LM4843 datasheet(13/19 Pages) NSC

13 / 19 page

Application Information (Continued)

Higher value capacitors need more time to reach a quiescent

DC voltage (usually V

DD/2) when charged with a fixed cur-

rent. The amplifier’s output charges the input capacitor

through the feedback resistor, R

f. Thus, pops can be mini-

mized by selecting an input capacitor value that is no higher

than necessary to meet the desired −6dB frequency.

As shown in Figure 2, the input resistor (R

IR,RIL = 20k) ( and

the input capacitor (C

IR,CIL = 0.33µF) produce a −6dB 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, the input coupling capacitor, using Equation

(7), is 0.063µF. The 0.33µF input coupling capacitor shown

in Figure 2 allows the LM4843 to drive a high efficiency, full

range speaker whose response extends below 30Hz.

OPTIMIZING CLICK AND POP REDUCTION

PERFORMANCE

The LM4843 contains circuitry that minimizes turn-on and

shutdown transients or “clicks and pops”. For this discus-

sion, turn-on refers to either applying the power supply volt-

age or when the shutdown mode is deactivated. While the

power supply is ramping to its final value, the LM4843’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 opera-

tional. Although the BYPASS pin current cannot be modified,

changing the size of C

B alters the device’s turn-on time and

the magnitude of “clicks and pops”. Increasing the value of

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

presents a tradeoff: as the size of C

B increases, the turn-on

time increases. There is a linear relationship between the

size of C

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

turn-on times for various values of C

0.01µF

2ms

0.1µF

20ms

0.22µF

44ms

0.47µF

94ms

1.0µF

200ms

MICRO-POWER SHUTDOWN

The voltage applied to the SHUTDOWN pin controls the

LM4843’s shutdown function. Activate micro-power shut-

down by applying V

DD to the SHUTDOWN pin. When active,

the LM4843’s micro-power shutdown feature turns off the

amplifier’s bias circuitry, reducing the supply current. The

logic threshold is typically V

DD/2. The low 0.7 µA typical

shutdown current is achieved by applying a voltage that is as

near as V

DD as possible to the SHUTDOWN pin. A voltage

that is less than V

DD may increase the shutdown current.

There are a few ways to control the micro-power shutdown.

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

microprocessor, or a microcontroller. When using a switch,

connect an external 10k

Ω pull-up resistor between the

SHUTDOWN pin and V

DD. Connect the switch between the

SHUTDOWN pin and ground. Select normal amplifier opera-

tion by closing the switch. Opening the switch connects the

SHUTDOWN pin to V

DD through the pull-up resistor, activat-

ing micro-power shutdown. The switch and resistor guaran-

tee that the SHUTDOWN pin will not float. This prevents

unwanted state changes. In a system with a microprocessor

or a microcontroller, use a digital output to apply the control

voltage to the SHUTDOWN pin. Driving the SHUTDOWN pin

with active circuitry eliminates the need for a pull up resistor.

DC VOLUME CONTROL

The LM4843 has an internal stereo volume control whose

setting is a function of the DC voltage applied to the DC VOL

CONTROL pin.

The LM4843 volume control consists of 31 steps that are

individually selected by a variable DC voltage level on the

volume control pin. The range of the steps, controlled by the

DC voltage, are from 0dB - 78dB. Each attenuation step

corresponds to a specific input voltage range, as shown in

table 2.

To minimize the effect of noise on the volume control pin,

which can affect the selected attenuation level, hysteresis

has been implemented. The amount of hysteresis corre-

sponds to half of the step width, as shown in Volume Control

Characterization Graph (DS200133-40).

For highest accuracy, the voltage shown in the ’recom-

mended voltage’ column of the table is used to select a

desired attenuation level. This recommended voltage is ex-

actly halfway between the two nearest transitions to the next

highest or next lowest attenuation levels.

The attenuation levels are 1dB/step from 0dB to -6dB, 2dB/

step from -6dB to -36dB, 3dB/step from -36dB to -47dB,

4dB/step from -47db to -51dB, 5dB/step from -51dB to

-66dB, and 12dB to the last step at -78dB.

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