LM4838 datasheet(17/36 Pages) NSC

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Application Information (Continued)

INPUT CAPACITOR VALUE SELECTION

Amplifying the lowest audio frequencies requires a high

value input coupling capacitor (0.33µF in Figure 2), but high

value capacitors 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 below

150 Hz. Applications using speakers with this limited fre-

quency response reap little improvement by using a large

input capacitor.

Besides effecting system cost and size, the input coupling

capacitor has an affect on the LM4838’s click and pop per-

formance. When the supply voltage is first applied, a tran-

sient (pop) is created as the charge on the input capacitor

changes from zero to a quiescent 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 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 0.053µF. The 0.33µF input coupling capacitor shown

in Figure 2 allows the LM4838 to drive a high efficiency, full

range speaker whose response extends below 30Hz.

OPTIMIZING CLICK AND POP REDUCTION

PERFORMANCE

The LM4838 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 LM4838’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, changing the size of C

B alters the device’s turn-on

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

value of C

B reduces the magnitude of turn-on pops. How-

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

B increases, the

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

tween 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 INTERFACE

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 LM4838 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 when using these outputs. Typical values for

the output coupling capacitors are 0.33µF to 1.0µF. If polar-

ized coupling capacitors are used, connect their "+" termi-

nals to the respective output pin, see Figure 2.

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 (R

FR,RFL) are shown in Fig-

ure 2 and they set each input amplifier’s gain to -1. Use

Equation 7 to determine the input and feedback resistor

values for a desired gain.

-A

VR =RFR/RIR and - AVL =RFL/RIL

(8)

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

that channel. Although the single ended output 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.

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 LM4838’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 (R

BEEP) are shown

as 200k

Ω devices in Figure 2. 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 LM4838’s shutdown mode must be deactivated

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

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