LM48 datasheet(14/24 Pages) TI1

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LM4808

SNAS051D – FEBRUARY 2000 – REVISED MAY 2013

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SELECTING PROPER EXTERNAL COMPONENTS

Optimizing the LM4808's performance requires properly selecting external components. Though the LM4808

operates well when using external components with wide tolerances, best performance is achieved by optimizing

component values.

The LM4808 is unity-gain stable, giving a designer maximum design flexibility. The gain should be set to no more

than a given application requires. This allows the amplifier to achieve minimum THD+N and maximum signal-to-

noise ratio. These parameters are compromised as the closed-loop gain increases. However, low gain demands

input signals with greater voltage swings to achieve maximum output power. Fortunately, many signal sources

such as audio CODECs have outputs of 1VRMS (2.83VP-P). Please refer to the AUDIO POWER AMPLIFIER

DESIGN section for more information on selecting the proper gain.

Input and Output Capacitor Value Selection

Amplifying the lowest audio frequencies requires high value input and output coupling capacitors (CI and CO in

Figure 1). A high value capacitor can be expensive and may compromise space efficiency in portable designs. In

many cases, however, the speakers used in portable systems, whether internal or external, have little ability to

reproduce signals below 150Hz. Applications using speakers with this limited frequency response reap little

improvement by using high value input and output capacitors.

Besides affecting system cost and size, Ci has an effect on the LM4808's click and pop performance. The

magnitude of the pop is directly proportional to the input capacitor's size. Thus, pops can be minimized by

selecting an input capacitor value that is no higher than necessary to meet the desired

−3dB frequency.

As shown in Figure 1, the input resistor, RI and the input capacitor, CI, produce a −3dB high pass filter cutoff

frequency that is found using Equation 3. In addition, the output load RL, and the output capacitor CO, produce a

-3db high pass filter cutoff frequency defined by Equation 4.

fI-3db=1/2πRICI

(3)

fO-3db=1/2πRLCO

(4)

Also, careful consideration must be taken in selecting a certain type of capacitor to be used in the system.

Different types of capacitors (tantalum, electrolytic, ceramic) have unique performance characteristics and may

affect overall system performance.

Bypass Capacitor Value

Besides minimizing the input capacitor size, careful consideration should be paid to the value of the bypass

capacitor, CB. Since CB determines how fast the LM4808 settles to quiescent operation, its value is critical when

minimizing turn-on pops. The slower the LM4808's outputs ramp to their quiescent DC voltage (nominally 1/2

VDD), the smaller the turn-on pop. Choosing CB equal to 1.0µF or larger, will minimize turn-on pops. As discussed

above, choosing Ci no larger than necessary for the desired bandwidth helps minimize clicks and pops.

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LM48 Datasheet(PDF) 14 Page - Texas Instruments