LM4841 datasheet(18/31 Pages) NSC

18 / 31 page

Application Information (Continued)

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 LM4841’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

DD/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 are shown as

200k

Ω devices in Figure 2. 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 amplifi-

ers. The LM4841’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

LM4841’s shutdown function. Activate micro-power shut-

down by applying ground (logic low) to the SHUTDOWN pin.

When activated, the LM4841’s micro-power shutdown fea-

ture turns off the amplifier’s bias circuitry, reducing the sup-

ply current. On the demo board, the micro-power shutdown

feature is controlled by a single pole switch that connects the

shutdown pin to either V

DD, for normal operation, or directly

to ground to enable shutdown. In a system with a micropro-

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

control voltage to the SHUTDOWN pin.

MODE FUNCTION

The LM4841’s MODE function has 2 states controlled by the

voltage applied to the MODE pin. In Mode 0 (mode pin at

GND), the HP Sense has no effect on the gain setting (only

the Gain Select Input Controls either internal or external

gain). In Mode 1 (mode pin tied high), the HP Sense and

Gain Select both can toggle between Internal and External

Gain. See ’Truth Table for Logic Inputs’ on page 5.

MUTE FUNCTION

The LM4841 mutes the amplifier and DOCK outputs when

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

LM4841 will amplify a system alert (beep) signal whose

magnitude satisfies the BEEP DETECT circuitry. Applying

0V to the MUTE pin returns the LM4841 to normal, unmuted

operation. Prevent unanticipated mute behavior by connect-

ing the MUTE pin to V

DD or ground. Do not let the mute pin

float.

20028087

FIGURE 5. Headphone Sensing Circuit (MT & MH Packages)

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