Datasheet

LM4938
+5V
RPU
100 k:
RS
100 k:
C
OUT
220 PF
C
OUT
220 PF
R
L
1 k:
R
L
1.5 k:
Headphone Jack
HP
Sense
-LOUT
-ROUT
LM4938
SNAS245B –FEBRUARY 2005–REVISED MAY 2013 www.ti.com
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 10kΩ pull-up resistor
between the SHUTDOWN pin and V
DD
. 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 V
DD
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 LM4938'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 LM4938 to effectively 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 LM4938 mutes the amplifier and DOCK outputs when V
DD
is applied to the MUTE pin. Even while muted,
the LM4938 will amplify a system alert (beep) signal whose magnitude satisfies the BEEP DETECT circuitry.
Applying 0V to the MUTE pin returns the LM4938 to normal, unmuted operation. Prevent unanticipated mute
behavior by connecting the MUTE pin to V
DD
or ground. Do not let the mute pain float.
Figure 45. Headphone Sensing Circuit
HP SENSE FUNCTION ( Head Phone In )
Applying a voltage between 4V and V
DD
to the LM4938'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 45 shows the implementation of the LM4938's headphone 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 LM4938 into bridged mode operation. The output coupling capacitor
blocks the amplifier's half supply DC voltage, protecting the headphones.
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