LM4849 datasheet(15/19 Pages) NSC

15 / 19 page

Application Information (Continued)

(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 1 allows the LM4849 to drive high efficiency, full

range speaker whose response extends below 30Hz.

OPTIMIZING CLICK AND POP REDUCTION

PERFORMANCE

The LM4849 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 LM4849’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

Bypass alters the device’s turn-on time

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

of C

Bypass reduces the magnitude of turn-on pops. However,

this presents a tradeoff: as the size of C

Bypass increases, the

turn-on time increases. There is a linear relationship be-

tween the size of C

Bypass and the turn-on time. Here are

some typical turn-on times for various values of C

Bypass:

Bypass

0.01µF

2ms

0.1µF

20ms

0.22µF

44ms

0.47µF

94ms

1.0µF

200ms

RIGHT AND LEFT LINE OUT

Applications such as notebook computers can take advan-

tage of a line output signal to connect to external devices

such as monitors or audio/visual equipment that sends or

receives line level signals. The LM4849 has two outputs

which connect to the outputs of the internal input amplifiers

that drive the volume control inputs. These input amplifiers

can drive loads of >1k

Ω (such as powered speakers) with a

rail-to-rail signal. Since the output signal present on the

RIGHT Line Out and LEFT Line Out pins is biased to V

DD/2,

coupling capacitors need to be connected in series with the

load. Typical values for the coupling capacitors are 0.33µF to

1.0µF. If polarized coupling capacitors are used, connect

their "+" terminals to the respective output pin.

Since the Line Out outputs precede the internal volume

control, the signal amplitude will be equal to the input sig-

nal’s magnitude and cannot be adjusted. However, the input

amplifier’s closed-loop gain can be adjusted using external

resistors. These 20K resistors are shown in Figure 1 (R1 -

R8 ) and they set each input amplifier’s gain to -1. Use

Equation 8 to determine the input and feedback resistor

values for a desired gain.

-A

v =R2 /R3

(8)

Adjusting the input amplifier’s gain sets the minimum gain for

that channel. Although the single ended outputs of the

Bridge Output Amplifiers can be used to drive line level

outputs, it is recommended that the Right & Left Line Out-

puts simpler signal path be used for better performance.

STEREO-INPUT MULTIPLEXER (STEREO MUX)

The LM4849 has two stereo inputs. The MUX CONTROL pin

controls which stereo input is active. Applying 0V to the MUX

CONTROL pin selects stereo input 1. Applying V

DD to the

MUX CONTROL pin selects stereo input 2.

MICRO-POWER SHUTDOWN

The voltage applied to the SHUTDOWN pin controls the

LM4849’s shutdown function. Activate micro-power shut-

down by applying V

DD to the SHUTDOWN pin. When active,

the LM4849’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.

The Logic Level Truth Table shows the logic signal levels

that activate and deactivate micro-power shutdown and

headphone amplifier operation.

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.

LM4849 Datasheet(PDF) 15 Page - National Semiconductor (TI)

LM4849 Datasheet(HTML) 15 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

Datasheet Download

Link URL