turn on pops, the bypass capacitor CB plays the most important role, since it

determines how fast the UTC LM4811 turns on. The slower the UTC LM4811’s outputs ramp to their quiescent DC

voltage (nominally 1/2 VDD ), the smaller the turn on pop. While the device will function properly, (no oscillations or

motorboating), with CB equal to 1μF, the device will be much more susceptible to turn on clicks and pops. Thus, a

value of CB equal to 1μF or larger is recommended in all but the most cost sensitive designs.

In addition, the type of capacitors should be given enough consideration since different types of capacitors

(tantalum, electrolytic, ceramic) have unique performance characteristics and may affect overall system

performance.

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