LM4869 datasheet(16/20 Pages) NSC

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Application Information (Continued)

pin that is as near to GND as possible. A voltage that is

greater than GND may increase the shutdown current.

There are a few methods to control the micro-power shut-

down. These include using a single-pole, single-throw switch

(SPST), a microprocessor, or a microcontroller. When using

a switch, connect a 100k

Ω pull-down resistor between the

SHUTDOWN pin and GND and the SPST switch between

the SHUTDOWN pin and V

DD. Select normal amplifier op-

eration by closing the switch. Opening the switch applies

GND to the SHUTDOWN pin, activating micro-power shut-

down. The switch and resistor guarantee that the SHUT-

DOWN pin will not float. This prevents unwanted state

changes. In a system with a microprocessor or a microcon-

troller, use a digital output to apply the active-state voltage to

the SHUTDOWN pin. Driving the SHUTDOWN pin with ac-

tive circuitry eliminates the pull-down resistor.

Table 1. LOGIC LEVEL TRUTH TABLE FOR

SHUTDOWN OPERATION

SHUTDOWN

OPERATIONAL

MODE

High

Full Power, stereo

BTL amplifiers

Low

Micro-power

Shutdown

SELECTING PROPER EXTERNAL COMPONENTS

Optimizing the LM4869’s performance requires properly se-

lecting external components. Though the LM4869 operates

well when using external components with wide tolerances,

best performance is achieved by optimizing component val-

ues. The LM4869 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 am-

plifier to achieve minimum THD+N and maximum signal-to-

noise ratio. These parameters are compromised as the

closed-loop gain increases. However, low gain demands

input signals with greater voltage swings to achieve maxi-

mum output power. Fortunately, many signal sources such

as audio CODECs have outputs of 1V

RMS

(2.83V

P-P).

Please refer to the Audio Power Amplifier Design section for

more information on selecting the proper gain.

Input Capacitor Value Selection

Amplifying the lowest audio frequencies requires high value

input coupling capacitors (C

I,C2 and C3,C4) in Figure 1. 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 with

frequencies below 150Hz. Applications using speakers with

this limited frequency response reap little improvement by

using large input capacitor.

Besides effecting system cost and size, CI - C4 can also

affect on the LM4869’s turn-on and turn-off transient (’click

and pop’) performance. When the supply voltage is first

applied, a transient may be created as the charge on the

input capacitor changes from zero to a quiescent state. The

magnitude of the transient is proportional to the value of, and

more importantly, the mismatch between, the capacitors

connected to a given pair of inverting and non-inverting

inputs. The better the match, the less the transient magni-

tude.

Higher value capacitors need more time to reach a quiescent

DC voltage (usually V

DD/2) when charged with a fixed cur-

rent. This fixed current is supplied through amplifiers input

pins. Thus, selecting an input capacitor value that is no

higher than necessary to meet the desired -3dB frequency

will reduce turn-on time and help ensure that transients are

minimized.

The LM4869’s nominal input resistance (R

i)is25k

Ω (20kΩ,

minimum) and the input capacitor, C

i, form high pass filter

with a -3dB low frequency limit defined by equation (5).

-3dB = 1/2

π(25kΩ)C

(5)

As an example when using a speaker with a low frequency

limit of 150Hz, C

I, is 0.047µF. The 0.47µF CI shown in Figure

1 allows the LM4869 to drive high efficiency, full range

speaker whose response extends below 30Hz.

Bypass Capacitor Value Selection

Besides optimizing the input capacitor value, careful consid-

eration should be paid to value of C

B, the capacitor con-

nected between the BYPASS pin and ground. Since C

determines how fast the LM4869 settles to its quiescent

operating state, its value is critical when minimizing turn-on

transients. The slower the LM4869’s outputs ramp to their

quiescent DC voltage (nominally 1⁄2 V

DD), the smaller the

turn-on transient. Choosing C

B equal to 0.47µF along with a

small value of C

i (in the range of 0.047µF to 0.47µF), pro-

duces a transient-free turn-on and shutdown function. As

discussed above, choosing C

i no larger than necessary for

the desired bandwidth helps minimize turn-on transients.

OPTIMIZING OUTPUT TRANSIENT REDUCTION (CLICK

AND POP PERFORMANCE)

The LM4869 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 voltage is ramping to its final value, the LM4869’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 amplifier inputs

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 can not be

modified, changing the size of C

B alters the device’s turn-on

time and the magnitude of output transients. Increasing the

value of C

B reduces the magnitude of turn-on transients.

However, this presents a tradeoff: as the size of C

B in-

creases, the turn-on time increases. There is a linear rela-

tionships between the size of C

B + 2(CI) and the turn-on

time. The table shows some typical turn-on times for various

values of C

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