Application Information

SHUTDOWN FUNCTION

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

LM4882 contains a shutdown pin to externally turn off the

amplifier’s bias circuitry. This shutdown features turns the

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

The trigger point between a logic low and logic high level is

typically half supply. It is best to switch between ground and

supply to provide maximum device performance. By switch-

ing the shutdown pin to the V

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

disabled with shutdown pin voltages less than V

current may be greater than the typical value of 0.5 µA. In ei-

ther case, the shutdown pin should be tied to a definite volt-

age because leaving the pin floating may result in an un-

wanted shutdown condition. In many applications, a

microcontroller or microprocessor output is used to control

the shutdown circuitry which provides a quick smooth transi-

tion into shutdown. Another solution is to use a single-pole,

single-throw switch in conjunction with an external pull-up re-

sistor. When the switch is closed, the shutdown pin is con-

nected to ground and enables the amplifier. If the switch is

open, then the external pull-up resistor will disable the

LM4882. This scheme guarantees that the shutdown pin will

not float which will prevent unwanted state changes.

POWER DISSIPATION

Power dissipation is a major concern when using any power

amplifier and must be thoroughly understood to ensure a

successful design. Equation 1 states the maximum power

dissipation point for a single-ended amplifier operating at a

given supply voltage and driving a specified output load.

P

(1)

Even with this internal power dissipation, the LM4882 does

not require heat sinking over a large range of ambient tem-

perature. From Equation 1, assuming a 5V power supply and

an 4

Ω load, the maximum power dissipation point is

316 mW. The maximum power dissipation point obtained

must not be greater than the power dissipation that results

from Equation 2:

P

(2)

For the LM4882 surface mount package,

θ

T

of the system surroundings, Equation 2 can be used to find

the maximum internal power dissipation supported by the IC

packaging. If the result of Equation 1 is greater than that of

Equation 2, then either the supply voltage must be de-

creased, the load impedance increased or T

the typical application of a 5V power supply, with an 4

Ω load,

the maximum ambient temperature possible without violating

the maximum junction temperature is approximately 83˚C

provided that device operation is around the maximum

power dissipation point. Power dissipation is a function of

output power and thus, if typical operation is not around the

maximum power dissipation point, the ambient temperature

may be increased accordingly. Refer to the Typical Perfor-

mance Characteristics curves for power dissipation infor-

mation for lower output powers.

POWER SUPPLY BYPASSING

As with any power amplifier, proper supply bypassing is criti-

cal for low noise performance and high power supply rejec-

tion. The capacitor location on both the bypass and power

supply pins should be as close to the device as possible. As

displayed in the Typical Performance Characteristics sec-

tion, the effect of a larger half supply bypass capacitor is im-

proved low frequency PSRR due to increased half-supply

stability. Typical applications employ a 5V regulator with

10 µF and a 0.1 µF bypass capacitors which aid in supply

stability, but do not eliminate the need for bypassing the sup-

ply nodes of the LM4882. The selection of bypass capaci-

tors, especially C

quency PSRR, click and pop performance as explained in

the section, Proper Selection of External Components

section, system cost, and size constraints.

PROPER SELECTION OF EXTERNAL COMPONENTS

Selection of external components when using integrated

power amplifiers is critical to optimize device and system

performance. While the LM4882 is tolerant of external com-

ponent combinations, consideration to component values

must be used to maximize overall system quality.

The LM4882 is unity gain stable and this gives a designer

maximum system flexibility. The LM4882 should be used in

low gain configurations to minimize THD+N values, and

maximize the signal to noise ratio. Low gain configuartions

require large input signals to obtain a given output power. In-

put signals equal to or greater than 1 Vrms are available

from sources such as audio codecs. Please refer to the sec-

tion, Audio Power Amplifier Design, for a more complete

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. Both the input coupling capacitor, C

the output coupling capacitor, C

filters which limit low frequency response. These values

should be chosen based on needed frequency response for

a few distinct reasons.

CLICK AND POP CIRCUITRY

The LM4882 contains circuitry to minimize turn-on and turn-

off transients or “clicks and pops.” In this case, turn-on refers

to either power supply turn-on or the device coming out of

shutdown mode. When the device is turning on, the amplifi-

ers are internally muted. An internal current source ramps up

the voltage of the bypass pin. Both the inputs and outputs

track the voltage at the bypass pin. The device will remain

muted until the bypass pin has reached its half supply volt-

age, 1/2 V

vice will become fully operational, where the gain is set by

the external resistors.

Although the bypass pin current source cannot be modified,

the size of C

time and the level of “clicks and pops.” By increasing the

value of C

ever, the tradeoff for using a larger bypass capacitor is an in-

crease in turn-on time for the device. There is a linear rela-

tionship between the size of C

are some typical turn-on times for a given C

C

B

T

ON

0.01 µF

20 ms

0.1 µF

200 ms

0.22 µF

420 ms

0.47 µF

900 ms

In order to eliminate “clicks and pops,” all capacitors must be

discharged before turn-on. Rapid on/off switching of the de-

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