LM4867MT datasheet(15/26 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 4). 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 150Hz. 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 LM4867’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 and is between 0.14C

B and 0.20CB.

A shown in

Figure 4, 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).

−3dB = 1/(2πRINCI)

(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 4 allows the LM4867 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 LM4867 settles to quiescent operation, its value is critical

when minimizing turn-on pops. The slower the LM4867’s

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

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

1.0µF along with a small value of C

i (in the range of 0.1µF to

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

tion. As discussed above, choosing C

i no larger than neces-

sary for the desired bandwidth helps minimize clicks and

pops. C

B’s value should be in the range of 5 times to 7 times

the value of C

i. This ensures that output transients are

eliminated when power is first applied or the LM4867 re-

sumes operation after shutdown.

OPTIMIZING CLICK AND POP REDUCTION

PERFORMANCE

The LM4867 contains circuitry that eliminates turn-on and

shutdown transients (“clicks and pops“) and transients that

could occur when switching between BTL speakers and

single-ended headphones. For this discussion, turn-on re-

fers 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 LM4867’s internal amplifiers

are configured as unity gain buffers and are disconnected

from the -OUT and +OUT pins. 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 amplifi-

ers 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 and the amplifier out-

puts are reconnected to the -OUT and +OUT pins. Although

the BYPASS pin current cannot be modified, changing the

size of C

B alters the device’s turn-on time. 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

3ms

0.1µF

30ms

0.22µF

63ms

0.47µF

134ms

1.0µF

300ms

2.2µF

630ms

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“.

DS200013-24

FIGURE 7. Headphone Circuit

(Pin numbers in ( ) are for the 20-pin MTE and MT packages.)

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