LM4817 datasheet(14/21 Pages) NSC

14 / 21 page

Application Information (Continued)

SELECTING PROPER EXTERNAL COMPONENTS

Optimizing the LM4817’s performance requires properly se-

lecting external components. Though the LM4817 operates

well when using external components with wide tolerances,

best performance is achieved by optimizing component val-

ues.

The LM4817 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 LM4817’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 LM4817 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 LM4817 settles to quiescent operation, its value is critical

when minimizing turn−on pops. The slower the LM4817’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 LM4817 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 LM4817’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-

sents a tradeoff: as the size of C

B increases, the turn-on time

increases. There is a linear relationship between the size of

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

for various values of C

0.01µF

20 ms

0.1µF

200 ms

0.22µF

440 ms

0.47µF

940 ms

1.0µF

2 Sec

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".

NO LOAD STABILITY

The LM4817 may exhibit low level oscillation when the load

resistance is greater than 10k

Ω. This oscillation only occurs

as the output signal swings near the supply voltages. Pre-

vent this oscillation by connecting a 5k

Ω between the output

pins and ground.

LM4817 датащи(PDF) 14 Page - National Semiconductor (TI)

LM4817 датащи(HTML) 14 Page - National Semiconductor (TI)

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21

датащи скачать

ссылки URL