Datasheet

LM4936
SNAS269A –APRIL 2005–REVISED APRIL 2013 www.ti.com
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 LM4936 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 20kΩ (R
DOCK1
, R
DOCK2
) are shown in Figure 3 and they set each input amplifier's
gain to -1. Use Equation 8 to determine the input and feedback resistor values for a desired gain.
- A
VR
= R
DOCK1
/R
IN1
and - A
VL
= R
DOCK2
/R
IN2
(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
LM4936'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 values 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 LM4936's shutdown mode must be deactivated before a system alert signal is applied to BEEP
IN pin.
MICRO-POWER SHUTDOWN
Shutdown mode is activated when a digital 0 is loaded into the Shutdown bit, B0. When active, the LM4936's
micro-power shutdown feature turns off the amplifier's bias circuitry reducing supply current to a typical 0.7μA.
Loading a digital 1 into B0 disables shutdown mode. When the LM4936 has power applied, all register bits will
have a default value of 0. Because of this, the LM4936 will be in shutdown mode when power is applied.
MODE FUNCTION
The LM4936's Mode function has two states controlled by bit B2. A digital 0 in bit B2 disables the volume control
and forces the LM4936 to function as a fixed gain amplifier. The gain selection is determined by the GAIN SEL
bit (B3) While in the fixed gain mode the volume setting has no effect on the output. When a digital 1 is loaded
into B2 the output level is determined by the volume control bits. See Table 4 for volume settings.
MUTE FUNCTION
The LM4936 mutes the amplifier and DOCK outputs when a digital 1 is loaded in bit B1. Even while muted, the
LM4936 will amplify a system alert (beep) signal whose magnitude satisfies the BEEP DETECT circuitry. Loading
a digital 0 into B1 returns the LM4936 to normal operation.
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