Datasheet

LM4938
www.ti.com SNAS245B –FEBRUARY 2005–REVISED MAY 2013
unity until the voltage on the BYPASS pin reaches 1/2 V
DD
. As soon as the voltage on the BYPASS pin is
stable, the device becomes fully operational. Although the BYPASS pin current cannot be modified, changing the
size of C
B
alters the device's turn-on time and the magnitude of “clicks and pops”. Increasing the value of C
B
reduces the magnitude of turn-on pops. However, this presents a tradeoff: as the size of C
B
increases, the turn-
on time increases. There is a linear relationship between the size of C
B
and the turn-on time.
DOCKING STATION INTERFACE
Applications such as notebook computers can take advantage of a docking station to connect to external devices
such as monitors or audio/visual equipment that sends or receives line level signals. The LM4938 has two
outputs, Right Dock and Left Dock, which connect to outputs of the internal input amplifiers that drive the volume
control inputs. These input amplifiers can drive loads of >1kΩ (such as powered speakers) with a rail-to-rail
signal. Since the output signal present on the RIGHT DOCK and LEFT DOCK pins is biased to V
DD
/2, coupling
capacitors should be connected in series with the load when using these outputs. Typical values for the output
coupling capacitors are 0.33µF to 1.0µF. If polarized coupling capacitors are used, connect their "+" terminals to
the respective output pin, see Figure 3.
Since the DOCK outputs precede the internal volume control, the signal amplitude will be equal to the input
signal's magnitude and cannot be adjusted. However, the input amplifier's closed-loop gain can be adjusted
using external resistors. These 20k resistors (R
FR
, R
FL
) are shown in Figure 3 and they set each input amplifier's
gain to -1. Use Equation 7 to determine the input and feedback resistor values for a desired gain.
- A
VR
= R
FR
/R
IR
and - A
VL
= R
FL
/R
IL
(8)
Adjusting the input amplifier's gain sets the minimum gain for that channel. Although the single ended output of
the Bridge Output Amplifiers can be used to drive line level outputs, it is recommended that the R & L Dock
Outputs simpler signal path be used for better performance.
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
LM4938'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 V
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 (R
BEEP
) are shown as 200kΩ devices in Figure 3. 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 LM4938's shutdown mode must be deactivated before a system alert signal is applied to
BEEP IN pin.
If the “Beep” feature is not needed, remove the two Beep Resistors (200k) and Beep input capacitor (.33μf).
Then, tie the Beep input pin (#11) to ground. Note that the Beep Circuit is designed to operate with only a square
wave input from a control source.
MICRO-POWER SHUTDOWN
The voltage applied to the SHUTDOWN pin controls the LM4938's shutdown function. Activate micro-power
shutdown by applying V
DD
to the SHUTDOWN pin. When active, the LM4938'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
0.5 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.
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