Unlike the shutdown function, the headphone control func-

tion does not provide the level of current conservation that is

required for battery powered systems. Since the quiescent

current resulting from the headphone control function is

1000 times more than the shutdown function, the residual

currents in the device may create a pop at the output when

coming out of the headphone control mode. The pop effect

may be eliminated by connecting the headphone sensing

output to the shutdown pin input as shown in Figure 4. This

solution will not only eliminate the output pop, but will also

utilize the full current conservation of the shutdown function

by reducing I

shutdown. This configuration also allows the designer to use

the control inputs as either two headphone control pins or a

headphone control pin and a shutdown pin where the lowest

level of current consumption is obtained from either function.

Figure 5 shows the implementation of the LM4860’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-IN1 pin to be approximately 50 mV when there are

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

age at the HP-IN1 pin enables the LM4860 to amplify AC

signals. Resistor R3 limits the amount of current flowing out

of the HP-IN1 pin when the voltage at that pin goes below

ground resulting from the music coming from the headphone

amplifier. The output coupling cap protects the headphones

by blocking the amplifier’s half-supply DC voltage. The ca-

pacitor also protects the headphone amplifier from the low

voltage set up by resistors R1 and R2 when there aren’t any

headphones plugged into the system. The tricky point to this

setup is that the AC output voltage of the headphone ampli-

fier cannot exceed the 2.0V HP-IN1 voltage threshold when

there aren’t any headphones plugged into the system, as-

suming that R1 and R2 are 100k and 1k, respectively. The

LM4860 may not be fully shutdown when this level is ex-

ceeded momentarily, due to the discharging time constant of

the bias-pin voltage. This time constant is established by the

two 50k resistors (in parallel) with the series bypass capaci-

tor value.

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-IN1 pin,

enabling the headphone function and disabling the LM4860

amplifier. The headphone amplifier then drives the head-

phones, whose impedance is in parallel with resistor R2.

Since the typical impedance of headphones are 32

Ω, resis-

tor R2 has negligible effect on the output drive capability.

Also shown in Figure 5 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

conductors and the Sleave is the common ground return.

One control pin contact for each headphone jack is sufficient

to indicate to control inputs that the user has inserted a plug

into a jack and that another mode of operation is desired.

For a system implementation where the headphone amplifier

is designed using a split supply, the output coupling cap, C

C

and resistor R2 of Figure 5, can be eliminated. The function-

ality described earlier remains the same, however.

In addition, the HP-SENSE pin, although it may be con-

nected to the SHUTDOWN pin as shown in Figure 4, may

still be used as a control flag. It is capable of driving the input

to another logic gate or approximately 2 mA without serious

loading.

01198807

FIGURE 4. HP-SENSE Pin to

SHUTDOWN Pin Connection

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