LM4839 datasheet(16/34 Pages) NSC

16 / 34 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 −3dB frequency.

As shown in

Figure 1, the input resistors (RIN = 20K) and the

input capacitosr (CIN = 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

Figure 1 allows the LM4839 to drive high efficiency, full

range speaker whose response extends below 30Hz.

OPTIMIZING CLICK AND POP REDUCTION

PERFORMANCE

The LM4839 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 LM4839’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

DOCKING STATION

Applications such as notebook computers can take advan-

tage of a docking station to connect to external devices such

as monitors or audio/visual equipment that sends or receives

line level signals. The LM4839 has two outputs, Right Dock

and Left Dock which connect to outputs of the internal input

amplifiers that drive the volume control inputs. These input

amplifiers can drive loads of >1k

Ω (such as powered speak-

ers) with a rail-to-rail signal. Since the output signal present

on the RIGHT DOCK and LEFT DOCK pins is biased to

DD/2, coupling capacitors should 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 DOCK outputs precede the internal volume con-

trol, the signal amplitude will be equal to the input signal’s

magnitude and cannot be adjusted. However, the input am-

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

resistors. These 20K resistors are shown in Figure 1 (RIN,

RF ) 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 =RF /Ri

(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 theR&L Dock Outputs

simpler signal path be used for better performance.

STEREO-INPUT MULTIPLEXER (STEREO MUX)

The LM4839 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.

BEEP DETECT FUNCTION

Computers and notebooks produce a system ’beep’ signal

that drives a small speaker. The speaker’s auditory output

signifies that the system requires user attention or input. To

accommodate this system alert signal, the LM4839’s beep

input pin is a mono input that accepts the beep signal.

Internal level detection circuitry at this input monitors the

beep signal’s magnitude. When a signal level greater than

DD/2 is detected on the BEEP IN pin, the bridge output

amplifiers are enabled. The beep signal is amplified and

applied to the load connected to the output amplifiers. A valid

beep signal will be applied to the load even when MUTE is

active. Use the input resistors connected between the BEEP

IN pin and the stereo input pins to accommodate different

beep signal amplitudes. These resistors are shown as

200k

Ω devices in Figure 1. Use higher value resistors to

reduce the gain applied to the beep signal. The resistors

must be used to pass the beep signal to the stereo inputs.

The BEEP IN pin is used only to detect the beep signal’s

magnitude: it does not pass the signal to the output amplifi-

ers. The LM4839’s shutdown mode must be deactivated

before a system alert signal is applied to the BEEP IN pin.

MICRO-POWER SHUTDOWN

The voltage applied to the SHUTDOWN pin controls the

LM4839’s shutdown function. Activate micro-power shut-

down by applying V

DD to the SHUTDOWN pin. When active,

the LM4839’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.

Logic Level Truth Table shows the logic signal levels that

activate and deactivate micro-power shutdown and head-

phone amplifier operation.

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

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

LM4839 Datasheet(PDF) 16 Page - National Semiconductor (TI)

LM4839 Datasheet(HTML) 16 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 22 23 24 25 26 27 28 29 30 31 32 33 34

Datasheet Download

Link URL