pin. When active, the LM4924’s micro-power shutdown fea-

ture turns off the amplifier’s bias circuitry, reducing the sup-

ply current. The trigger point is 0.4V (max) for a logic-low

level, and 1.5V (min) for a logic-high level. The low 0.1µA

(typ) shutdown current is achieved by applying a voltage that

is as near as ground as possible to the SHUTDOWN pin. A

voltage that is higher than ground may increase the shut-

down current.

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 100k

Ω pull-up resistor between the

SHUTDOWN pin and V

SHUTDOWN pin and ground. Select normal amplifier opera-

tion by opening the switch. Closing the switch connects the

SHUTDOWN pin to ground, 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 microcontrol-

ler, use a digital output to apply the control voltage to the

SHUTDOWN pin. Driving the SHUTDOWN pin with active

circuitry eliminates the pull-up resistor.

SELECTING EXTERNAL COMPONENTS

Selecting proper external components in applications using

integrated power amplifiers is critical to optimize device and

system performance. While the LM4924 is tolerant of exter-

nal component combinations, consideration to component

values must be used to maximize overall system quality.

The LM4924 is unity-gain stable which gives the designer

maximum system flexibility. The LM4924 should be used in

low gain configurations to minimize THD+N values, and

maximize the signal to noise ratio. Low gain configurations

require large input signals to obtain a given output power.

Input signals equal to or greater than 1V

from sources such as audio codecs. Very large values

should not be used for the gain-setting resistors. Values for

R

Ω. Please refer to the

section, Audio Power Amplifier Design, for a more com-

plete explanation of proper gain selection

Besides gain, one of the major considerations is the closed-

loop bandwidth of the amplifier. The input coupling capacitor,

C

quency response. This value should be chosen based on

needed frequency response and turn-on time.

SELECTION OF INPUT CAPACITOR SIZE

Amplifiying the lowest audio frequencies requires a high

value input coupling capacitor, C

be expensive and may compromise space efficiency in por-

table designs. In many cases, however, the headphones

used in portable systems have little ability to reproduce

signals below 60Hz. Applications using headphones with this

limited frequency response reap little improvement by using

a high value input capacitor.

In addition to system cost and size, turn-on time is affected

by the size of the input coupling capacitor Ci. A larger input

coupling capacitor requires more charge to reach its quies-

cent DC voltage. This charge comes from the output via the

feedback Thus, by minimizing the capacitor size based on

necessary low frequency response, turn-on time can be

minimized. A small value of Ci (in the range of 0.1µF to

0.39µF), is recommended.

USING EXTERNAL POWERED SPEAKERS

The LM4924 is designed specifically for headphone opera-

tion. Often the headphone output of a device will be used to

drive external powered speakers. The LM4924 has a differ-

ential output to eliminate the output coupling capacitors. The

result is a headphone jack sleeve that is connected to V

O3

instead of GND. For powered speakers that are designed to

have single-ended signals at the input, the click and pop

circuitry will not be able to eliminate the turn-on/turn-off click

and pop. Unless the inputs to the powered speakers are fully

differential the turn-on/turn-off click and pop will be very

large.

AUDIO POWER AMPLIFIER DESIGN

A 30mW/32

Ω Audio Amplifier

Given:

Power Output

30mWrms

Load Impedance

32

Ω

Input Level

1Vrms

Input Impedance

20k

Ω

A designer must first determine the minimum supply rail to

obtain the specified output power. By extrapolating from the

Output Power vs Supply Voltage graphs in the Typical Per-

formance Characteristics section, the supply rail can be

easily found.

Since 3.3V is a standard supply voltage in most applications,

it is chosen for the supply rail in this example. Extra supply

voltage creates headroom that allows the LM4924 to repro-

duce peaks in excess of 30mW without producing audible

distortion. At this time, the designer must make sure that the

power supply choice along with the output impedance does

no violate the conditions explained in the Power Dissipa-

tion section.

Once the power dissipation equations have been addressed,

the required differential gain can be determined from Equa-

tion 2.

(2)

From Equation 2, the minimum A

the desired input impedance is 20k

Ω, and with A

a ratio of 1:1 results from Equation 1 for R

are chosen with R

Ω and R

Ω.

The last step in this design example is setting the amplifier’s

pass band magnitude variation limit, the low frequency re-

sponse must extend to at least one-fifth the lower bandwidth

limit and the high frequency response must extend to at least

five times the upper bandwidth limit. The gain variation for

desired limit. The results are an

f

(3)

and an

f

(4)

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