Application Information

BEEP IN FUNCTION

The Beep In pin (pin 14) is a mono input, for system beeps,

that is mixed into the left and right input. This Beep In pin will

allow an input signal to pass through to the Sum Out and

Line/HP output pins. The minimum potential for the input of

the Beep In signal is 300mV. Beep in signals less than

300mV

circuitry provides left-right signal isolation to prevent

crosstalk at the summed input. As shown in the Fig. 2, it is

required that a resistor and capacitor is placed in series with

the Beep In pin and the node tied to V

sistor. The recommended value for the input resistor is be-

tween 120k

Ω to 10kΩ and the input capacitor is between

.22Fµ and .47µF. The input resistor can be changed to vary

the amplitude of the beep in signal. Higher values of the in-

put resistor will reduce the amplifier gain and attenuate the

beep in signal. In cases where system beeps are required

when the system is in a suspended mode, the LM4834 must

be brought out of shutdown before the beep in signal is input.

SHUTDOWN FUNCTION

In order to reduce power consumption while not in use, the

LM4834 contains a shutdown pin to externally turn off the

bias circuitry. The LM4834 will shutdown when a logic high is

placed on the shutdown pin. The trigger point between a

logic low and logic high level is typically half supply. It is best

to switch between ground and the supply V

maximum device performance. By switching the shutdown

pin to V

mized. While the device will be disabled with shutdown pin

voltages less than V

the typical value of 0.6 µA.The shutdown pin should not be

floated, since this may result in an unwanted shutdown con-

dition.

In many applications, a microcontroller or microprocessor

output is used to control the shutdown circuitry which pro-

vides a quick, smooth transition into shutdown. Another solu-

tion is to use a single-pole, single-throw switch in conjuction

with an external pull-up resistor. When the switch is

closed,the shutdown pin is connected to ground and enables

the amplifier. If the switch is open, then the external pull-up

resistor will shutdown the LM4834. This scheme prevents

the shutdown pin from floating.

MODE FUNCTION

The LM4834 was designed to operate in two modes. In

mode 0 (lineout mode),where the mode pin (pin 17) is given

a logic level low, the attenuation at the Line/HP outputs are

fixed at a gain of 1.4. In mode 1 (headphone mode), where

the mode pin is given a logic level high, the attenuation of the

Line/HP outputs is controlled through the DC voltage at pin

22. The signal levels of the Left and Right Sum Out pins are

always controlled by the DC potential at pin 22 regardless of

the mode of the IC. In mode 0, pin 5 and pin 24 are line out

drivers. In mode 1, pin 5 and pin 24 are headphone drivers.

MUTE FUNCTION

By placing a logic level high on the mute pin (pin 15), the

Right and Left Sum Out pins will be muted. If the LM4834 is

in the headphone mode, the HP/Line out pins as well as the

Sum Out pins are muted. The mute pin must not be floated.

HP SENSE FUNCTION

The LM4834 possesses a headphone sense pin (pin 16) that

mutes the bridged amplifier, when given a logic high, so that

headphone or line out operation can occur while the bridged

connected load will be muted.

Figure 3 shows the implementation of the LM4834’s head-

phone control function using a single-supply.The voltage di-

vider of R1, R2, R4, and R5 sets the voltage at the HP sense

pin (pin 16) to be approximately 50 mV when there are no

headphones plugged into the system. This logic-low voltage

at the HP sense pin enables bridged power amplifier. Resis-

tor R4 limits the amount of current flowing out of the HP

sense pin when the voltage at that pin goes below ground re-

sulting from the music coming from the headphone amplifier.

Resistor R1, R4, and R5 form a resistor divider that prevents

false triggering of the HP sense pin when the voltage at the

output swings near the rail, since V

When a set of headphones are plugged into the system, the

contact pin of the headphone jack is disconnected from the

signal pin, interrupting the voltage divider set up by resistors

R1, R2, R4, and R5. Resistor R1 then pulls up the HP sense

pin, enabling the headphone function and disabling the

bridged amplifier. The headphone amplifier then drives the

headphones, whoseimpedance is in parallel with resistor R2

and R3. Also shown in

Figure 3 are the electrical connec-

tions for the headphone jack and plug. A 3-wire plug consists

of a Tip, Ring and Sleeve, where the Tip and Ring are signal

carrying conductors and the Sleeve is the common ground

return. One control pin contact for each headphone jack is

sufficient to indicate that the user has inserted a plug into a

jack and that another mode of operation is desired.

The LM4834 can be used to drive both a bridged 8

Ω internal

speaker and a pair of 32

Ω speakers without using the HP

sense pin. In this case the HP sense is controlled by a micro-

processor or a switch.

DC VOLUME CONTROL

The DC voltage at the DC Volume Control pin (pin 22) deter-

mines the attenuation of the Sum Out and Line/HP amplifi-

ers. If the DC potential of pin 22 is at 4V the internal ampli-

fiers are set at a gain of 1.4 (2.9dB). The attenuation of the

amplifiers increase until 0V is reached. The attenuator range

DS100015-31

FIGURE 3. Headphone Input Circuit

9

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