LM4838 datasheet(18/36 Pages) NSC

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Application Information (Continued)

MICRO-POWER SHUTDOWN

The voltage applied to the SHUTDOWN pin controls the

LM4838’s shutdown function. Activate micro-power shut-

down by applying V

DD to the SHUTDOWN pin. When active,

the LM4838’s micro-power shutdown feature turns off the

amplifier’s bias circuitry, reducing the supply current. The

logic threshold is typically V

DD/2. The low 0.7 µA typical

shutdown current is achieved by applying a voltage that is as

near as V

DD as possible to the SHUTDOWN pin. A voltage

that is less than V

DD may increase the shutdown current.

There are a few ways to control the micro-power shutdown.

These include using a single-pole, single-throw switch, a

microprocessor, or a microcontroller. When using a switch,

connect an external 10k

Ω pull-up resistor between the

SHUTDOWN pin and V

DD. Connect the switch between the

SHUTDOWN pin and ground. Select normal amplifier opera-

tion by closing the switch. Opening the switch connects the

SHUTDOWN pin to V

DD through the pull-up resistor, activat-

ing micro-power shutdown. The switch and resistor guaran-

tee that the SHUTDOWN pin will not float. This prevents

unwanted state changes. In a system with a microprocessor

or a microcontroller, use a digital output to apply the control

voltage to the SHUTDOWN pin. Driving the SHUTDOWN pin

with active circuitry eliminates the need for a pull up resistor.

MODE FUNCTION

The LM4838’s MODE function has 2 states controlled by the

voltage applied to the MODE pin. Mode 0, selected by

applying 0V to the MODE pin, forces the LM4838 to effec-

tively function as a "line-out," unity-gain amplifier. Mode 1,

which uses the internal DC controlled volume control is

selected by applying V

DD to the MODE pin. This mode sets

the amplifier’s gain according to the DC voltage applied to

the DC VOL CONTROL pin. Unanticipated gain behavior can

be prevented by connecting the MODE pin to V

DD or ground.

Note: Do not let the mode pin float.

MUTE FUNCTION

The LM4838 mutes the amplifier and DOCK outputs when

DD is applied to the MUTE pin. Even while muted, the

LM4838 will amplify a system alert (beep) signal whose

magnitude satisfies the BEEP DETECT circuitry. Applying

0V to the MUTE pin returns the LM4838 to normal, unmuted

operation. Prevent unanticipated mute behavior by connect-

ing the MUTE pin to V

DD or ground. Do not let the mute pain

float.

HP SENSE FUNCTION ( Head Phone In )

Applying a voltage between 4V and V

DD to the LM4838’s

HP-IN headphone control pin turns off the amps that drive

the Left out "+" and Right out "+" pins. This action mutes a

bridged-connected load. Quiescent current consumption is

reduced when the IC is in this single-ended mode.

Figure 3 shows the implementation of the LM4838’s head-

phone control function. With no headphones connected to

the headphone jack, the R1-R2 voltage divider sets the

voltage applied to the HP SENSE pin at approximately

50mV. This 50mV puts the LM4838 into bridged mode op-

eration. The output coupling capacitor blocks the amplifier’s

half supply DC voltage, protecting the headphones.

The HP-IN threshold is set at 4V. While the LM4838 operates

in bridged mode, the DC potential across the load is essen-

tially 0V. Therefore, even in an ideal situation, the output

swing cannot cause a false single-ended trigger. Connecting

headphones to the headphone jack disconnects the head-

phone jack contact pin from R2 and allows R1 to pull the HP

Sense pin up to V

DD through R4. This enables the head-

phone function, turns off both of the "+" output amplifiers,

and mutes the bridged speaker. The remaining single-ended

amplifiers then drive the headphones, whose impedance is

in parallel with resistors R2 and R3. These resistors have

negligible effect on the LM4838’s output drive capability

since the typical impedance of headphones is 32

Ω.

Figure 3 also shows the suggested headphone jack electri-

cal connections. The jack is designed to mate with a three-

wire plug. The plug’s tip and ring should each carry one of

the two stereo output signals, whereas the sleeve should

carry the ground return. A headphone jack with one control

pin contact is sufficient to drive the HP-IN pin when connect-

ing headphones.

A microprocessor or a switch can replace the headphone

jack contact pin. When a microprocessor or switch applies a

voltage greater than 4V to the HP-IN pin, a bridge-connected

speaker is muted and the single ended output amplifiers 1A

and 2A will drive a pair of headphones.

20013304

FIGURE 3. Headphone Sensing Circuit

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