SHUTDOWN FUNCTION

In order to reduce power consumption while not in use, the

LM4891 contains a shutdown pin to externally turn off the

amplifier’s bias circuitry. This shutdown feature turns the

amplifier off when a logic high is placed on the shutdown pin.

By switching the shutdown pin to V

current draw will be minimized in idle mode. While the device

will be disabled with shutdown pin voltages more than

1.0V

value of 0.1µA. (Idle current is measured with the shutdown

pin tied to V

In many applications, a microcontroller or microprocessor

output is used to control the shutdown circuitry to provide a

quick, smooth transition into shutdown. Another solution is to

use a single-pole, single-throw switch in conjunction with an

external pull-up resistor. When the switch is closed, the

shutdown pin is connected to ground which enables the

amplifier. If the switch is open, then the external pull-up

resistor to V

antees that the shutdown pin will not float thus preventing

unwanted state changes.

PROPER SELECTION OF EXTERNAL COMPONENTS

Proper selection of external components in applications us-

ing integrated power amplifiers is critical to optimize device

and system performance. While the LM4891 is tolerant of

external component combinations, consideration to compo-

nent values must be used to maximize overall system qual-

ity.

The LM4891 is unity-gain stable which gives the designer

maximum system flexibility. The LM4891 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 1 Vrms are available

from sources such as audio codecs. 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

first order high pass filter which limits low frequency re-

sponse. This value should be chosen based on needed

frequency response for a few distinct reasons.

Selection Of Input Capacitor Size

Large input capacitors are both expensive and space hungry

for portable designs. Clearly, a certain sized capacitor is

needed to couple in low frequencies without severe attenu-

ation. But in many cases the speakers used in portable

systems, whether internal or external, have little ability to

reproduce signals below 100 Hz to 150 Hz. Thus, using a

large input capacitor may not increase actual system perfor-

mance.

In addition to system cost and size, click and pop perfor-

mance is effected by the size of the input coupling capacitor,

C

reach its quiescent DC voltage (nominally 1/2 V

charge comes from the output via the feedback and is apt to

create pops upon device enable. Thus, by minimizing the

capacitor size based on necessary low frequency response,

turn-on pops can be minimized.

Besides minimizing the input capacitor size, careful consid-

eration should be paid to the bypass capacitor value. Bypass

capacitor, C

turn-on pops since it determines how fast the LM4891 turns

on. The slower the LM4891’s outputs ramp to their quiescent

DC voltage (nominally 1/2 V

Choosing C

(in the range of 0.1 µF to 0.39 µF), should produce a virtually

clickless and popless shutdown function. While the device

will function properly, (no oscillations or motorboating), with

C

to turn-on clicks and pops. Thus, a value of C

1.0 µF is recommended in all but the most cost sensitive

designs.

AUDIO POWER AMPLIFIER DESIGN

A 1W/8

Ω AUDIO AMPLIFIER

Given:

Power Output

1 Wrms

Load Impedance

8

Ω

Input Level

1 Vrms

Input Impedance

20 k

Ω

Bandwidth

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. A second way to determine the minimum sup-

ply rail is to calculate the required V

and add the output voltage. Using this method, the minimum

supply voltage would be (V

V

age vs Supply Voltage curve in the Typical Performance

Characteristics section.

(2)

5V is a standard voltage, in most applications, chosen for the

supply rail. Extra supply voltage creates headroom that al-

lows the LM4891 to reproduce peaks in excess of 1W with-

out producing audible distortion. At this time, the designer

must make sure that the power supply choice along with the

output impedance does not violate the conditions explained

in the Power Dissipation section.

Once the power dissipation equations have been addressed,

the required differential gain can be determined from Equa-

tion 3.

(3)

A

From Equation 3, the minimum A

Since the desired input impedance was 20 k

Ω, and with a

A

R

Ω and R

Ω. The final design step is to

address the bandwidth requirements which must be stated

as a pair of −3 dB frequency points. Five times away from a

−3 dB point is 0.17 dB down from passband response which

f

f

www.national.com

10