LM4841 datasheet(17/31 Pages) NSC

17 / 31 page

Application Information (Continued)

SELECTING PROPER EXTERNAL COMPONENTS

Optimizing the LM4841’s performance requires properly se-

lecting external components. Though the LM4841 operates

well when using external components with wide tolerances,

best performance is achieved by optimizing component val-

ues.

The LM4841 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 amplifier to

achieve minimum THD+N and maximum signal-to-noise ra-

tio. These parameters are compromised as the closed-loop

gain increases. However, low gain circuits demand input

signals with greater voltage swings to achieve maximum

output power. Fortunately, many signal sources such as

audio CODECs have outputs of 1V

RMS (2.83VP-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 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 LM4841’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

IA,RIB = 20k) ( and

the input capacitor (C

IA,CIB = 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 LM4841 to drive a high efficiency, full

range speaker whose response extends below 30Hz.

TURN ON Characteristics

The LM4841 contains advanced circuitry that minimizes

turn-on and shutdown transients or “clicks and pops”. For

this discussion, turn-on refers to either applying the power

supply voltage or when the shutdown mode is deactivated.

While the power supply is ramping to its final value, the

LM4841’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 LM4841 is ready to

be fully turned on. To turn the device on, the input signal

must exceed 22mV

rms.

This is accomplished through a

threshold detect circuit that enables all appropriate output

amplifiers after the 22mVrms limit is reached. Until this

threshold is reached, some of the amplifiers remain in a

tri-state mode. This insures that there is no current flowing

through to the speakers or headphones during power up.

Without current flow, the speakers or headphones remain

silent. During headphone mode, A+, B-, and B+ are in tri-

state mode during power up. During speaker mode, A+ and

B+ are in tri-state mode during power up.

Although the BYPASS pin current cannot be modified,

changing the size of C

BYP alters the device’s turn-on time. As

the size of C

BYP increases, the turn-on time increases. There

is a linear relationship between the size of C

BYP and the

turn-on time. Here are some typical turn-on times for various

values of C

BYP:

BYP

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 LM4841 has two outputs, Dock A and

Dock B, which connect to outputs of the internal input am-

plifiers 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 Dock A and Dock B pins are biased to V

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 polarized coupling capaci-

tors are used, connect their ’+’ terminals to the respective

output pin.

Since the Dock outputs precede the internal volume control,

the signal amplitude will be equal to the input signal’s mag-

nitude and cannot be adjusted. However, the input amplifi-

er’s closed-loop gain can be adjusted using external resis-

tors. These 20k resistors (R

FA and RFB) are shown in Figure

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

v =RF /RIN

(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 the A & B Dock Outputs simpler signal

path be used for better performance.

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