LM4914 datasheet(12/15 Pages) NSC

12 / 15 page

Application Information (Continued)

easily satisfies the LM4914’s requirement for a dual switch

headphone jack. For applications that require an SPDIF

interface in the stereo headphone jack, use a Foxconn

2F1138-TJ-TR.

SELECTING EXTERNAL COMPONENTS

Input Capacitor Value Selection

Amplifying the lowest audio frequencies requires high value

input coupling capacitors (C

1 and C2 in Figure 2). A high

value capacitor can be expensive and may compromise

space efficiency in portable designs. In many cases, how-

ever, the speakers used in portable systems, whether inter-

nal or external, have little ability to reproduce signals below

150Hz. Applications using speakers with this limited fre-

quency response reap little improvement by using large

input capacitor.

The LM4914’s advanced output transient suppression cir-

cuitry has eliminated the need to select the input capacitor’s

value in relation to the BYPASS capacitor’s value as was

necessary in some previous Boomer amplifiers. The value of

1 and C2 are now strictly determined by the desired low

frequency response.

As shown in Figure 2, the input resistors (R

1 and R2) and the

input capacitors (C

1 and C2) produce a high pass filter cutoff

frequency that is found using Equation (7).

C =1/2

πR

iCi

(7)

As an example, when using a speaker with a low frequency

limit of 150Hz, and input resistances (R

2 and R3) of 16.8k

Ω,

using Equation (7), input capacitances (C

1 and C2) are

0.063µF. The 0.1µF capacitors (C

1 and C2) shown in Figure

2 allow the LM4914 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

B, the capacitor con-

nected to the BYPASS pin. Since C

B determines how fast

the LM4914 settles to quiescent operation, its value is critical

when minimizing turn-on pops. The slower the LM4914’s

outputs ramp to their quiescent DC voltage (nominally V

DD/

2), the smaller the turn-on pop. Choosing C

B equal to 1.0µF

along with a small value of Ci (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 LM4914 re-

sumes operation after shutdown.

OPTIMIZING CLICK AND POP REDUCTION

PERFORMANCE

The LM4914 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

micro-power shutdown mode is deactivated.

As the V

DD/2 voltage present at the BYPASS pin ramps to its

final value, the LM4914’s internal amplifiers are configured

as unity gain buffers and are disconnected from the RO/MO+

and LO/MO- pins. An internal current source charges the

capacitor connected between the BYPASS pin and GND 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 V

DD/2. As soon as the voltage on the

bypass pin is stable, the device becomes fully operational

and the amplifier outputs are reconnected to their respective

output 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

and the turn-on time. Here are some typical turn-on times for

various values of C

B(µF)

ON (ms)

0.01

0.1

0.22

0.47

1.0

200

2.2

420

200634A2

FIGURE 4. Headphone Circuit

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