muting it. The output coupling capacitors blocks the amplifi-

er’s half supply DC voltage, protecting the headphones from

the VDD/2 DC output voltage.

Figure 3 also shows the suggested headphone jack electri-

cal connections. The jack is designed to mate with a three-

wire plug. The plug’s tip and adjacent ring should carry the

left and right channel stereo signals, respectively. The sleeve

furthest from the tip should carry the ground return. The

Switchcraft 35RAPC4BH3 five-terminal headphone jack

easily satisfies the LM4913’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 capacitor (Ci 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 150Hz. Applications

using speakers with this limited frequency response reap

little improvement by using large input capacitor.

The LM4913’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

CI is now strictly determined by the desired low frequency

response.

As shown in Figure 2, the input resistor (Ri) and the input

capacitor (Ci) produce a high pass filter cutoff frequency that

is found using Equation (7).

f

πR

(7)

As an example when using a speaker with a low frequency

limit of 150Hz, Ci, using Equation (7) is 0.063µF. The 1.0µF

Ci shown in Figure 2 allows the LM4913 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 CB, the capacitor con-

nected to the BYPASS pin. Since CB determines how fast

the LM4913 settles to quiescent operation, its value is critical

when minimizing turn-on pops. The slower the LM4913’s

outputs ramp to their quiescent DC voltage (nominally VDD/

2), the smaller the turn-on pop. Choosing CB 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 Ci no larger than neces-

sary for the desired bandwidth helps minimize clicks and

pops. CB’s value should be in the range of 5 times to 7 times

the value of Ci. This ensures that output transients are

eliminated when power is first applied or the LM4913 re-

sumes operation after shutdown.

OPTIMIZING CLICK AND POP REDUCTION

PERFORMANCE

The LM4913 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 VDD/2 voltage present at the BYPASS pin ramps to

its final value, the LM4913’s internal amplifiers are config-

ured 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 VDD/2. As soon as the voltage on

the bypass pin is stable, the device becomes fully opera-

tional and the amplifier outputs are reconnected to their

respective output pins. Although the BYPASS pin current

cannot be modified, changing the size of CB alters the

device’s turn-on time. There is a linear relationship between

the size of CB and the turn-on time. Here are some typical

turn-on times for various values of CB:

200617A2

FIGURE 4. Headphone Circuit

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