LM4936 datasheet(20/28 Pages) NSC

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Application Information (Continued)

0.01µF

2ms

0.1µF

20ms

0.22µF

44ms

0.47µF

94ms

1.0µF

200ms

DOCKING STATION INTERFACE

Applications such as notebook computers can take advan-

tage 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 >1k

Ω (such as powered speak-

ers) with a rail-to-rail signal. Since the output signal present

on the RIGHT DOCK and LEFT DOCK pins is biased to

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 polar-

ized coupling capacitors are used, connect their "+" termi-

nals to the respective output pin, see Figure 2.

Since the DOCK outputs precede the internal volume con-

trol, the signal amplitude will be equal to the input signal’s

magnitude and cannot be adjusted. However, the input am-

plifier’s closed-loop gain can be adjusted using external

resistors. These 20k

Ω (R

DOCK1,RDOCK2) are shown in Fig-

ure 2 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 =RDOCK1/RIN1 and - AVL =RDOCK2/RIN2

(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 theR&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

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 200k

Ω values 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 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.

HP SENSE FUNCTION ( Headphone In )

Applying a voltage between the V

IH threshold shown in the

graph found in the Typical Performance Characteristics and

DD to the LM4936’s HP SENSE control pin or loading a

digital 1 into the HP Control bit (B4) will change the output

mode. The ’+’ outputs will change to be in phase with the ’-’

outputs instead of 180 degrees out of phase. This action

mutes a bridged-connected load since the differential volt-

age across the load is now close to 0V. The HP SENSE pin

over rides the HP Control bit. See Table 2 for more info.

Quiescent current consumption is reduced when the IC is in

this single-ended mode.

Figure 3 shows the implementation of the LM4936’s head-

phone control function. With no headphones connected to

the headphone jack, the R1-R2 voltage divider sets the

20117805

FIGURE 3. Headphone Sensing Circuit

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