LM4888 datasheet(17/22 Pages) NSC

17 / 22 page

Application Information (Continued)

150 Hz. Applications using speakers with this limited fre-

quency response reap little improvement by using large

input capacitor.

Besides effecting system cost and size, C1 and C2 have an

effect on the LM4888’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

quiescent state. The magnitude of the pop is directly propor-

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

need more time to reach a quiescent DC voltage (usually

DD/2) when charged with a fixed current. The amplifier’s

output charges the input capacitor through the feedback

resistors, R2 and R8. 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 1, the input resistors (R1,4,5, and 6) and

the input capacitors, C1 and C2 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

1, using Equation (7) is 0.053µF. The .33µF

1 shown in Figure 1 allows the LM4888 to drive high

efficiency, 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

6, the capacitor con-

nected to the BYPASS pin. Since C

6 determines how fast the

LM4888 settles to quiescent operation, its value is critical

when minimizing turn-on pops. The slower the LM4888’s

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

DD), the smaller the turn-on pop. Choosing C6 equal to

1.0 µF along with a small value of C

1 (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

1 no larger than

necessary for the desired bandwith helps minimize clicks

and pops. Connecting a 1µF capacitor, C

6, between the

BYPASS pin and ground improves the internal bias voltage’s

stability and improves the amplifier’s PSRR.

OPTIMIZING CLICK AND POP REDUCTION

PERFORMANCE

The LM4888 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. When the part is

turned on, 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

6 alters the device’s turn-on

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

value of C

6 reduces the magnitude of turn-on pops. How-

ever, this presents a tradeoff: as the size of C

6 increases, the

turn-on time increases. There is a linear relationship be-

tween the size of C

6 and the turn-on time. Here are some

typical turn-on times for various values of C

0.01µF

30ms

0.1µF

40ms

0.22µF

60ms

0.47µF

80ms

1.0µF

140 ms

In order eliminate “clicks and pops”, all capacitors must be

discharged before turn-on. Rapidly switching V

DD on and off

may not allow the capacitors to fully discharge, which may

cause “clicks and pops”.

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 4.35V for a 1W output

at 1% THD+N. This is easily met by the commonly used 5V

supply voltage. The additional voltage creates the benefit of

headroom, allowing the LM4888 to produce peak output

power in excess of 1.3W at 5V of V

DD and 1% THD+N

without clipping or other audible distortion. The choice of

supply voltage must also not create a situation that violates

maximum power dissipation as explained above in the

Power Dissipation section.

After satisfying the LM4888’s power dissipation require-

ments, the minimum differential gain needed to achieve 1W

dissipation in an 8

Ω load is found using Equation (10).

(10)

Thus, a minimum gain of 2.83 allows the LM4888’s to reach

full output swing and maintain low noise and THD+N perfor-

mance. For this example, let A

VD =3.

The amplifier’s overall gain (non 3D mode) is set using the

input (R1 and R9) and feedback resistors R2 and R8. With

the desired input impedance set at 20k

Ω, the feedback

resistor is found using Equation (11).

2/R1 =AVD/2

(11)

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