LM4874 datasheet(16/21 Pages) NSC

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Application Information (Continued)

Table 1. Gain Settings and Input Resistance

GAIN 0 GAIN 1

Selected Gain

(dB)

Input Resistance

90k

Ω

70k

Ω

15.6

45k

Ω

21.6

25k

Ω

POWER SUPPLY BYPASSING

As with any power amplifier, proper supply bypassing is

critical for low noise performance and high power supply

rejection. The capacitors connected to the bypass and power

supply pins should be placed as close to the LM4874 as

possible. The capacitor connected between the bypass pin

and ground improves the internal bias voltage’s stability,

producing improved PSRR. The improvements to PSRR

increase as the bypass pin capacitor value increases.

Typical applications employ a 5V regulator with 10µF and a

0.1µF filter capacitors that aid in supply stability. Their pres-

ence, however, does not eliminate the need for bypassing

the LM4874’s supply pins. The selection of bypass capacitor

values, especially C

B, depends on desired PSRR require-

ments, click and pop performance (as explained in theSe-

lecting External Components section), system cost, and

size constraints.

MICRO-POWER SHUTDOWN

The LM4874 features an active-low micro-power shutdown

mode. The voltage applied to the SHUTDOWN pin controls

the LM4874’s shutdown function. Activate micro-power shut-

down by applying 0V to the SHUTDOWN pin. The logic

threshold is typically 0.4V for a logic low and 1.5V for a logic

high. When active, the LM4874’s micro-power shutdown

feature turns off the amplifier’s bias circuitry, disables the

internal V

DD/2 generator, and forces the amplifier outputs

into a high impedance state. The result is greatly reduced

power supply current. The low 0.1µA typical shutdown cur-

rent is achieved by applying a voltage to the SHUTDOWN

pin that is as near to GND as possible. A voltage that is

greater than GND may increase the shutdown current.

There are a few methods to control the micro-power shut-

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

(SPST), a microprocessor, or a microcontroller. When using

a switch, connect a 100k

Ω pull-down resistor between the

SHUTDOWN pin and GND and the SPST switch between

the SHUTDOWN pin and V

DD. Select normal amplifier op-

eration by closing the switch. Opening the switch applies

GND to the SHUTDOWN pin, activating micro-power shut-

down. The switch and resistor guarantee that the SHUT-

DOWN pin will not float. This prevents unwanted state

changes. In a system with a microprocessor or a microcon-

troller, use a digital output to apply the active-state voltage to

the SHUTDOWN pin. Driving the SHUTDOWN pin with ac-

tive circuitry eliminates the pull-down resistor.

Table 2. Logic Level Truth Table for Shutdown

Operation

SHUTDOWN

OPERATIONAL

MODE

High

Full Power, stereo

BTL amplifiers

Low

Micro-power

Shutdown

SELECTING PROPER EXTERNAL COMPONENTS

Optimizing the LM4874’s performance requires properly se-

lecting external components. Though the LM4874 operates

well when using external components with wide tolerances,

best performance is achieved by optimizing component val-

ues. The LM4874 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 am-

plifier to achieve minimum THD+N and maximum signal-to-

noise ratio. These parameters are compromised as the

closed-loop gain increases. However, low gain demands

input signals with greater voltage swings to achieve maxi-

mum output power. Fortunately, many signal sources such

as audio CODECs have outputs of 1V

RMS

(2.83V

P-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 high value

input coupling capacitors (C

I,C2 and C3,C4) in Figure 1. A

high value capacitor 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 with

frequencies below 150Hz. Applications using speakers with

this limited frequency response reap little improvement by

using large input capacitor.

Besides effecting system cost and size, CI - C4 can also

affect on the LM4874’s turn-on and turn-off transient (’click

and pop’) performance. When the supply voltage is first

applied, a transient may be created as the charge on the

input capacitor changes from zero to a quiescent state. The

magnitude of the transient is proportional to the value of, and

more importantly, the mismatch between, the capacitors

connected to a given pair of inverting and non-inverting

inputs. The better the match, the less the transient magni-

tude.

Higher value capacitors need more time to reach a quiescent

DC voltage (usually V

DD/2) when charged with a fixed cur-

rent. This fixed current is supplied through amplifiers input

pins. Thus, selecting an input capacitor value that is no

higher than necessary to meet the desired -3dB frequency

will reduce turn-on time and help ensure that transients are

minimized.

The LM4874’s nominal input resistance (R

i)is25k

Ω (20kΩ,

minimum) and the input capacitor, C

i, form high pass filter

with a -3dB low frequency limit defined by equation (5).

-3dB = 1/2

π(25kΩ)C

(5)

As an example when using a speaker with a low frequency

limit of 150Hz, C

I, is 0.047µF. The 0.47µF CI shown in Figure

1 allows the LM4874 to drive high efficiency, full range

speaker whose response extends below 30Hz.

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