TABLE 1. Logic Level Truth Table for SHUTDOWN, HP-IN, and MUX Operation

SHUTDOWN

PIN

HP-IN PIN

MUX CHANNEL

SELECT PIN

OPERATIONAL MODE

(MUX INPUT CHANNEL #)

Logic Low

Logic Low

Logic Low

Bridged Amplifiers (1)

Logic Low

Logic Low

Logic High

Bridged Amplifiers (2)

Logic Low

Logic High

Logic Low

Single-Ended Amplifiers (1)

Logic Low

Logic High

Logic High

Single-Ended Amplifiers (2)

Logic High

X

X

Micro-Power Shutdown

MUTE FUNCTION

The LM4849 mutes the amplifier and DOCK outputs when

V

pin returns the LM4849 to normal, unmated operation. Pre-

vent unanticipated mute behavior by connecting the MUTE

pin to V

HP SENSE FUNCTION ( Head Phone In )

Applying a voltage between 4V and V

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 2 shows the implementation of the LM4849’s head-

phone 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 LM4849 into bridged mode operation.

The output coupling capacitor blocks the amplifier’s half

supply DC voltage, protecting the headphones. (The HP-IN

threshold is set at 4V). While the LM4849 operates in

bridged mode, the DC potential across the load is essentially

0V. Therefore, even in an ideal situation, the output swing

cannot cause a false single-ended trigger. Connecting head-

phones to the headphone jack disconnects the headphone

jack contact pin from R2 and allows R1 to pull the HP Sense

pin up to V

function, turns off both of the "+" output amplifiers and mutes

the bridged speaker. The amplifier then drives the head-

phones, whose impedance is in parallel with resistors R2

and R3. These resistors have negligible effect on the

LM4849’s output drive capability since the typical impedance

of headphones is 32

Ω.

Figure 2 also shows the suggested headphone jack electri-

cal connections. The jack is designed to mate with a three-

wire plug. The plug’s tip and ring should each carry one of

the two stereo output signals, whereas the sleeve should

carry the ground return. A headphone jack with one control

pin contact is sufficient to drive the HP-IN pin when connect-

ing headphones.

A microprocessor or a switch can replace the headphone

jack contact pin. When a microprocessor or switch applies a

voltage greater than 4V to the HP-IN pin, a bridge-connected

speaker is muted and the single ended output amplifiers will

drive a pair of headphones.

DC VOLUME CONTROL

The LM4849 has an internal stereo volume control whose

setting is a function of the DC voltage applied to the DC VOL

CONTROL pin.

The LM4849 volume control consists of 31 steps that are

individually selected by a variable DC voltage level on the

volume control pin. The range of the steps, controlled by the

DC voltage, are from 0dB - 78dB. Each gain step corre-

sponds to a specific input voltage range, as shown in table 2.

To minimize the effect of noise on the volume control pin,

which can affect the selected gain level, hysteresis has been

implemented. The amount of hysteresis corresponds to half

of the step width, as shown in Volume Control Characteriza-

tion Graph (DS200133-40).

For highest accuracy, the voltage shown in the ’recom-

mended voltage’ column of the table is used to select a

desired gain. This recommended voltage is exactly halfway

between the two nearest transitions to the next highest or

next lowest gain levels.

The gain levels are 1dB/step from 0dB to -6dB, 2dB/step

from -6dB to -36dB, 3dB/step from -36dB to -47dB, 4dB/step

from -47db to -51dB, 5dB/step from -51dB to -66dB, and

12dB to the last step at -78dB.

20076704

FIGURE 2. Headphone Sensing Circuit (MH Pinout)

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