LM4838 datasheet(21/44 Pages) TI

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LM4838

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SNAS131F – JANUARY 2001 – REVISED MARCH 2013

BEEP DETECT FUNCTION

Computers and notebooks produce a system “beep“ signal that drives a small speaker. The speaker's auditory

output signifies that the system requires user attention or input. To accommodate this system alert signal, the

LM4838's beep input pin is a mono input that accepts the beep signal. Internal level detection circuitry at this

input monitors the beep signal's magnitude. When a signal level greater than VDD/2 is detected on the BEEP IN

pin, the bridge output amplifiers are enabled. The beep signal is amplified and applied to the load connected to

the output amplifiers. A valid beep signal will be applied to the load even when MUTE is active. Use the input

resistors connected between the BEEP IN pin and the stereo input pins to accommodate different beep signal

amplitudes. These resistors (RBEEP) are shown as 200kΩ devices in Figure 6. Use higher value resistors to

reduce the gain applied to the beep signal. The resistors must be used to pass the beep signal to the stereo

inputs. The BEEP IN pin is used only to detect the beep signal's magnitude: it does not pass the signal to the

output amplifiers. The LM4838's shutdown mode must be deactivated before a system alert signal is applied to

BEEP IN pin.

MICRO-POWER SHUTDOWN

The voltage applied to the SHUTDOWN pin controls the LM4838's shutdown function. Activate micro-power

shutdown by applying VDD 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 VDD/2. The low

0.7 µA typical shutdown current is achieved by applying a voltage that is as near as VDD as possible to the

SHUTDOWN pin. A voltage that is less than VDD 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 VDD. Connect the switch between the SHUTDOWN pin and ground. Select

normal amplifier operation by closing the switch. Opening the switch connects the SHUTDOWN pin to VDD

through the pull-up resistor, activating micro-power shutdown. The switch and resistor ensure 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 effectively function as a "line-out," unity-gain amplifier.

Mode 1, which uses the internal DC controlled volume control is selected by applying VDD 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 VDD or ground. Note: Do not let the mode pin

float.

MUTE FUNCTION

The LM4838 mutes the amplifier and DOCK outputs when VDD 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 connecting the MUTE pin to VDD or ground. Do not let the mute pain float.

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