KB4863 datasheet(14/19 Pages) KINGBOR

14 / 19 page

Application Information (Continued)

phone jack contact pin from -OUTA and allows R1 to pull the

HP Sense pin up to V

DD. This enables the headphone func-

tion, turns off Amp2A and Amp2B, and mutes the bridged

speaker. The amplifier then drives the headphones, whose

impedance is in parallel with resistor R2 and R3. These

resistors have negligible effect on the KB4863’s output drive

capability since the typical impedance of headphones is

Ω.

Figure 2 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 ring should each carry

one of the two stereo output signals, whereas the sleeve

should carry the ground return. A headphone jack with one

control pin contact is sufficient to drive the HP-IN pin when

connecting headphones.

A microprocessor or a switch can replace the headphone

jack contact pin. When a microprocessor or switch applies a

voltage greater than 4V to the HP-IN pin, a bridge-connected

speaker is muted and Amp1A and Amp2A drive a pair of

headphones.

SELECTING PROPER EXTERNAL COMPONENTS

Optimizing the KB4863’s performance requires properly se-

lecting external components. Though the KB4863 operates

well when using external components with wide tolerances,

best performance is achieved by optimizing component val-

ues.

The KB4863 is unity-gain stable, giving a designer maximum

design flexibility. The gain should be set to no more than a

given application requires. This allows the amplifier to

achieve minimum THD+N and maximum signal-to-noise ra-

tio. These parameters are compromised as the closed-loop

gain increases. However, low gain demands input signals

with greater voltage swings to achieve maximum output

power. Fortunately, many signal sources such as audio CO-

DECs have outputs of 1V

RMS (2.83VP-P). Please refer to the

Audio Power Amplifier Design section for more informa-

tion on selecting the proper gain.

Input Capacitor Value Selection

Amplifying the lowest audio frequencies requires high value

input coupling capacitor (C

i in Figure 1). 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 KB4863’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 1, 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 1 allows the KB4863 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 KB4863 settles to quiescent operation, its value is critical

when minimizing turn−on pops. The slower the KB4863’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.

OPTIMIZING CLICK AND POP REDUCTION

PERFORMANCE

The KB4863 contains circuitry to minimize turn-on and shut-

down 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 KB4863’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, chang-

ing the size of C

B alters the device’s turn-on time and the

magnitude of ’clicks and pops’. Increasing the value of C

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

FIGURE 2. Headphone Circuit

KingborTechnologyCo.,Ltd

TEL:(86)0755-83095458 FAX:(86)0755-88364052

KB4863

14/19

Rev: 1.1

2005-12-05

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