LM4873 datasheet(12/19 Pages) NSC

12 / 19 page

Application Information (Continued)

sistor will disable the LM4873. This scheme guarantees that

the shutdown pin will not float, thus preventing unwanted

state changes.

HP-IN FUNCTION

The LM4873 possesses a headphone control pin that turns

off the amplifiers which drive +OutA and +OutB so that

single-ended operation can occur and a bridged connected

load is muted. Quiescent current consumption is reduced

when the IC is in this single-ended mode.

Figure 3 shows the implementation of the LM4873’s head-

phone control function using a single-supply headphone am-

plifier. The voltage divider of R1 and R2 sets the voltage at

the HP-IN pin (pin 16) to be approximately 50 mV when there

are no headphones plugged into the system. This logic-low

voltage at the HP-IN pin enables the LM4873 and places it in

bridged mode operation. Resistor R4 limits the amount of

current flowing out of the HP-IN pin when the voltage at that

pin goes below ground resulting from the music coming from

the headphone amplifier. The output coupling capacitors pro-

tect the headphones by blocking the amplifier’s half supply

DC voltage.

When there are no headphones plugged into the system and

the IC is in bridged mode configuration, both loads are es-

sentially at a 0V DC potential. Since the HP-IN threshold is

set at 4V, even in an ideal situation, the output swing cannot

cause a false single-ended trigger.

When a set of headphones are plugged into the system, the

contact pin of the headphone jack is disconnected from the

signal pin, interrupting the voltage divider set up by resistors

R1 and R2. Resistor R1 then pulls up the HP-IN pin, en-

abling the headphone function. This disables the second

side of the amplifier thus muting the bridged speakers. The

amplifier then drives the headphones, whose impedance is

in parallel with resistors R2 and R3. Resistors R2 and R3

have negligible effect on output drive capability since the

typical impedance of headphones are 32

Ω. Also shown in

Figure 3 are the electrical connections for the headphone

jack and plug. A 3-wire plug consists of a Tip, Ring and

Sleave, where the Tip and Ring are signal carrying conduc-

tors and the Sleave is the common ground return. One con-

trol pin contact for each headphone jack is sufficient to indi-

cate to control inputs that the user has inserted a plug into a

jack and that another mode of operation is desired.

The LM4873 can be used to drive both a pair of bridged 8

Ω

speakers and a pair of 32

Ω headphones without using the

HP-IN pin. In this case the HP-IN would not be connected to

the headphone jack but to a microprocessor or a switch. By

enabling the HP-IN pin, the 8

Ω speakers can be muted.

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 LM4873 is tolerant to a

variety of external component combinations, consideration

to component values must be used to maximize overall sys-

tem quality.

The LM4873 is unity-gain stable, giving the designer maxi-

mum system performance. The LM4873 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

bandwidth 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 fre-

quency response for a few distinct reasons.

DS100993-24

FIGURE 3. Headphone Circuit

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