LM4873 datasheet(17/25 Pages) NSC

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Application Information (Continued)

Input Capacitor Value Selection

Amplifying the lowest audio frequencies requires high value

input coupling capacitor (C

i in Figure 2). A high value capaci-

tor 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 150 Hz. Applications

using speakers with this limited frequency response reap

little improvement by using large input capacitor.

Besides effecting system cost and size, C

i has an affect on

the LM4873’s click and pop performance. When the supply

voltage is first applied, a transient (pop) is created as the

charge on the input capacitor changes from zero to a quies-

cent state. The magnitude of the pop is directly proportional

to the input capacitor’s size. Higher value capacitors need

more time to reach a quiescent DC voltage (usually V

DD/2)

when charged with a fixed current. The amplifier’s output

charges the input capacitor through the feedback resistor,

f. Thus, pops can be minimized by selecting an input

capacitor value that is no higher than necessary to meet the

desired −3dB frequency.

A shown in Figure 2, the input resistor (R

I) and the input

capacitor, C

I produce a −3dB high pass filter cutoff frequency

that is found using Equation (7).

(7)

As an example when using a speaker with a low frequency

limit of 150Hz, C

i, using Equation (4) is 0.063µF. The 1.0µF

i shown in Figure 2 allows the LM4873 to drive high effi-

ciency, full range speaker whose response extends below

30Hz.

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 LM4873 settles to quiescent operation, its value is critical

when minimizing turn-on pops. The slower the LM4873’s

outputs ramp to their quiescent DC voltage (nominally 1/2

DD), the smaller the turn-on pop. Choosing CB equal to

1.0 µF along with a small value of C

i (in the range of 0.1 µF

to 0.39 µF), produces a click-less and pop-less shutdown

function. As discussed above, choosing C

i no larger than

necessary for the desired bandwith helps minimize clicks

and pops.

OPTIMIZING CLICK AND POP REDUCTION

PERFORMANCE

The LM4873 contains circuitry that minimizes turn-on and

shutdown transients or “clicks and pop”. For this discussion,

turn-on refers to either applying the power supply voltage or

when the shutdown mode is deactivated. While the power

supply is ramping to its final value, the LM4873’s internal

amplifiers are configured as unity gain buffers. An internal

current source changes the voltage of the BYPASS pin 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 1/2 V

DD . As soon as the voltage on the

bypass pin is stable, the device becomes fully operational.

Although the BYPASS pin current cannot be modified,

changing the size of C

B alters the device’s turn-on time and

the magnitude of “clicks and pops”. Increasing the value of

B reduces the magnitude of turn-on pops. However, this

presents a tradeoff: as the size of C

B increases, the turn-on

time increases. There is a linear relationship between the

size of C

B and the turn-on time. Here are some typical

turn-on times for various values of C

0.01µF

20ms

0.1µF

200ms

0.22µF

440ms

0.47µF

940ms

1.0µF

2sec

In order 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”. In a single-ended configuration, the output

is coupled to the load by C

OUT. This capacitor usually has a

high value. C

OUT discharges through internal 20k

Ω resistors.

Depending on the size of C

OUT, the discharge time constant

can be relatively large. To reduce transients in single-ended

mode, an external 1k

Ω–5kΩ resistor can be placed in par-

allel with the internal 20k

Ω resistor. The tradeoff for using

this resistor is increased quiescent current.

NO LOAD STABILITY

The LM4873 may exhibit low level oscillation when the load

resistance is greater than 10k

Ω. This oscillation only occurs

as the output signal swings near the supply voltages. Pre-

vent this oscillation by connecting a 5k

Ω between the output

pins and ground.

AUDIO POWER AMPLIFIER DESIGN

Audio Amplifier Design: Driving 1W into an 8

Ω Load

The following are the desired operational parameters:

Power Output:

RMS

Load Impedance:

Ω

Input Level:

rms

Input Impedance:

20k

Ω

Bandwidth:

100Hz−20kHz ± 0.25dB

The design begins by specifying the minimum supply voltage

necessary to obtain the specified output power. One way to

find the minimum supply voltage is to use the Output Power

vs Supply Voltage curve in the Typical Performance Char-

acteristics section. Another way, using Equation (8), is to

calculate the peak output voltage necessary to achieve the

desired output power for a given load impedance. To ac-

count for the amplifier’s dropout voltage, two additional volt-

ages, based on the Dropout Voltage vs Supply Voltage in the

Typical Performance Characteristics curves, must be

added to the result obtained by Equation (8). The result in

Equation (9).

(8)

≥ (V

OUTPEAK +(VODTOP +VODBOT))

(9)

The Output Power vs Supply Voltage graph for an 8

Ω load

indicates a minimum supply voltage of 4.6V. This is easily

met by the commonly used 5V supply voltage. The additional

voltage creates the benefit of headroom, allowing the

LM4873 to produce peak output power in excess of 1W

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