The Automatic Switching Circuit is based upon a single

control pin common in many headphone jacks which forms a

normally closed switch with one of the output pins. The out-

put of this circuit (the voltage on pin 5 of the LM4880) has

two states based on the position of the switch. When the

switch inside the headphone jack is open, the LM4880 is en-

abled and the LM4862 is disabled since the NMOS inverter

is on. If a headphone jack is not present, it is assumed that

the internal speakers should be on and the external speak-

ers should be off. Thus the voltage on the LM4862 shutdown

pin is low and the voltage on the LM4880 shutdown pin is

high.

The operation of this circuit is rather simple. With the switch

closed, R

gate voltage of less than 50 mV. The gate voltage keeps the

NMOS inverter off and R

LM4880 to the supply voltage. This shuts down the LM4880

and places the LM4862 in its normal mode of operation.

When the switch is open, the opposite condition is produced.

Resistor R

the inverter and produces a logic low signal on the shutdown

pin of the LM4880. This state enables the LM4880 and

places the LM4862 in shutdown mode.

Only one channel of this circuit is shown in

Figure 2 to keep

the drawing simple but a typical application would be a

LM4880 driving a stereo headphone jack and two LM4862’s

driving a pair of internal speakers. If a single internal speaker

is required, one LM4862 can be used as a summer to mix

the left and right inputs into a mono channel.

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 LM4862 is tolerant of

external component combinations, consideration to compo-

nent values must be used to maximize overall system qual-

ity.

The LM4862 is unity-gain stable which gives a designer

maximum system flexibility. The LM4862 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. 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. The input coupling capacitor, C

first order high pass filter which limits low frequency re-

sponse. This value should be chosen based on needed fre-

quency 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 attenua-

tion. But in many cases the speakers used in portable sys-

tems, whether internal or external, have little ability to repro-

duce signals below 100–150 Hz. Thus using a large input

capacitor may not increase system performance.

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

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

C

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

create pops upon device enable. Thus, by minimizing the ca-

pacitor 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 LM4862 turns

on. The slower the LM4862’s outputs ramp to their quiescent

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 or larger is recommended in all but the most cost sen-

sitive designs.

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