LM4910 datasheet(11/22 Pages) NSC

11 / 22 page

Application Information (Continued)

headphone operation (32

Ω impedance) using a 3.3V supply

the maximum power dissipation is only 138mW. Therefore,

power dissipation is not a major concern.

POWER SUPPLY BYPASSING

As with any amplifier, proper supply bypassing is important

for low noise performance and high power supply rejection.

The capacitor location on the power supply pins should be

as close to the device as possible.

Typical applications employ a 3.3V regulator with 10µF tan-

talum or electrolytic capacitor and a ceramic bypass capaci-

tor which aid in supply stability. This does not eliminate the

need for bypassing the supply nodes of the LM4910. A

bypass capacitor value in the range of 0.1µF to 1µF is

recommended for C

MICRO POWER SHUTDOWN

The voltage applied to the SHUTDOWN pin controls the

LM4910’s shutdown function. Activate micro-power shut-

down by applying a logic-low voltage to the SHUTDOWN

pin. When active, the LM4910’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 volt-

age that is higher than ground may increase the shutdown

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

DD. Connect the switch between the

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 LM4910 is tolerant of exter-

nal component combinations, consideration to component

values must be used to maximize overall system quality.

The LM4910 is unity-gain stable which gives the designer

maximum system flexibility. The LM4910 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

rms are available

from sources such as audio codecs. Very large values

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

i and Rf should be less than 1M

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. To a large extent, the band-

width is dictated by the choice of external components

shown in Figure 1. The input coupling capacitor, C

i, forms a

first order high pass filter which limits low frequency re-

sponse. 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

i. A high value capacitor can

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 LM4910 is designed specifically for headphone opera-

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

drive external powered speakers. The LM4910 has a differ-

ential output to eliminate the output coupling capacitors. The

result is a headphone jack sleeve that is connected to V

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

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 LM4910 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)

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